Getting the Biggest Bang for Your Air-Sealing Buck

Most new homes are leaky. In the typical new home, significant volumes of air enter through cracks near the basement rim joists and exit through ceiling holes on the building’s top floor. These air leaks waste tremendous amount of energy.

air_seal_bangIn recent years, after years of prodding by building scientists, code officials have finally taken a few stabs at addressing air leakage. (For more information on recent code changes related to building airtightness, see New Air Sealing Requirements in the 2009 International Residential Code and An Overview of the 2012 Energy Code.)

Of course, some builders have focused on energy efficiency for years, and many of these builders own a blower door. If you have your own blower door, you have probably learned by trial and error which cracks matter most.

However, the vast majority of contractors build homes without any feedback from a blower door. If these builders want to improve the airtightness of the homes they build, they probably don’t know where to start.

Get the big holes first

The first step is to make sure that there aren’t any really big holes in your homes. (Joe Lstiburek calls these “the Joe-sized holes”; they’re the holes that are big enough for Joe to crawl through.) You may be thinking, “Can a house really have holes that big?” The answer, sadly, is “Yes, it really can.”

Let’s raise the bar just a little, and make a list of holes that are big enough for a cat to walk through. These include:

  • Holes in the air barrier behind zero-clearance metal fireplaces.
  • Unsealed holes above kitchen soffits.
  • Unsealed holes above dropped ceilings.
  • Attic access hatches or pull-down attic stairs without any weatherstripping.
  • Unsealed utility chases that connect basements with attics.
  • Holes behind bathtubs installed on exterior walls.

Once these holes are patched — in most cases, using OSB, plywood, rigid foam, or ThermoPly — what’s next? If you are a Passive house builder aiming to achieve 0.6 ach50, the answer is simple: every conceivable crack in the home’s thermal barrier needs to be sealed. In some cases, Passive house builders use a redundant approach — for example, using both caulk and a gasket.

If you are a production builder, you probably don’t have the time or inclination to approach air sealing with a fastidious attention to detail that Passive house builders employ. So perhaps you buy a case of caulk and begin by sealing the cracks between double studs and double top plates. Or maybe you focus on sealing the cracks around windows. When you run out of caulk, you might call it a day.

Does this approach make sense? Not really.

Quantifying the results of sealing measures

Dave Wolf, a senior research and development project leader at Owens Corning, has completed a study to determine which cracks and holes result in the “biggest bang for your air-sealing buck.” Wolf’s research had two components: laboratory measurements of air leakage through several 8 ft. by 8 ft. mockups of building assemblies, and field research at a 1,400-square-foot Owens Corning test house. (To measure the results of different air-sealing measures, the researchers used a blower door: “All joints were selectively sealed and/or unsealed for measuring their contribution to the overall air leakage of the house.”)

Wolf concluded that the five most important areas for builders to focus their air-sealing efforts are:

  • Cracks at recessed can lights in the top-floor ceiling.
  • Cracks between duct boots in the top-floor ceiling and the ceiling drywall.
  • Cracks between the top plates of top-floor partitions and the partition drywall.
  • Leakage through walls separating a house from an attached garage.
  • Cracks in the rim-joist area.

A few comments on Wolf’s findings:

  • The researchers did not test leaks around floor-mounted duct boots.
  • The researchers did not test leaks at cracks between ceiling drywall and bath exhaust fan housings. Wolf speculates that these leaks might be worse than the leaks around ceiling-mounted duct boots, because with most bath fans, “There isn’t necessarily a flange. Unlike with a duct boot, it is a flangeless opening.”
  • Since leakage was measured by a blower door, the reported results exaggerate the importance of leaks near the neutral pressure plane, and didn’t properly evaluate the way the stack effect disproportionately depressurizes the lowest areas of a house and pressurizes the highest areas of a house. Is spite of this fact, four out of five of the highlighted areas are either down low (the basement rim joist) or up high (recessed cans, ceiling duct boots, and top-plate cracks). My conclusion: these leakage areas are even more important than this research indicates. So be sure to seal these areas!
  • Walls between a house and an attached garage made the list due to the fact that these walls leak more than other walls — not due to any concerns over indoor air quality, nor to the fact that air in a garage is often contaminated. Why are these walls leaky? “These are the only exterior walls where you have drywall on both sides,” Wolf told me. “Drywall is a flimsy material compared to OSB sheathing or plywood. When you mechanically fasten drywall on the outside of the studs, the crack where it mates with the framing is not as tight as what you get with OSB or plywood.” These walls are good candidates for the Airtight Drywall Approach.

At the opposite end of the spectrum are the cracks that take a lot of sealant and a lot of time to seal, without reducing total air leakage by very much. This group — let’s call it the “why bother?” list — includes vertical sheathing joints, the cracks between double top plates, and the cracks between the wall sheathing and the framing of window rough openings.

Air-sealing tips

For more information on ways to seal air leaks in these areas, check out the resources listed below.

Recessed Can Lights: “In most cases, these innocent-looking circles are actually holes in your ceiling. Not only do recessed can lights leak air, but warm lightbulbs also make the situation worse, turning the holes into small chimneys. The heat source accelerates the stack effect, speeding up the flow of air.”

Duct boots. “Practically no sealing takes place [at duct boots]. … It’s also not just a single issue at the boots, it’s two specific locations: between the boot and the metal frame and more importantly, between the ceiling drywall (or subfloor) and the metal frame. … Why this is not the HVAC contractors fault is the timing of construction activities. The time to perform this sealing usually doesn’t align with their typical site visits. Should it be the painters? Drywallers? Insulators? Site supervisors? Punch out specialists? I think the answers will vary, and it will be up to the individual builders to decide when this sealing should take place given their particular build process.”

Cracks between partition top plates and drywall. “Between the drywall and the top plate of the wall, we have a few different choices to seal up. I tend to go for an acoustical sealant. The stuff is very sticky, and it doesn’t completely cure. It will stay sticky and be able to move with the materials.”

Wall between a house and an attached garage. “The air-sealing details to isolate an attached garage are not easy.”

Rim joists: “The next step was to insulate and air seal at the rim joist, so that the thermal boundary was continuous to the bottom of the subfloor above. I chose to do this with a two-component spray polyurethane kit.”

A few caveats

In a PowerPoint presentation describing his research, Wolf provides a helpful summary of his work, as well as a list of caveats:

  • The study provides a “blower‐door‐centric point of view. All of the … results are prioritized based on the effect on whole‐house leakage, not  thermal comfort, IAQ, etc.”
  • It best to try to “seal all the joints, if you can. The sealing of all joints/openings is important, although some are more important than others.”
  • It makes sense to “get the big holes first. This study focuses on small joints/openings only (i.e., the big holes are presumed to be blocked & sealed).”
  • “Sometimes the cladding matters. The wall cladding is assumed to be air permeable (e.g., vinyl, fiber-cement, wood siding, or brick, not stucco or stone veneer).”
  • “These results are for general guidance. These results should be considered directional, not absolute, since construction quality varies from house to house.”
  • “Don’t abandon common sense. If you can see daylight through a joint, it should be sealed, regardless of what this study may indicate.”

Drywall is the unsung hero of air-sealing efforts

When weatherization contractors try to seal leaks in an existing home, the work is known as “blower-door-directed air sealing.” Veterans of this type of work know that some time-consuming measures have little effect on a home’s air leakage rate, while other simple measures produce good results. Here are two lessons that weatherization contractors have learned:

  • It always makes sense to seal the big holes first.
  • Holes near the bottom of the house (in a crawl space or basement) and holes near the top of the house (in the ceiling of the top floor) matter much more than holes near the center of the house (an area that is also known as the “neutral pressure plane”).

Dave Wolf’s research looks at these issues from the perspective of a new home builder rather than a weatherization contractor. His results give important guidance to builders who are just beginning to think about air sealing.

The research results correlate well with long-standing advice on air-sealing. “When you stack up these results with typical air-sealing advice, the list passes the gut check for what has been said for a long time,” Wolf told me. “That might be deflating, since we just found out what the industry already knew. But what is novel about this is that we have quantified the amount of leakage per unit length for all of these joints. We know for a top-plate-to-drywall crack how many cfm50 per foot that the crack leaks. And we can take those parameters to do calculations: ‘Is it worth it to me to seal this particular joint?’ With the information we have, you can calculate that if you have 100 feet of this type of joint, the sealing ought to be worth about so much to me from a blower-door standpoint. We can use the information to help make strategic decisions.”

Wolf’s research demonstrated the contribution that drywall makes to the airtightness of wall assemblies. “When we tested naked walls without drywall in place, they leaked a lot,” Wolf told me. “Sometimes you could even see daylight through the cracks. But when you put the drywall on, the leakage went down dramatically. The drywall is providing a secondary air barrier. There might be five different locations in the wall cavity where air is leaking in, but the air is having a hard time getting past the drywall. The pressure in the wall cavity builds up, and you don’t have a 50 pascal pressure difference across your sheathing; you do across the drywall, but not across the sheathing. It took making the measurements for me to see the effect of this traffic jam in the wall cavity.”

If you care about airtightness, it’s probably time for you to bring your painter a box of donuts. “When the painter comes along, the builder holds the painter accountable,” said Wolf. “The painter has to make sure that finishes look magnificent, and the work includes caulking the trim pieces to the drywall. What the painter is inadvertently doing is improving the airtightness of the drywall layer. We would never advocate using painter’s caulk as part of your air barrier. It is not the right approach. But it is interesting that it does have an impact. The painter is playing a role in improving the airtightness of the wall.”

by Martin Holladay
August 23, 2013
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Radiant Barriers

How it Works: Radiant Barriers

Want to get builders into a heated discussion? Bring up radiant barriers.radiant-barrier

Promoted as a method for reducing cooling costs by mitigating solar heat gain through walls and attics, these shiny (typically aluminum) surfaces have as many detractors as proponents. The dispute is not about the science of radiant-heat transmission; it’s whether applying that science to your roof will make a difference on your utility bill.

First, the science
Heat energy moves through houses (and everything else) in three ways: conduction (when objects touch), convection (through air movement), and radiation (through an airspace or vacuum via electromagnetic waves). Insulation, thermal breaks, and air-sealing prevent conduction and convection; radiant barriers prevent heat transfer through radiation only.

Emissivity (or emittance) measures how much radiant energy a material emits. It’s rated on a scale from 0 to 1; the lower the value, the less energy emitted. Radiant barriers by definition have an emissivity of 0.1 or less, emitting 10% or less of the radiant energy striking them.

Are they radiating or reflecting?
Both, actually. Surfaces that are highly reflective to long-wave (heat) energy are also low-emitting. Aluminum, for example, reflects 97% of the long-wave radiation that hits it, emitting 3% into the airspace on the other side. Remember, though, we’re talking about invisible, long-wave radiation—not visible light. White paint, for example, does a great job of reflecting light but a poor job of blocking long-wave heat transfer. The key point is this: A material can look reflective and not be a good radiant barrier, and a good radiant barrier will work whether or not the shiny side is facing the heat source—as long as it is facing an airspace.

Fine. But do they work?
Current research supports radiant barriers in attics as a viable strategy for reducing cooling loads in hot climates. This reduction, however, is limited to solar gain from the attic—about 22% of a home’s cooling load. So even though research has found that radiant barriers can deflect 40% of incoming attic heat, the net savings represents only 8% to 10% of a home’s total cooling costs.

The benefit is even sketchier in northern homes, where summer heat gain is less of a concern and the barriers may limit beneficial winter solar gain. Although radiant barriers may help to retain winter heat, most winter heat loss through attics is due to convection (rising air), not radiation—making proper insulation and air-sealing far more effective.

Here’s how radiant barriers work.

Where radiant barriers work—and where they don’t
Because radiation occurs in an airspace, radiant barriers won’t work unless they face an airspace. Pressed between two surfaces, a radiant barrier becomes a heat conductor. In addition, because heat moves toward cold, there also must be a difference in temperature between materials. Radiant barriers, then, are of negligible benefit in well-insulated homes.

Vented attics can be good places for radiant barriers because they contain large airspaces and take the brunt of solar-heat gain. Because walls are subject to less solar gain and because heat transfer through them relies more on conduction and convection than radiation, radiant barriers in walls provide less benefit. Although barriers placed on attic floors can work, surface dust will eventually hinder performance.

New constructionnew-construction-1
In a new vented attic, radiant-barrier sheathing can be used (left), or a foil barrier can be draped over the rafters or trusses before sheathing is installed (below left). In the second example, the airspace created in the rafter bays enhances the foil’s effect by increasing






Radiant-barrier foil can be attached to the rafter sides (below) or to the faces (top). The latter method curtails heat transfer through the rafters and is preferred. In both cases, gaps at the top and bottom promote ventilation. Double-sided foil provides a small increase in effectiveness.

by Debra Judge Silber
From Fine Homebuilding 236, pp. 18-19 May 16, 2013
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Whole-House Fans vs. Powered Ventilators

What’s the Difference?: Whole-House Fans vs. Powered Ventilators

Whole-house fansWhole House Ventilator
A whole-house fan is an attic-mounted fan that exhausts air from a home at night, when the heat of the day has passed and the outdoor temperature has dropped enough to feel comfortable. The main advantage of using a whole-house fan instead of an air conditioner is to save energy. A whole-house fan usually draws between 200w and 700w, in contrast to a central air conditioner, which draws 2000w to 5000w.

The fan pulls air from a central hallway and blows it into the attic. Because whole-house fans are relatively powerful—usually rated between 2000 cfm and 6000 cfm—they quickly exhaust hot indoor air, allowing cooler outdoor air to enter through downstairs windows. Once the house has cooled, the fan can be turned off and the windows closed. Keeping windows closed from early morning until evening prevents cool air inside the house from escaping.

Because a whole-house fan pulls all the hot air from the house into the attic, the fan won’t work effectively unless the attic has large openings to exhaust the hot air. It’s better to have too much vent area than not enough. A good rule of thumb is to have 1 sq. ft. of net free vent area for every 750 cfm of fan capacity. The net free vent area is the total size of the vents, minus the area covered by obstructions such as louvers and insect screens. The vent area can be made up of a combination of soffit vents, ridge vents, and gable vents.

Where does a wholehouse fan make sense?Ventilator-Whole House
Whole-house fans make sense in areas with cool nights. If you live in a place where the temperature stays in the 80s all night long, a whole-house fan won’t help you much. However, even if you need to seal your house and turn on your air conditioner during the hottest months of summer, a whole-house fan may be useful during the spring and fall, when nights are cool but days can be hot.

Whole-house fans don’t make sense for homes in neighborhoods where security concerns prevent homeowners from leaving their windows open. They also don’t make sense for homes with a gas water heater or furnace in the attic; such a powerful fan can extinguish the pilot light or interfere with the draft.

Because they depressurize a home, whole-house fans can cause atmospherically vented appliances—for example, a gas-fired water heater—to backdraft. If the homeowner remembers to open plenty of windows before turning on the fan, backdrafting probably won’t occur. However, the best way to avoid backdrafting problems in a house with a whole-house fan is to make sure that the appliances have sealed combustion or direct venting rather than atmospheric venting.

Whole-house fans represent a big hole in your ceiling—a hole that is likely to leak a lot of heat during the winter unless it is properly sealed with an insulated cover.

Powered attic ventilatorsAttic Ventilator
Powered attic ventilators are usually mounted on a sloped roof, but they also can be installed in the gable wall of an attic. Most are controlled by a thermostat so that they turn on when the attic gets hot.

The intent of a powered attic ventilator is to exhaust hot air from the attic and replace it with cooler outdoor air. The idea is to save energy by reducing the run-time of your air conditioner. Installers evidently hope that a powered attic ventilator will save more energy than the electricity required to run the fan.

Although the logic behind powered attic ventilators is compelling to many hot-climate homeowners, these devices can cause a host of problems. Here’s the basic one: A powered attic ventilator will depressurize your attic, and it’s hard to predict where the makeup air will come from. Although the arrows in some sales brochures show outdoor air entering the attic through the soffit vents, that’s not what usually happens.

In an online Q&A post for Advanced Energy, a nonprofit corporation in North Carolina, building-science consultant and researcher Arnie Katz says this: “In most of the houses we’ve tested, the attic fans were drawing some of their air from the house, rather than from the outside. In other words, they are cooling the attic by drawing air-conditioned air out of your house and into the attic. Air-conditioning the attic is not recommended by anyone I know as an effective strategy for reducing your bills.”

Katz also mentions a more alarming problem with powered attic ventilators: “In one house we tested, we measured substantial levels of carbon monoxide (CO) in the daughter’s bedroom in the basement. The CO was coming from the water heater next to the bedroom, which was backdrafting. The daughter had been suffering from flulike symptoms for some time. The backdrafting was caused by the powered attic vent fan.”

What do I do if my attic is too hot?Ventilator-Attic Fan

A hot attic isn’t necessarily a problem. If you don’t have any ductwork or HVAC equipment up there, who cares how hot it gets? After all, you should have a thick layer of insulation on your attic floor to isolate your hot attic from your cool house.

If you do have ductwork or HVAC equipment in your attic, the designer and builder of your home made a major mistake. Solutions include:

  • Moving your ductwork and HVAC equipment to the interior of your home.
  • Sealing leaky duct seams and adding insulation on top of your ductwork.
  • Moving the insulation from your attic floor to the sloped roof assembly, creating an unvented conditioned attic.

If you believe that your house has a hot ceiling during the summer, the solution is not a powered attic ventilator. The solution is to seal any air leaks in the ceiling and to add more insulation to the attic floor.

by Martin Holladay
From Fine Homebuilding 235 (Houses), pp. 30-32 April 25, 2013
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Hiding Ducts in Conditioned Space

Ducts, furnaces, and air handlers belong inside a house’s conditioned spacehiding-ducts-1

Ducts, furnaces, and air handlers belong inside a house’s conditioned space. The best locations for ducts are insulated basements, sealed crawlspaces, or unvented conditioned attics. If placing ducts in these locations won’t work, they can also be installed in open-web floor trusses (in a two-story house with a centrally located mechanical room) or in some type of soffit, dropped ceiling, or chase. In this “Energy-Smart Details” article, senior editor Martin Holladay explains how to follow each method, and detailed drawings show how to hide ducts in a chase and in a ceiling. Before ducts are installed in a chase, the chase needs to be lined with an airtight barrier such as drywall or plywood with sealed seams. An alternative to hiding ducts in a dropped ceiling is to recess them into special roof trusses, called plenum trusses. Holladay concludes by emphasizing that even when ducts are inside a house’s conditioned space, their seams should still be sealed. Otherwise, remote registers may not get sufficient airflow, and leaks may pressurize soffits or joist bays, forcing conditioned air outdoors through cracks in the rim-joist area.

By now, most conscientious builders and designers know that ducts, furnaces, and air handlers belong inside a house’s conditioned space. Researchers have shown that ductwork in unconditioned attics or vented crawlspaces wastes about 20% of the output of a furnace or air conditioner. If the duct system is unusually leaky and poorly insulated—or as occasionally happens, if some of the ducts are crushed, ripped, or completely disconnected—the energy waste will be far higher.

The furnace or air handler’s location determines where ducts should be installed. The best locations for ducts are in insulated basements, sealed crawlspaces, or unvented conditioned attics. If these places won’t work, ducts can be installed in openweb floor trusses or in some type of soffit, dropped ceiling, or chase.

Ducts in open-web floor trusses
If you are building a two-story house with a centrally located mechanical room, it often makes sense to put ducts in open-web floor trusses. Follow these steps:
• Any duct leaks will pressurize the joist bays, so be sure to air-seal the rim joist carefully to keep conditioned air indoors.
• Order joists at least 16 in. deep to make room for insulation and ductwork. Account for this when designing stairs.
• If you are building a tight, well-insulated house with high-performance windows, locate supply registers near the center of the house instead of at the perimeter. This will keep duct runs short.
• Communicate and coordinate with the HVAC contractor, the plumber, and the electrician. These subcontractors will all be competing for the same joist space.

Hide ducts in a chase
Make sure that duct chases don’t have any leaks or air pathways that communicate with the outdoors. Before any ducts are installed, the chases need to be lined with an airtight barrier—for example, drywall, plywood, or OSB with sealed seams.








Ducts in soffits or dropped ceilings
In a single-story home, it often makes sense to install ducts in soffits or in a dropped ceiling. This design works especially well in a house with a central hallway flanked by bedrooms on both sides. In a house with a more complicated floor plan, duct soffits can be built along the top of any wall. If the house is designed with 9-ft. ceilings, there will be plenty of room to lower the ceiling height where necessary.

For this approach to work, you’ll need to air-seal the soffit before the ducts are installed. If the drywall crew can’t come to the site twice, the soffit will probably be sealed by the framers with OSB and caulk. If this step is done poorly, however, the house will have a major air leak.



Ducts in a special roof truss
It’s possible to order special roof trusses, called plenum trusses, designed to accommodate a duct chase near the attic floor. The resulting chase is variously called a raised HVAC coffer and an inverted soffit. In a house that has an 8-ft. ceiling, these special roof trusses make more sense than site-built soffits. Like a site-built soffit, a chase within a roof truss must be air-sealed carefully before the ducts are installed, which means either that the drywall hangers and tapers have to come to the job site twice, or that your framing crew has to be trained to do the required air-sealing.

These roof trusses create a bump in the attic floor, complicating the work of the attic-insulation crew. The chase’s walls may be vertical or sloping; in either case, protect the insulation on the walls of the chase with an attic-side air barrier.

You still need to seal the duct seams
In years past, it was commonly assumed that as long as ducts were installed inside a home’s conditioned envelope, you didn’t need to seal the duct seams. These days, however, most energy consultants insist that HVAC contractors seal duct seams with mastic, even when all the ducts are inside the conditioned space. There are at least two good reasons for this practice. First, if ductwork is leaky, remote registers may get insufficient airflow, leading to comfort complaints. Second, duct leaks in soffits or joist bays can pressurize these hidden compartments, forcing conditioned air outdoors through cracks in the rim-joist area.

Is all this work really worth the hassle?
Placing your ducts indoors has several benefits:
• It may be possible to downsize the air conditioner and furnace, which saves money.
• Room-to-room temperature differences will probably be reduced, which improves occupant comfort.
• Energy bills should be significantly lower.

In spite of these obvious benefits, bringing the ducts inside the conditioned envelope is usually a headache for the builder, and the necessary details raise costs. Open-web trusses will probably cost more than I-joists, and deep joists may require a longer stairway. For a successful job, the general contractor will need to budget time for facilitating coordination between all of the trades, including the framers, the HVAC contractor, the plumber, the electrician, and the drywallers.

by Martin Holladay
Fine Homebuilding 233, pp. 86-87 January 10, 2013
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Two Ways to Insulate Attic Kneewalls

Kneewalls—short walls under sloped ceilings—are common in story-and-a-half homes and in bonus rooms above garages. Although kneewalls can help turn attics into living space, they often present insulation challenges. Most builders install fiberglass batts between the studs and some type of blown insulation between the floor joists. This quick-and-dirty approach doesn’t work well because air from the soffit vents flows freely through the unprotected batts. Because these kneewalls usually have no sheathing on the back side, the fiberglass batts eventually fall out of the stud bays and flop over onto the floor. Whether they flop over or not, though, cold air can enter the living space through unsealed electrical boxes or cracks around access doors.
Many builders also forget to add blocking between the floor joists under the kneewall, so cold air has an easy path into the uninsulated joist bays that separate the first floor from the second floor. A leaky kneewall also can provide a pathway for escaping warm air to heat up roof sheathing, which can contribute to ice dams.

There are two ways to insulate triangular attics behind kneewalls. The traditional approach is to insulate the kneewall and the attic floor behind the kneewall. This method can be made to work, but the necessary air-sealing details are demanding and fussy. A better approach is to insulate the roof slope above the attic. This brings the triangular attic into the home’s conditioned space and is therefore the only method to use if the attic includes ducts or plumbing pipes.

Good: air-seal and insulate the kneewall
If you plan to insulate a kneewall and the attic floor behind the kneewall, protect the insulation with an adjacent air barrier. The air barrier should have no leaks, especially in the areas where the floor meets the wall and the wall meets the roof.
This approach requires a lot of blocking under the bottom plate of the kneewall and between the rafters above the top plate of the kneewall. If the roof is vented, the rafter blocking should extend to the ventilation baffles; if the roof is not vented, this blocking should extend all the way to the roof sheathing. Most builders use rigid foam as blocking, although it’s also possible to use solid lumber. In either case, seal the perimeter of the blocking with caulk or canned foam. After you install the blocking, the kneewall and the attic floor can be insulated. If you are using air-permeable insulation (fiberglass, cellulose cellulose, or mineral wool), it’s essential to include an air barrier on the back side of the kneewall. The best material for this purpose is rigid foam; acceptable alternatives include OSB, ThermoPly, or drywall. It’s also possible to use housewrap, but it’s less effective. Seal all panel seams with caulk or compatible tape. If you install rigid foam on the back side of the kneewalls, remember that thick foam is more likely to keep stud bays warm and free of condensation than thin foam.
An insulated kneewall should be air-sealed as if it were an exterior wall. This means that all penetrations (including electrical boxes and duct penetrations) should be made as airtight as possible.
Access doors need to be detailed like exterior doors. These doors need to include thick insulation (for example, several layers of rigid foam glued to the back of the door), a tight threshold, good weatherstripping, and a latch that pulls the door tightly shut.

Better: insulate the sloped ceiling
It’s usually easier and more effective to insulate the sloped ceiling rather than the kneewall. In a traditional story-and-a-half Cape, the insulation should extend from the rafter bird’s mouths to somewhere above the flat ceiling above the second floor.
Sloped roof assemblies can be insulated with a wide variety of insulation materials, including spray foam, fiberglass batts, or blown-in cellulose. If you choose an air-permeable insulation like fiberglass or cellulose, most building codes require the roof assembly to include a ventilated air gap directly above or directly below the roof sheathing. If you’re installing spray foam, the roof assembly can be unvented.
Air-permeable insulation should be protected by a durable air barrier on both sides of the insulation. The top-side air barrier is usually some type of panel forming the bottom of the ventilation chute, while the bottom-side air barrier can be drywall, OSB, ThermoPly, or rigid foam. To ensure a tight air barrier, the seams of these panels should be sealed. It’s also important to caulk the joint between these panels and the kneewall top plate.
Make sure that you install enough insulation to meet or exceed minimum code requirements. The 2012 IRC requires at least R-30 in climate zone 1, R-38 in zones 2 and 3, and R-49 in zones 4 and higher.

Keep the attic outside

1. Keep the attic outside
Insulating a kneewall and the attic floor behind the kneewall is possible but tricky. This method of insulation won’t work if the triangular attic includes ducts or plumbing pipes. An insulated kneewall is effectively an exterior wall, so you’ll need a layer of rigid foam, drywall, OSB, or ThermoPly on the back side of the kneewall to prevent cold air from degrading the performance of the insulation. Remember to seal air leaks at any electrical boxes in the kneewall and ceiling below the insulated floor.


Bring the attic inside

2. Bring the attic inside
Insulating between the rafters simplifies the air-sealing details. Now that the kneewall is located within the home’s thermal enclosure, it’s no longer necessary to air-seal electrical boxes or access doors in the kneewall. Don’t forget to verify that the R-value of the roof insulation meets minimum code requirements.


Remodeling: The devil’s triangle
A new construction project has many obvious advantages over a retrofit project. If you’re building a new home with a kneewall, it makes sense to insulate between the rafters before the kneewall is framed.

Weatherization workers who insulate older homes refer to an attic behind a kneewall as “the devil’s triangle.” These tight environments aren’t conducive to high-quality work. In some cases, these areas are so tight that it’s hard to crawl in there, much less swing a hammer. Even when a worker can enter with a tool belt, a staple gun, and a cordless drill, it probably will be hard to get full-size sheets of drywall or OSB into position.

If a spray-foam contractor is willing to take on the job, count your blessings, but check with your local building official to find out whether the foam needs to be protected by a layer of drywall.

If you have no choice but to crawl into a tight space to install sheet goods such as drywall or foil-faced foam, one trick is to score the facing on one side of each sheet so that it can be folded in half to fit through a tight opening.

If you have to insulate a triangular attic that is too small to crawl into, it may be necessary to fill the space with dense-packed cellulose. Because this technique doesn’t allow for roof ventilation, it is somewhat controversial. Nevertheless, this approach has been used successfully for years by weatherization contractors in New England. According to Bill Hulstrunk, technical manager at National Fiber, “We do it only if the area is so small that we can’t get a body in there. In other words, the kneewall has to be 18-in. high or less. The first time the tube is inserted, it sits on the bottom of the floor. Then after a while, you pull out the tube and reinsert it higher up to top everything off.”

by Martin Holladay
Fine Homebuilding 230, pp. 88-89 September 6, 2012
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5 Proven Ways to Optimize Framing

Advanced framing can save materials and time while boosting your homes’ efficiencies. Here are five techniques that can be adopted independently.

As builders look for ways to cut costs while meeting ever-more-stringent energy codes, many are turning to advanced framing methods. Also known as “optimum value engineering (OVE),” advanced framing techniques optimize material usage to cut down on waste, eliminate redundancies, reduce labor, and increase a home’s energy efficiency, while maintaining structural integrity.

APA-The Engineered Wood Association recently published a 24-page Advanced Framing Construction Guide that provides an overview of some of the techniques, including those that can be used toward Energy Star certification. To help builders make the transition to new building methods, advanced framing can be implemented in stages.
Here is a look at 5 advanced framing techniques that builders can adopt one at a time as they ease into new, more efficient building methods.

Each of these concepts focuses on increasing cavity insulation and reducing thermal bridging, thereby providing overall higher whole-wall R-values. Furthermore, by reducing waste and making better use of framing materials, these sustainable techniques achieve points in green building programs such as the National Green Building Standard (ICC-700) and LEED for Homes.

1. Corners
Insulated corners eliminate the isolated cavity found in conventional three- or four-stud corners, making it easier to install insulation and providing for more cavity insulation space.

Advanced framing wall corners can include insulated three-stud corners, sometimes referred to as “California corners” (Figure 1), or two-stud corner junctions with ladder blocking, drywall clips, or an alternative means of supporting interior or exterior finish (Figure 2).




2. 2×6 Framing Placed 24 Inches On Center
Framing members are traditionally spaced 16 inches on center (o.c.). Advanced framing methods increase member spacing, typically to 24 inches o.c., effectively trimming the number of required studs by about one-third (Figure 3). Walls built with 2×6 wood framing spaced 24 inches o.c. have deeper, wider insulation cavities than conventional 2×4 framing spaced 16 inches o.c., thereby increasing the amount of insulation inside the wall and improving the whole-wall R-value. For example, an advanced-framed R-20 code-compliant wall system provides a greater whole-wall R-value than conventionally framed 2×6 walls with studs spaced 16 inches o.c. or code-minimum 2×4 walls sheathed with foam.


With reduced framing members and larger cavities, the efficiency of other trades will be enhanced. More space between framing members means fewer studs for plumbers and electricians to drill through and fewer cavities for insulators to fill. If a builder is switching from a 2×4 stud wall to a 2×6 stud wall, the decreased number of pieces typically offsets the cost of the deeper framing members, providing more efficient insulation, often for the same or less cost.

3. Ladder Junction Tee Intersections
Advanced framing ladder junctions are used at wall intersections with 2x blocking at 24 inches o.c. vertical spacing (Figure 4). This method requires less than 6 feet of blocking material in a typical 8-foot-tall wall. In conventional walls, interior wall intersections include a stud at each side of the intersecting wall, which can require as much as 16 feet of stud lumber, plus additional blocking material. Blocking can also often be made up of lower-grade lumber or lumber scraps (cutoffs from plates or other framing), thus reducing waste and disposal costs.

















The advanced framing ladder junction method, when used at junctions between interior and exterior walls, provides a cavity that can be easily insulated, versus conventional, three-stud interior wall intersections that may contain voids that are rarely insulated.

Wall intersections that feature a continuous drywall application minimize air infiltration by reducing the amount of joints in the drywall.

Drywall clips can be used in place of ladder blocking. Drywall is not fastened to the clips; it is held against the clips by the installation of drywall to the adjacent wall. In all cases, it is recommended to install at least one ladder block at the mid-height of the wall to restrain the adjacent stud in a straight plain.

4. Insulated Headers
Advanced framing headers offer increased energy efficiency by replacing framing material with space for cavity insulation inside the header. Advanced framing headers are sized for the loads they carry and are often installed in single plies rather than double, as shown in Figure 5. Sizing for single-ply lumber headers is covered prescriptively in the 2012 IRC Table R602.7.1.

















Single-ply engineered wood headers may be calculated based on tributary loads applied to the header. To do so, determine the live load and total load in pounds per linear foot and refer to a published standard, such as the American Wood Council’s Wood Frame Construction Manual (WFCM) for One- and Two-Family Dwellings. Headers at openings in non-load-bearing walls are not required (Figure 6). The top of the opening can typically be framed with a flatwise member the same dimension as the wall studs.


Site-built wood structural panel box headers are another simple code-prescribed header solution that provides full-depth cavity insulation. They may be used as load-bearing headers in exterior wall construction, when built in accordance with 2012 IRC Figure R602.7.2 Typical Wood Structural Panel Box Header Construction and Table R602.7.2 Maximum Spans for Wood Structural Panel Box Headers.

Typically built with 15/32 Performance Category wood structural panel sheathing installed over minimum 2×4 framing, wood structural panel box headers provide more cavity insulation space than dimensional lumber headers. Types of wood structural panel box headers are shown in Figure 7.


Wood structural panels can be installed on one side (panel installed on the exterior side) or both sides of the header. In most cases, one-sided is the best option (if meeting the structural requirements specified in the IRC Table) because installation of interior finishes may be impaired by wood structural panels on the interior side of the wall. On the exterior side, wood structural panel box headers are a perfect complement to continuous wood structural panel wall sheathing, because the sheathing for the header also acts as part of the continuous sheathing.

The 2012 IRC Table R602.7.2 allows a 15-inch, one-sided wood structural panel box header to span 4-foot-wide openings for homes up to 28 feet wide, and 3-foot-wide or narrower openings for homes up to 32 feet wide in single-story construction with a clear-span truss roof or two-story construction with the floor and roof supported by interior bearing walls. Openings up to 4 feet wide require only a single stud at the sides of the rough opening, eliminating the need for jack studs, thereby providing another opportunity to replace framing members with cavity insulation.

Where nominal ceiling height is 8 feet or higher, an overall height of 15 inches allows for installation of 6-foot, 8-inch interior and exterior doors.

5. Single Top Plate/In-Line Framing
Single top plate construction, which in a single step eliminates hundreds of lineal feet of lumber per home to further reduce thermal bridging and increase cavity insulation, requires vertical framing alignment, in which framing members are “stacked” to create a direct load path (Figure 8). This approach requires a single — or master — framing layout for all members at all framing levels.


When designing a master framing layout, start with the layout of the roof framing members, which is generally dictated by roof design and geometry, followed by the layout of the framing members below. Although this will be a change in approach for framing carpenters who are accustomed to working up from the foundation, addressing the roof first will simplify load calculation for the designer and maximize material efficiency.

The type of roof design will impact the master framing layout. For example, hip roof design will usually require a different starting point for framing member layout than gable roof design. In hip roof construction, common rafters and hip jack rafters typically layout from the nominal center line intersection of the hip(s) with the ridge. In gable roof construction, common rafter layout typically commences from one of the end walls of the structure.

Framing member layout will also be dictated by the type of roof construction. Truss roofs will often require a different framing member layout than framed roofs. When trusses are specified, the trusses should be stacked directly above the wall studs. There is no member offset, hence the truss and wall stud layout will be the same.

For more information on these and other advanced framing techniques, download APA’s Advanced Framing Construction Guide at and see other resources at APA’s

Profession Builder – August 2012
By Bob Clark – Senior Engineer Wood Specialist,
APA-The Engineered Wood Association
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Ledger Flashing

What’s the Difference: Ledger Flashing

Proper flashing of a deck ledger is crucial to preventing water from getting behind the ledger and damaging it and the sheathing underneath, a key cause of premature deck failure. Ledgers can be flashed with many materials. Among the most common are copper, vinyl, galvanized steel, stainless steel, and aluminum, although not all of these are easy to find in all locations. FHB editorial adviser Mike Guertin, an expert deck builder, says that all these materials can flash a deck ledger adequately. “The design and installation of the flashing material are more important than the material when it comes to successful durability,” he explains. Installing one of these materials correctly, however, requires knowing what makes it different from the rest. All except for stainless steel and copper are available in both rolls and factory-bent panels. Rolled products are best bent on a sheet-metal brake.

This traditional flashing material is durable and compatible with CA- and ACQ-treated lumber. It is expensive, however. At least two manufacturers sell flashing with a sheet of copper on one side and a sheet of polypropylene on the other. Not only is it a good bit cheaper ($45 to $68 for a 12-in. by 20-ft. roll) than straight copper, but it also can be formed by hand.
14-in. by 10-ft. roll: $110

Vinyl lasts indefinitely and won’t dent. If treated with a UV inhibitor, it also resists fading and cracking. As with other vinyl products, vinyl flashing moves with changes in temperature. To accommodate this movement, cut slotted holes for fasteners (stainless steel or hot-dipped galvanized) if they’re not already there, and nail the fasteners loosely. Vinyl flashing is generally available in white and tan.
14-in. by 30-ft. roll: $20 15⁄8-in. by 8-ft. panel: $10

Galvanized Steel
Using a peel-and-stick membrane is a good idea with galvanized-steel flashing, whose zinc-covered surface degrades over time, especially in coastal environments. Frequent application of rock salt and certain other deicing chemicals on a deck also can speed up corrosion. For the best protection, choose G185 galvanized steel, which has 1.85 oz. of zinc coating per sq. ft.
14 in. by 25 ft. roll: $19 15⁄8-in. by 8-ft. panel: $20

Stainless Steel
Stainless steel is Guertin’s favorite material for flashing deck ledgers. In his coastal New England environment, stainless steel holds up very well on homes near salt water. It does the same on decks that receive salt during the winter to melt ice.
15⁄8-in. by 8-ft. panel: $30

Aluminum was the most common flashing material until CA and ACQ replaced CCA for treating lumber. Both chemicals contain more copper than CCA, which means that they corrode aluminum flashing. Separating the aluminum from the lumber with a peel-and-stick membrane is one way around this problem, but it adds a step to the installation process, not to mention an additional cost. Still, aluminum is inexpensive and readily available, so for some builders, particularly those in regions that receive little rainfall, this extra step may be worth it.
14-in. by 10-ft. roll: $6.50 15⁄8-in. by 8-ft. panel (painted): $21

by Don Burgard
From Fine Homebuilding 228, pp. 34, May 17, 2012
Posted in Construction Tips | Comments Off on Ledger Flashing

Fire-Resistant I-Joists

Weyerhaeuser – Flak Jacket I-Joists

Upcoming changes to the International Residential Code (IRC) pertaining to the fire protection of floor assemblies could limit your ability to use I-joists for some flooring systems, unless you want to cover the bottom of the joists with drywall or install a sprinkler system. (Given that only three states have adopted the residential-sprinkler requirement in the IRC, that’s unlikely to be the case.) Another alternative is to use a new Weyerhaeuser product called Flak Jacket, which complies with the new one-hour flame rating.

Flak Jacket I-joists are coated with a proprietary covering that puffs up when flames lick at it. “It’s an intumescent,” says Glen Robak, senior engineer for new products at Weyerhaeuser. “It’s similar to the stuff you might have seen sprayed on steel beams in commercial buildings.” An intumescent is a substance that swells when exposed to heat and keeps flames away from the material it covers.

As the material on Flak Jacket expands, it prevents flames from reaching the 3⁄8-in.-thick I-joist web, which would burn through a lot faster than the 2x chords. When you look at the Flak Jacket joists, you’ll see that the intumescent doesn’t cover all sides of each piece. “It doesn’t need to,” says Robak. “When the material expands, it swells out from the web and encapsulates the chords.”

Weyerhaeuser is market-testing the product and expects it to be widely available by the beginning of 2013. The price for Flak Jacket isn’t set in stone yet, but a company spokesperson says it will be a lot less than the cost of the time and materials to hang gypsum board on the underside of a floor system to comply with the new code

Posted in Code Updates, Construction Tips | Comments Off on Fire-Resistant I-Joists

The Trouble With Building Science

Few people understand it. Nobody agrees what it is, how to learn about it, or who’s responsible for it. It has never been more important

When builders began to insulate houses in the 1920s and 1930s, the exterior paint began to peel. Many painters concluded that insulation draws moisture and refused to paint a house if it was insulated. By 1938, the problem was common enough that Architectural Record published an article titled “Preventing Condensation in Insulated Structures.” The author, an architect named Tyler Stewart Rogers, argued that insulation was not the problem; indoor humidity was. He proposed a two-part solution: vapor barriers and attic ventilation.

Unfortunately, Rogers jumped to prescriptive solutions without fully understanding the problem, says Bill Rose, research architect at the University of Illinois and one of our country’s most respected building scientists. Rogers didn’t account for the effects of temperature on wood siding, and he didn’t address rain leaking in, which Rose says is “the greatest source of water in building envelopes.”

Vapor barriers and attic ventilation did not stop exterior paint from peeling, much to the delight of generations of asbestos-, aluminum-, and vinyl-siding salesmen. Nonetheless, within five years of Rogers’s article being published, his recommendations had been written into our earliest building codes, and the fledgling discipline of residential building science was off to its rocky start.

Seventy years later, our houses are bigger, more complicated, more airtight, insulated to higher levels, and dependent on ever-pricier fossil-fuel energy. Hence, the stakes of building science—comfort, health, durability, and energy bills—are higher than ever. Despite that, most architects, builders, and code officials still don’t understand moisture movement through houses. To make matters worse, there’s no easy way for them to learn about it.

Building science followed advances in comfort
Our efforts to make homes more comfortable—indoor plumbing, thermal insulation, central heating—and the problems resulting from those efforts gave rise to the first generation of building scientists, though they mostly called themselves engineers.

After World War II, man-made building materials such as plywood and dual-pane windows tightened up our houses, making them less drafty and more comfortable. But the air leaks that existed in houses were not all bad. For one thing, warm air leaking into walls and roofs helped to dry any moisture that was already there, whether from water pipes, humidity, or bad flashing. Perhaps more important, those random air leaks also were ventilating our houses. Fresh air entered houses through leaks (infiltration) around foundations and floors, while stale air exited through holes (exfiltration) in walls and roofs.

Despite those changes, our houses continued to perform reasonably well through the 1960s. We didn’t have major problems with rot or mold. And then in the 1970s, the energy crisis hit.

Building science moves to the forefront
When the cost of heating our homes skyrocketed, so did our motivation to heat them more efficiently. We began to experiment—passive solar, active solar, superinsulation, double walls, Larson trusses, envelope houses—which meant that we had to ask what works, what doesn’t, and why. As a result, scientists became interested in houses.

In 1977, an engineer at Princeton University named Gautam Dutt was crawling through attics to figure out why real houses were losing three to seven times more heat than his models predicted. According to Martin Holladay of Green Building Advisor, his eureka moment occurred when he pulled back some insulation and found a huge air leak through an unsealed utility chase. Dutt is credited with discovering the thermal bypass, which led to the realization that hidden air leaks were a far more serious problem than the obvious ones around windows and doors that had been the focus until then. From that point on, sealing hidden air leaks became a priority in the quest for energy efficiency and lower utility bills. Within a few years, the first blower doors were being sold commercially and used to find air leaks and to test homes for airtightness.

We also had our first catastrophic failures in the 1970s, as some  supertight houses became uninhabitable within a year due to mold and rot. Those failures helped us to realize that while tight houses save  energy, they also need ventilation. Also during the 1970s, the U.S.  Department of Energy was established, and scientists at atomic research  labs such as Oak Ridge in Tennessee and Lawrence Berkeley in California  began to study houses. Residential building science in the United States was ready to emerge as a serious, formal discipline.

It didn’t happen, though. In the mid-1980s, oil prices dropped, interest in energy efficiency waned, research funding was cut, and residential  building science lost critical momentum, at least in the United States.  In Canada and many European countries, including Germany and Sweden,  interest in building science (Europeans call it building physics) continued unabated, spurred on largely by government funding. In 1983,  for instance, the U.S. home-building industry was 20 times bigger than  Sweden’s, but the Swedish Council for Building Research spent more than  three times more on building research than the U.S. Department of  Housing and Urban Development. That same year, the National Research  Council Canada published Canada’s first textbook on building science, Building Science for a Cold Climate, by Neil B. Hutcheon and Gustav O.P. Handegord. Nearly 30 years later, an American equivalent still hasn’t been published.

Mold, asthma, and construction defects on the rise
Today, we are on the threshold of another major push for increased airtightness and more insulation in houses. Whether it’s the 2012 International Energy Conservation Code (IECC), Energy Star 3.0, Passive House certification, net-zero houses, or simply the movement to improve the efficiency of existing homes, the result is the same— tighter homes—and it has some experts worried.

Rose Grant is a research architect in the Building Technology Research Unit for State Farm Insurance and a former colleague of Bill Rose’s at the University of Illinois. “I think we are on the cusp of some serious building-science issues,” Grant says, “and mold is the canary in the coal mine.” In 2001, mold claims on homeowners’ policies cost insurance companies $1.3 billion, five times more than in the previous year. In 2002, they more than doubled again, exceeding $3 billion. It’s hard to say what happened after 2002 because most insurance companies began excluding mold from coverage.

In the past, experts argued about whether mold posed a serious health threat, but according to a 2007 study funded by the EPA, “Of the 21.8 million people reported to have asthma in the U.S., approximately 4.6 million cases are estimated to be attributable to dampness and mold exposure in the home.” The same study goes on to say, “The national annual cost of asthma that is attributable to dampness and mold exposure in the home is estimated to be $3.5 billion.” Those are just health costs; they don’t include mold remediation. The authors also estimate that dampness or mold is present in 47% of homes.

Dampness and mold could be signs of a maintenance problem. But a Feb. 10, 2011, article from Bloomberg BusinessWeek reports a doubling of construction defects per housing unit from 2000 through 2005 compared with the previous six years. The article references a 2007 University of Florida study in which 69% of the 17,000 defect claims reviewed were found to be associated with moisture penetration.

You could argue that bad flashing or failure to overlap building paper correctly results from poor workmanship, not a failure to understand building science, but the distinction may not matter. As we move toward higher-performance houses, not only do we have to get the weather-resistive barrier right, but we also have to bring a high level of craftsmanship to it. To do that, everybody involved in construction—frame carpenters, plumbers, electricians, HVAC installers, insulators, roofers, siding crews—needs an understanding of basic building science that just doesn’t exist on most job sites today.

You could argue that bad flashing or failure to overlap building paper correctly results from poor workmanship, not a failure to understand building science, but the distinction may not matter. As we move toward higher-performance houses, not only do we have to get the weather-resistive barrier right, but we also have to bring a high level of craftsmanship to it. To do that, everybody involved in construction—frame carpenters, plumbers, electricians, HVAC installers, insulators, roofers, siding crews—needs an understanding of basic building science that just doesn’t exist on most job sites today.

The cost of ignorance
To stay comfortable and to reduce energy costs, we’re adding more and more insulation to our homes and sealing air leaks with a vengeance. Without a thorough understanding of building science, though, you easily can trap moisture in walls and roofs, which can lead to peeling paint, mold, rot, and asthma. Ignore air leaks and you’ll pay a stiff energy penalty year after year.

Building science is not well defined
Building science is still an immature discipline, and its scope is not well defined. The narrowest definition focuses on heat, air, and moisture transfer in the building enclosure because that’s where most of the problems are. The broader definition also includes lighting and daylighting, acoustics, fire prevention, and structure.

Regardless of the definition, one of the mostimportant things that building science brings to residential construction is an emphasis on the house as a system. As houses have become increasingly complicated over the years, so too has the network of specialty trades among which we divvy up construction responsibilities. This division of labor makes it difficult for any one person to monitor how everything works (or doesn’t work) together. For example, an electrician installs the bathroom vent fan, a carpenter cuts in the dryer vent, a kitchen specialist hangs the range hood, an HVAC contractor puts in the furnace, a plumber installs the gas water heater, and a mason builds the chimney. Who’s in charge of the home’s ventilation?

Good building science not only requires that all the parts and pieces of a house work together, but it also demands that they be figured out ahead of time. The person doing the figuring matters less. It can be the architect, the builder, an energy specialist, or even a bona-fide building scientist, assuming you can find one.

Although the terms building science and building scientist are not well defined, they are certainly well used. Joseph Lstiburek, a founder of Building Science Corp. (BSC), an architecture and consulting firm near Boston, is perhaps the person in this country most qualified to call himself a building scientist, but he’s so frustrated by all the people misusing the term that he now refers to himself as an engineer.

For John Straube, a partner of Lstiburek’s at BSC, the dividing line between a person with a basic understanding of building and an actual building scientist is the ability to predict performance before it happens and to explain performance quantitatively afterward. “I would ask that a building scientist be able to calculate or predict things—R-values, heat loss, dew point,” Straube says.

Unfortunately, it’s not easy to become a building scientist. Auburn, Penn State, and the University of Minnesota, among others, all have programs in building science. MIT, USC, and UC Berkeley offer master’s degrees in building science. But, says Lstiburek in his typically candid way, “That’s total crap. They have no connection to real building science.” Eric Burnett, who taught building science for 20 years in Canada, was frustrated during the 10 years he spent trying to establish the program at Penn State. “One of the problems is the failure of current architectural and civil-engineering faculty to embrace the teaching of building science,” Burnett says. “They have other priorities.”

There are people working on the problem, however. The National Institute of Building Sciences has a committee devoted to enhancing education across the United States in building science and technology. Paul Totten, a practicing engineer in Washington D.C., is chairman of that committee. He says, “We’re way behind Canada and almost every European country.” One of the committee’s goals over the next five to ten years is to have “full-scale building-science master’s and Ph.D. programs with some consistency in what’s being taught. Right now, heat, air, and moisture transfer aren’t emphasized enough.”

But even a degree is just the beginning. Straube says, “There’s no way to prove that windows leak based on physics. The way we know windows leak is by experience. It’s dangerous when people learn the physics and don’t have the experience.” If we’re expecting hordes of young building scientists to come pouring out of universities and help us to fix all our houses, we’re going to have to wait awhile.

Architects should be trained in building science
Because the goal of building science is to predict how a house will perform, it makes sense that architects and designers should understand it, but building science isn’t emphasized in most architecture schools. Katrin Klingenberg, a German architect now living in Illinois and the head of the Passive House Institute US, says that when she looked into the level of science training for architects in this country, “I was flat-out shocked.” In Germany, she says, architecture students had to take six courses of building science over two years, with exams. If you didn’t pass the exams in three tries, “you had to go and find yourself a different job.”

Many U.S. architects today are becoming certified Passive House consultants because the nine-day training program includes so much building science. “We’re basically re-educating a whole generation of architects,” Klingenberg says. In fact, Carnegie Mellon University and the University of Oregon are looking to partner with the Passive House Institute US and incorporate parts of its certification program into their curricula.

Rachel Wagner is one architect who has taken the Passive House consultant training. She describes the teaching of building science in architecture schools as “woefully inadequate.” Wagner thinks that part of the solution is to make building science a section of the Architect Registration Examination. “Unless getting your license, your accreditation, depends on it, it’s not going to stick. It’s not going to be taken seriously,” Wagner says.

Despite the fact that architects are involved in few residential projects (maybe 5%), Lstiburek thinks the key to improving knowledge of building science is to fix architectural education. “Architects divorced themselves from the technology of construction,” he says. “If they were doing their jobs, I’d be out of business.” He believes that if you start with the architects, the rest of the industry will follow.

Is builder licensing the answer?
Producing more building scientists and educating architects in building science, however important, will not change the way houses are built. To do that, builders need to be educated. Pat Huelman, director of the Cold Climate Housing Program at the University of Minnesota, says, “We could have the best design, the best specs, we could have the right mousetrap, but if the person building it doesn’t understand what it’s supposed to do, it may not work when you’re done.” Paul Totten agrees. “The folks actually building the buildings need to be very deep in this subject,” he says. “Just making some minor errors in the field may cost you all of the performance that you should have gotten out of the building.”

At least some people in Oregon think the answer is builder licensing with a continuing-education requirement that includes building science. Legislation to that effect passed in 2009 and began to phase in last fall. When asked what prompted the legislation, Jon Chandler, CEO of the Oregon Home Builders Association, says that during the legislative session in 2007, “builders got pummeled in the press over construction defects—mold claims, water-intrusion claims, and so on. There was a solid week of front-page, above-the-fold articles. That was the tipping point.”

Not everyone agrees that contractor licensing and continuing education are the answer. Back in the 1990s, Minnesota had a requirement similar to Oregon’s, and Huelman was one of the people who taught the building-science courses. “I started to lose a little faith,” Huelman says, “because the owner of the company, or some delegate, was going to the class and learning about building science or energy, but that wasn’t traveling down to the guy who was putting in the window or to the siding contractor who was messing up the housewrap.”

Mark LaLiberte, who helped to set up the training programs in both states, hesitates to recommend any solution that will burden builders with more regulation, but he does advocate continuing education, especially to address building-science issues. “It’s the only solution that will bring builders to the point where they say, ‘I’m going to do this because it’s my reputation, it’s my business, and I’m a professional.’” He wants builders to seek that education on their own.

One thing everybody agrees on is that building science, just like the devil, is in the details. That’s why Straube says, “If I had to pick anybody to give training to, it would always be the site supervisor first.” Here and there, in fits and starts, some builders are getting trained, at conferences and online, through green-building certification programs, through Energy Star and Building America, but no single program is comprehensive or sufficient. The quality of the education offered varies considerably, and hucksters have set up shop to exploit this critical need. Even the most conscientious builders have a hard time learning what they need to know about building science.

How water gets into houses After rain and plumbing leaks, airborne moisture is the biggest source of water in walls and roofs, which is why sealing air leaks—creating an air barrier—is so important. The difference between (and relative importance of) air barriers and vapor retarders is probably the most widely misunderstood concept in high-performance home building.

Performance-based codes would help
“Code development isn’t predicated on good building science,” Huelman says. “It’s a political negotiation.” He explains that it often takes several years for a building failure to show itself. Then it takes several years to develop the language in the code that leads to a fix for the problem. It then takes several more years before the code is adopted, and another several years before the code officials are sufficiently trained. “You’re 10 to 15 years behind the eight ball,” Huelman says.

Given the complexity of the code-changing process, Totten worries about another risk. He points out, for example, that when you change code requirements for the airtightness of homes, you also have to change the codes for ventilation rates. “If we have a lag on one, particularly the ventilation rate, we’re going to create a whole pool of new problems.”

Perhaps the biggest issue with codes from the standpoint of building science is that they are prescriptive. They suggest that if you vent the crawlspace or if you install insulation with the correct R-value, you won’t have a moisture problem. However, success depends on how well you install the insulation and on how well you seal the air barrier, which is why Sweden, for example, has gone to performance-based energy codes. Like the Passive House standard, Sweden’s energy codes limit total energy use per square meter and specify an air-change rate. Meeting these requirements puts considerable pressure on builders to understand the science and to get the details right.

Whatever happens with our codes, our building inspectors need to understand building science thoroughly. They are the ones assessing the quality of the energy details before they’re covered up. Inspectors also have the authority to allow substitutions for code requirements, which can be dangerous without a deep understanding of building science. But it is no easier, and no more likely, for inspectors to educate themselves than it is for builders.

Let’s admit that it’s complicated
At some point in the past 30 years, without fanfare and without most of us ever acknowledging it, our houses crossed a threshold of complexity. They became dynamic systems whose construction and performance goes beyond the abilities and understanding of many of us in the industry. Most architects, builders, and code officials still can’t explain the difference between a vapor retarder and an air barrier. It’s not that we’re stupid. We know plenty of other things, but we haven’t had to know about building science. Now we do.

Creating homes that are comfortable, healthful, durable, and efficient means learning to build airtight, highly insulated houses. We can’t keep complaining that it’s too expensive. We can’t keep saying, “Houses need to breathe,” and then use that as an excuse to be careless about how we put them together. And we can’t be lazy and rely on prescriptive solutions. Those of us who build houses really need to understand the science of how they work. We have to take responsibility for educating ourselves.

by Kevin Ireton
From Fine Homebuilding 227, pp. 76-81 April 26, 2012
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Paint Primers

What’s the Difference: Primers

Learn what the difference is between latex, oil-based and shellac-based primers and how to choose the right one for the job.

For most paint jobs, both inside and outside the house, latex paint is preferred. Latex emits fewer odors and VOCs than oil-based paint, and it cleans up much more easily. In one area, however, oil-based products still hold their own over latex: primers. With interior applications and exterior spot-priming, there’s another option: shellac-based primers. All three primers can be used under latex topcoats. Choose a primer based on the condition of the substrate to be primed and its location. Before you open the can, though, be sure to prep the surface you’ll be priming. No primer will perform its best if the surface hasn’t been thoroughly scraped, sanded, or otherwise readied for painting.


Latex primers bond well with new wood or with old wood that has been sanded and is in good condition. They remain flexible after drying, which allows them to move with exterior wood during expansion and contraction. Latex primers are formulated to leave a smooth film on a smooth surface. However, that film won’t develop properly if the temperature is below 50°F or above 90°F. Latex primers don’t perform as well on weathered or damaged wood. They also don’t block knots or the tannins in cedar and redwood, and they raise wood grain more than oil-based or shellac-based primers. Cleanup is with water.
Cost per gallon: $14 to $23


Oil-based primers are better than latex primers at sealing nail heads, covering knots in bare wood, and blocking tannin bleeding and other stains. They are also better for sealing bare window muntins that will be covered with oil-based window putty. Oil-based primers penetrate wood more thoroughly than latex primers, making them better at preparing weathered wood for paint. The best penetration is achieved by a slow-drying primer, but the trade-off is a longer wait before topcoating. When speed is more important than degree of penetration, oil-based primers that can be topcoated in one to two hours are available. Oil-based primers continue to harden as they age, which can be a problem with exterior applications. As the wood underneath expands and contracts, the primer remains rigid, weakening the bond with the wood and with the topcoat. Cleanup is with mineral spirits.
Cost per gallons: $17 to $23


Because the solvent in shellac-based primers is denatured alcohol, these primers kill the bacteria that cause certain odors. They are also great at covering knots and heavy stains. “If we have a problem stain from a knot bleeding through or a water stain that oil-based primer will not stop, then we turn to shellac-based primer,” says painter Philip Hansell. Because shellac softens in high temperatures, its use on exterior surfaces is limited to spot-priming. On the other end of the thermometer, shellac-based products are the only primers that can be applied in freezing temperatures. They don’t penetrate wood deeply, but are the fastest-drying primers available; a topcoat usually can be applied after 45 minutes. Cleanup is with denatured alcohol or ammoniated detergent.
Cost per gallon: $40 to $65

by Don Burgard
From Fine Homebuilding 226, pp. 34 March 8, 2012
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How to Insulate a Cold Floor

Properly air-sealed floors exposed on the underside to outdoor temperatures will have better energy performance

Floors that are exposed on the underside to outdoor temperatures are often poorly insulated. Such floors can be found in homes with post foundations, in rooms over garages, and in cantilevered bays.

In many cases, these floors are casually insulated with fiberglass batts and covered with OSB or plywood. If the batts aren’t thick enough to fill the joist bays, they may fall away from the subfloor and rest on the OSB below. At that point, the floor assembly has an R-value of 0.

This type of floor usually leaks a lot of air, and the floor feels cold all winter long. If the floor includes plumbing, frozen pipes are a distinct possibility.

Bonus-room floors

There are two important reasons to do a careful job of air-sealing the floor of a bonus room over a garage: to improve the home’s energy performance and to prevent carbon monoxide from entering the room. Even after air-sealing, it’s essential that you install a CO detector.

When insulating a bonus-room floor, you’ll usually be working from below. If the area of the heated bonus room above the garage is smaller than the area of the garage ceiling, you’ll need to install blocking between the joists to provide an air barrier where the floor insulation stops. Locate the blocking under the kneewalls.

Blocking can be made from 2x lumber or rigid foam. If it is being installed between I-joists or floor trusses, rigid foam will be easier to cut into odd shapes than lumber. Whether you use 2x or rigid foam, it’s important to seal the perimeter of each piece of blocking with caulk or canned spray foam.

If the floor is the same size as the garage, it’s essential to air-seal the rim-joist area carefully. If ceiling joists extend from the house into the garage, you need to install blocking between the joists to maintain the home’s air barrier. Once the blocking has been installed, seal air leaks at the blocking and the rim-joist area on all four sides of the garage.

Air-seal, and hold insulation tight to subfloors The first step in energy-smart floor assemblies is air-sealing, which is particularly important in garages, where carbon monoxide and other toxic chemicals can leak into the living space above. Another significant detail is to make sure the insulation is held tight to the bottom of the subfloor; gaps negate the R-value of the insulation.

Floor above a garage This detail shows a garage bonus-room floor framed with open-web floor trusses. To prevent air leaks, it’s important to install caulk at all of the indicated locations. Ideally, the floor assembly won’t include any ducts. If there’s no way to avoid installing ductwork in the floor, specify deep floor joists, such as trusses, that provide plenty of room under the ducts for insulation, and be sure to include a continuous layer of rigid foam under the joists.

Smaller room over a large garage It is important to maintain a continuous thermal boundary between conditioned and unconditioned spaces. The weak link in this situation is the kneewalls. Install blocking (either wood or rigid foam) under the kneewalls, and insulate the walls, floor, and ceiling as shown.

Insulate ducts and plumbing, and put them close to the floor
If HVAC ducts are routed through the floor assembly, the floor joists must be deep enough to install several inches of insulation under the ducts. A floor with ducts also should be insulated with a continuous layer of thick rigid foam under the floor joists. To keep plumbing pipes from freezing, install them as close to the subfloor as possible, without insulation between the pipes and the subfloor. It’s also a good idea to install a layer of sealed rigid foam directly under the pipe. The rigid foam should extend the full width of the joist bay.

Cantilevered floors

If you’re about to insulate a cantilevered floor, the first step is to install blocking between the joists. The blocking should be located above the bearing wall below. It’s better for the blocking to be on the interior side of the bearing wall rather than the exterior side. Seal each piece of blocking at the perimeter to prevent air leaks.

You can insulate cantilevered floors many ways; the illustration shows rigid foam at the top and bottom of the joist bays. At the top of the joist bays, insert narrow rectangles of rigid foam between each joist, and secure that foam to the subfloor with a compatible adhesive. Seal the perimeter of each piece of foam with caulk or canned foam.

Once the upper layer of foam is installed, the joist bays can be insulated with either fiberglass batts or with dense-packed cellulose insulation. Under the floor assembly, install a continuous piece of rigid foam mechanically fastened to the joists and in an airtight manner. Protect the foam with plywood or solid soffit material.

Cantilevered floor The blocking above the bearing wall helps to define the home’s air barrier, so each piece of blocking needs to be sealed at the perimeter with caulk or canned spray foam. As long as both layers of rigid foam are installed with attention to airtightness, this type of cantilevered floor performs well.

5 Tips for a Thermally Efficient Floor

1. Air-sealing is just as important as insulating; it’s especially crucial to seal the rim-joist area.

2. Fiberglass batts are the worst type of insulation for this application. Spray foam performs well because it adheres to the subfloor and stops air leaks.

3. If you’re insulating with fiberglass or cellulose, the insulation must completely fill the joist bays, and the top of the insulation should be in full contact with the subfloor above. If you’re building an Energy Star house, these principles are mandatory elements of the Thermal Bypass Checklist.

4. It’s always a good idea to install a continuous layer of rigid foam on the underside of the floor joists, especially if the joist bays are insulated with fiberglass batts or cellulose. Rigid foam stops thermal bridging through the floor joists and helps with air-sealing, especially if the perimeter of each piece of foam is sealed with caulk or high-quality tape. One of the best tapes for sealing the seams of rigid foam is 3M All Weather Flashing Tape 8067. The rigid foam should be protected by a layer of plywood, OSB, or drywall (for a garage ceiling).

5. Whatever type of insulation you install, the total R-value of the floor assembly must be no less than minimum code requirements. The 2009 International Residential Code calls for a minimum of R-13 floor insulation in climate zones 1-2, R-19 in zones 3-4 (except marine 4), and R-30 in marine 4 and zones 5-8.

by Martin Holladay
From Fine Homebuilding 226, pp. 86-87 March 8, 2012
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Air-Sealing a Basement

Once you’ve sealed the air leaks in your attic floor—the cracks where warm air escapes from your house during the winter—it’s time to turn your attention to the basement or crawlspace, where cold air leaks in. If you turn off the basement lights and look for daylight, you might be surprised to find some large holes.

Weatherization contractors often use a blower-door test to help pinpoint leaks in the building envelope. Once your house is depressurized, you can use your bare hands to feel for air infiltration. The most common places to find air entering basements are around windows and doors and between the concrete foundation and the mudsill. But some air-leakage paths may surprise you: Air can even seep through the crack at the perimeter of your basement slab or through a sump in the basement floor. Here are five places to check for air leaks in your basement and some advice on how best to seal them.

Foundation walls
Walls made of poured concrete or concrete blocks are usually fairly tight. However, if your basement walls have any obvious cracks, fill them with silicone caulk. If the walls are made of stone and mortar, don’t use canned spray foam or caulk to seal cracks. Instead, remove any loose material from these areas, and repair them with mortar and small stones.

Use caulk or canned spray foam to seal leaks near wall penetrations for your electrical service, water service, cable service, or natural-gas service. Your home also may have penetrations for a fuel-oil filler pipe, an oil-tank vent, or a clothes-dryer vent. If basement access is awkward, some cracks may be easier to seal from the exterior.

In a tight basement Atmospherically vented appliances—for example, water heaters, furnaces, or boilers attached to old fashioned brick or metal chimneys—depend on air leaking through cracks in your walls to supply combustion air. If your basement is very tight, atmospherically vented appliances could be starved for air, and exhaust gases may struggle to exit through the chimney. That’s why the best appliances for tight homes are sealed combustion appliances equipped with ducts that supply outdoor air directly to the burners.

Because flue gases sometimes include carbon monoxide, it’s always important to be sure that your combustion appliances have adequate combustion air and that your chimneys draw well. If you plan to seal cracks in your basement, arrange for a combustion-safety test of any atmospherically vented appliances once air-sealing work is complete. Contact your gas utility or a home performance contractor certified by RESNET or BPI for more information on combustion-safety testing.

Rim joists
Air can leak through the crack between the top of the foundation wall and the mudsill, the crack between the mudsill and the rim joist, and the crack between the rim joist and the subfloor. The best way to seal leaks in the rim-joist area is with a high-quality caulk.

Once these cracks are caulked, you may want to reduce air leaks further by installing a layer of closed-cell spray foam at the rim-joist area using a two-component spray-foam kit.

Although spray foam is effective, it is expensive and sometimes messy. If you would rather not use it, you can insulate rim joists with rectangles of 2-in.-thick rigid foam (polyisocyanurate or extruded polystyrene). Seal the perimeter of each foam rectangle with caulk or canned spray foam. Don’t use fiberglass batts; they do nothing to slow airflow.

Two ways to air-seal and insulate a rim joist Rim joists are a common source of air leakage in basements and are often left uninsulated. The first step toward an energy-smart rim joist is to caulk gaps between the foundation wall and the mudsill, the mudsill and the rim joist, and the rim joist and the subfloor. As seen in the details shown here, you then can use rigid foam or spray foam to add another layer of air-sealing and to insulate the area.

Basement Air Seal with Batt InsulationRigid foam You can cut the pieces of rigid foam roughly and somewhat undersize because the perimeter of each rectangle should be sealed in place with canned spray foam. With the rim joist air-sealed and covered with rigid foam, you can now add cavity insulation like fiberglass batts or, better yet, a second layer of rigid foam.

 Basement Air Seal with Spray Foam




Spray foam Extend spray foam from the top of the foundation wall to the underside of the subfloor above. In addition to sealing leaks, 2 in. of cured foam will insulate to R-13. Most building codes, including the International Residential Code, allow spray foam installed at rim joists to remain exposed—without protection from a thermal barrier like drywall—as long as the foam is no thicker than 3-1⁄4 in.

Windows and doors
If your basement has old single-pane windows, they may need new glazing compound and weatherstripping. If the windows are in bad shape, consider replacing them with new double-glazed units.

If you rarely open your basement windows, consider sealing them shut with screws and caulk, or even covering them with rigid foam. (Of course, this advice applies only to small basement windows, not to any egress windows in a basement bedroom.) Don’t forget to caulk between the window frame and the concrete.

Every bulkhead entry needs a tight, weatherstripped exterior door at the base of the stairs. Be sure to caulk or to foam the gap between the door jamb and the foundation. If the door is warped or difficult to weatherstrip, it’s time to replace it. Most lumberyards can order a custom insulated entry door to fit any size opening. If you’re framing the rough opening, use pressure-treated lumber, and seal the frame to the concrete with canned spray foam.

Basement floors
After a new foundation is backfilled, the fill often shrinks away from the foundation, leaving a gap next to the basement wall that allows outdoor air to reach the footings. That’s one way for outdoor air to enter a home through cracks in the basement floor or sump pit. Even if a home has no shrink-age gap, air can still reach the footings, especially in areas with porous soil.

If there’s a crack at the perimeter of your basement slab, clean the crack with compressed air or a vacuum cleaner, and then fill it with caulk. If you have a sump pit without a tight lid, replace it with a new airtight lid. Sumps that have airtight lids are available from Jackel.

Basement ceilings
Chases and chimneys that extend from the basement to the attic should certainly be capped at the top, but the bottom of these chases should be sealed as well. After all, once air gets into a chase, it can move sideways into joist bays and partition walls until it finds an exit crack. This belt-and-suspenders approach is the best way to prevent the stack effect from stealing your home’s heat.

Cover large ceiling holes with plywood, drywall, or rigid foam, and seal the edges with caulk, spray foam, or housewrap tape. While you’re at it, check for holes under first-floor bathtubs or showers. Plumbers typically cut out a big piece of the subfloor to accommodate drain lines and traps; these air pathways should be sealed.Radon is a colorless, odorless, naturally occurring gas that can seep through soil into your basement. High radon levels can damage human health. While most homes have relatively low radon levels, some have dangerously high levels.

Sealing cracks in your basement can affect radon levels in your home, either for better or worse, depending on several factors. If necessary, a radonremediation contractor can install plastic pipes under your basement slab to lower the radonto safe levels.

The best way to determine whether your home needs radon-remediation work is to test the air in your home. For more information on radon testing and remediation, visit

For details on radon mitigation, see

by Martin Holladay
Drawings: Steve Baczek, Architect
From Fine Homebuilding 225, pp. 82-83 January 19, 2012
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How to Seal Air Leaks

How to Seal 4 Typical Hidden Air Leaks in Your House

A 1⁄8-in. crack doesn’t seem like much to worry about, but a 1⁄8-in. crack that runs the length of your house amounts to a square hole 8 in. on a side—a big-enough hole to toss a cat through. Because it is a crack, you can seal it with caulk, but not all holes in the building envelope are cracks that can be sealed with a tube of caulk. Some of them really are big enough to toss a cat through, and they need to be sealed with sheet goods such as drywall, rigid foam, or plywood.

Some common gaps and holes are listed in the Energy Star thermal-bypass checklist. Among the most common holes found in poorly sealed houses are holes near soffits, chases, and bathtubs. Green Building Advisor has a collection of 56 detail drawings that pertain to the Energy Star checklist. These drawings come from that collection.

Chases and soffits are raceways for air leaks
Older houses often have several big holes in the attic floor. Many are around open chases for ducts, electrical cables, or flues, often running unimpeded from the basement to the attic. When installing plumbing chases in new houses, install air-barrier sheathing first, and seal the gaps between intersecting walls with caulk. As the framing shrinks, these gaps become large. The top of an open plumbing chase in an existing house can be sealed with a variety of sheathing materials as long as seams and edges are sealed with caulk or tape.

If any chases for plumbing pipes, ducts, or flues originate in the basement, be sure to seal the chases at the bottom with the same techniques you used to seal the chases in the attic.

While you’re in the attic, check for any unsealed kitchen soffits. Such soffits are often built above a row of wall cabinets. In new construction, the ceiling drywall should be installed and taped before the soffit is framed. The first clue to a leaky soffit is often a piece of discolored fiberglass insulation. The discoloration is caused by escaping air that has carried dust upward, often for years.

In many existing houses, the sides and top of kitchen soffits are open to the framing cavities. In this case, you can install the neces-sary pieces of air-barrier sheathing from above. Don’t forget to seal the perimeter of the sheathing and any seams with durable tape, caulk, or spray foam. Once the soffit is airtight, don’t forget to replace the insulation.

Tubs hide big holes in floors and walls
 Tub installation When you’re installing a fiberglass tub/shower unit against an exterior wall, it’s essential to insulate and air-seal the wall before installing the tub. On new-construction jobs, remember to install a durable air-barrier material (for example, Thermo-ply sheathing) to cover the insulation.

In an existing house where builders omitted the insulation and the air barrier behind the tub, repairs are difficult. One solution (not the cheapest) is to remove some of the siding and sheathing so that spray-foam insulation can be installed from the outside. A better bang for the buck may be to address the problem as part of a larger bathroom remodel. If you’re contemplating a bathroom upgrade, it may be cheaper to demolish the tub/shower unit and seal the wall properly from the interior.

If your house has an unconditioned basement—in other words, if the basement ceiling is insulated—be sure to pull that insulation aside and check for air leaks under any first-floor bathtubs or showers. Plumbers typically cut out a big piece of the subfloor to accommodate drain lines and traps, but rarely repair the ruptured air barrier. If these cutouts are not sealed, your floor has a huge hole.

Keep fireplace heat inside
Prefab fireplace niche Prefabricated metal fireplaces, whether wood-burning or gasburning, are usually installed in a niche framed into an exterior wall. Unfortunately, many of these fireplace niches are poorly sealed against air leaks.

The best opportunity to prevent such leaks is before the fireplace is installed. After the niche is carefully insulated, the insulation should be covered with a durable, rigid air-barrier material, such as Thermo-ply sheathing, drywall, sheet metal, or OSB. All gaps and seams should be sealed with caulk, spray foam, or contractors’ tape. Where the chimney penetrates the niche ceiling, seal gaps with metal flashing and high-temperature silicone caulk.

To repair air-barrier defects behind an existing metal fireplace, it may be necessary to remove sections of siding and sheathing to provide access for the installation of spray polyurethane foam.

Fine Homebuilding222, pp. 90-91


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Roof Ventilation

A Crash Course in Roof Venting

Understand when to vent your roof, when not to,
and how to execute each approach successfully

So much information has been devoted to the subject of roof venting that it’s easy to become confused and to lose focus. So I’ll start by saying something that might sound controversial, but really isn’t: A vented attic, where insulation is placed on an air-sealed attic floor, is one of the most underappreciated building assemblies that we have in the history of building science. It’s hard to screw up this approach. A vented attic works in hot climates, mixed climates, and cold climates. It works in the Arctic and in the Amazon. It works absolutely everywhere—when executed properly.

Unfortunately, we manage to screw it up again and again, and a poorly constructed attic or roof assembly can lead to excessive energy losses, ice dams, mold, rot, and lots of unnecessary homeowner angst.

Here, I’ll explain how to construct a vented attic properly. I’ll also explain when it makes sense to move the thermal, moisture, and air-control layers to the roof plane, and how to detail vented and unvented roofs correctly.

Theory behind venting
The intent of roof venting varies depending on climate, but it is the same if you’re venting the entire attic or if you’re venting only the roof deck.

In a cold climate, the primary purpose of ventilation is to maintain a cold roof temperature to avoid ice dams created by melting snow and to vent any moisture that moves from the conditioned living space to the attic.

In a hot climate, the primary purpose of ventilation is to expel solar-heated hot air from the attic or roof to reduce the building’s cooling load and to relieve the strain on air-conditioning systems. In mixed climates, ventilation serves either role, depending on the season.

Vent the attic
A key benefit of venting the attic is that the approach is the same regardless of how creative your architect got with the roof. Because the roof isn’t in play here, it doesn’t matter how  many hips, valleys, dormers, or gables there are. It’s also easier and often less expensive to pile on fiberglass or cellulose insulation at the attic floor to hit target R-values than it is  to achieve a comparable R-value in the roof plane.

The success of this approach hinges on the ceiling of the top-level of the house being absolutely airtight before any insulation is installed. It’s also important to ensure that there isn’t anything in the attic except lots of  insulation and air—not the Christmas decorations, not the tuxedo you wore on your wedding day, nothing. Attic space can be used for storage, but only if you build an elevated platform above  the insulation. Otherwise, the insulation gets compressed or kicked around, which diminishes its R-value. Also, attic-access hatches are notoriously leaky. You can build an airtight entry  attic, but you should know that the more it is used, the leakier it gets.

How do people get this simple approach wrong? They don’t follow the rules. They punch a bunch of holes in the ceiling, they fill the holes with recessed lights that leak air, and they stuff mechanical systems with air handlers and a serpentine array of ductwork in the attic. The air leakage from these holes and systems is a major cause of ice dams in cold climates and a major cause of humidity problems in hot climates. It’s also an unbelievable energy waste no matter where you live.

Don’t think you can get away with putting ductwork in an unconditioned attic just because you sealed and insulated it. Ductsealing is faith-based work. You can only hope you’re doing a good-enough job. Even when you’re really diligent about air-sealing, you can take a system with 20% leakage and bring it down to maybe 5% leakage, and that’s still not good enough. With regard to recessed lights and other ceiling penetrations, it would be great if we could rely on the builder to air-seal all these areas. Unfortunately, we can’t be sure the builder will air-seal well or even air-seal at all. So we have to take some of the responsibility out of the builder’s hands and think of other options.

In a situation where mechanical systems or ductwork has to be in the attic space or when there are lots of penetrations in the ceiling below the attic, it’s best to bring the entire attic area inside the thermal envelope. This way, it’s not as big a deal if the ceiling leaks air or if the ducts are leaky and uninsulated.

Vent the roof deck
If the attic space is going to be conditioned, either for living or mechanical purposes, or if a home design calls for a vaulted ceiling, provision R806.3 in the International Residential Code calls for the roof deck above the space to be vented continuously from the eave to the ridge. This is easy to accomplish in simply constructed roofs and difficult, if not impossible, to accomplish in roofs that have hips, valleys, dormers, or  skylights that interrupt the rafter bays.

If you choose to vent the roof deck, then be serious about it and really vent it. The code calls for a minimum of 1 in. of airspace between the top of the insulation and the back of the roof sheathing. That’s not enough. For best performance, the airspace in the vent chute should be a minimum of 2 in. deep. Unless you’re bulk-filling rafter bays between 2×10 or 2×8 rafters with closed-cell spray foam, this approach will likely require you to fur out the rafters to accommodate additional insulation to achieve desired R-values. That can be a pain, but you won’t run into the problems associated with having too little air circulating under the roof. To be sure your roof is getting enough ventilation, there are simple calculations that you can follow.

Beyond the decreased capacity for insulation when venting the roof deck, venting the roof deck or the attic has some other drawbacks worth considering. In cold climates, snow can enter the soffit and ridge vents, melt, and potentially cause rot. Similarly, in coastal environments or in regions with lots of rain and wind, moisture can be forced into the vents and into the roof assembly. In hurricane-prone zones with frequent high-wind events, vented-soffit collapse can pressurize a building, which can cause windows to blow out and the roof to be blown off. Finally, in wildfire zones, floating embers can enter the vents and cause roof fires. If any of these issues are of concern, there is another option.

Create an unvented roof
Through provision R806.4, the IRC also allows you to build an unvented roof assembly. Unvented assemblies work particularly well on complex roofs that would be difficult or impossible to vent properly or on roofs where it would be difficult to insulate properly if the roof were vented.

It should be noted, however, that in high-snow-load areas, you still need a vented over-roof to deal with ice damming. In essence, you’re creating a hybrid vented/unvented roof system.The goal in an unvented roof is to keep the roof deck—the principal condensing surface in roof assemblies-sufficiently warm through the year to prevent condensation from occurring. In most climates, builders have to insulate the roof sheathing to prevent condensation from occurring within the assembly. The exception is hot-dry climates such as in Phoenix, where condensation isn’t as big an issue.

Condensation control is most often accomplished by installing rigid foam above the roof deck or by installing air-impermeable spray-foam insulation directly against the underside of the roof deck. The code also allows for air-permeable insulation, such as fiberglass or cellulose, to be used under the roof deck as long as rigid foam is used above the roof sheathing. Flash-and batt (or flash-fill) assemblies are also allowed. Any of these approaches can adequately prevent condensation from occurring within the roof when the rigid foam or spray foam is installed at the appropriate thickness.

If you’re spraying foam on the underside of the roof deck, be sure you’re using the right product. Closed-cell spray foam works in all climates, but especially well in climate zones 5 through 8, where high R-values are desired and where airimpermeable insulation also must be a vapor retarder. Lowdensity, open-cell foam is permissible, but in climate zones 5 and above, it has to be covered with a vapor-retarder coating, like rigid foam or painted drywall.

Also pay attention to roofing materials. Asphalt shingles require special attention when installed on unvented roof assemblies in hot-humid, mixed humid, and marine climates due to inward vapor drive. To keep moisture out of the roof assembly, a roofing underlayment with 1 perm or less (class-II vapor retarder) must be installed under the shingles. Also, check to be sure that you are in compliance with the manufacturer warranties when installing shingles over an unvented roof in all climates. Some manufacturers don’t warranty or offer only a limited warranty when their products are used over an unvented roof assembly.

Shingles that are installed on unvented roof assemblies operate at slightly higher temperatures, roughly 2°F to 3°F warmer than shingles on vented assemblies. This can reduce their service life by roughly 10%. You can vent the roof cladding, which will increase its  longevity, but the expense of fastening battens over the roof sheathing, then adding another layer of plywood over the battens as a nail base for the shingles, may not be worth the expense. After all, the shingle color and the roof orientation are much more significant concerns when it comes to shingle life.

Unvented roofs
Unvented roofs aren’t nearly as common as vented assemblies, and builders may not be familiar with detailing them correctly. While there are certainly a variety of ways to build an unvented roof assembly that performs well, here are three examples worth considering

The success of a vented attic or roof deck relies on its airtightness. The space above the top plate of exterior walls-at the bottom of each rafter bay-is especially important. Baffles placed in this area channel intake air into either the attic space or vent chutes, and also prevent insulation from falling into the soffit and blocking airflow.

Prevent condensation with the right amount of insulation
An unvented roof assembly is possible only if you keep the roof sheathing warm enough to prevent conditioned air from condensing against it. The map at right, which is based on table R806.4 of the IRC, lists the minimum R-values required to prevent condensation in unvented assemblies in various climate zones. The thickness of the insulation will vary depending on the type. These R-value requirements are intended only to prevent condensation and don’t supersede the code-required R-values for energy efficiency, which are also listed.

Site-built or prefab baffles?
The success of a vented attic or roof deck relies on its airtightness. The space above the top plate of exterior walls at the bottom of each rafter bay is especially important. Baffles placed in this area channel intake air into either the attic space or vent chutes, and also prevent insulation from falling into the soffit and blocking airflow.

Site-built: 2-in. chutes and baffles Cut 1-in.-thick rigid polyiso insulation into 2-in.- wide spacer strips, and glue them to the inside face of each rafter with a spray-foam adhesive like Pur Stick ( Cut the polyiso insulation to fit snugly in each rafter bay, and foam it in place against the spacer to create a 2-in. chute or baffle.

Size: Custom-cut polyiso foam
Cost: $23 per sheet
Source: Dow

Prefab: fast and functional The AccuVent soffit insulation baffle is made of rigid recycled plastic. It’s more durable than other foam-based products and installs quickly with staples. These baffles should still be air-sealed with spray foam, but they’re a good option if you’re looking for a stock product.

Size: 41 in. by 22 in.
Cost: $1.68 each
Source: Berger Building Products

Fine Homebuilding 212, pp. 68-72
by Joseph Lstiburek
Posted in Construction Tips | Comments Off on Roof Ventilation

Airtight Window Installations

 Installing a window is tricky. The first challenge is to do a good job with the flashing and water-management details. But once the window is fastened to the wall, you’re still not done: Now you have to pay attention to air-sealing. Every window is part of your home’s air barrier. For the air barrier to be complete, seal all air leaks between the wall and the window frame.

The most crucial gap to seal is the crack between the window frame and the rough opening. Before tackling that gap, however, remember to seal three other locations with caulk or peel-and-stick membrane.

The first location is under the sill pan. Whether you’ve used a length of clapboard to create a sloping sill or a manufactured sill pan, you’ll need to caulk it to the framing. If you are making a site-built sill pan out of for caulk at this location. The second location is under the win­ dow flanges—either all four flanges, if you are following the instructions pro­ vided by most window manufacturers, or under only the side and top flanges, if you follow the recommendations of most building scientists. Sealing the bottom flange is probably a bad idea because it prevents water leaks from draining out. The third location is behind the drywall at the perimeter of the rough opening. This is especially important if you are fol­ lowing the airtight-drywall approach.

Most window-installation steps—instal­ ling flashing, lapping the housewrap, and attaching the window—are done from the exterior. Once the window has been installed, it’s time to move indoors to address air-sealing details. Experts differ on whether any housewrap projecting into the interior of the rough opening should be trimmed back. For those who care about airtightness, the consensus seems to be that any housewrap between the window frame and the rough opening can interfere with air-sealing, so it makes sense to trim it back with a utility knife. The best time to do this is before the win­ dow is installed. Many materials—caulk, spray foam, gas­ kets, tape—work to seal the gap between a window and a rough opening. Just don’t use air movement. Regardless of the material you use, it’s important that your air-sealing work doesn’t prevent water from escaping. Keep caulk, spray foam, or gaskets near the interior side of the gap you are sealing.

If the gap is ¼ in. wide or less, it can be filled with caulk, with or without backer rod. Larger gaps should not be caulked. Most builders seal the gaps between a window frame  and the rough  opening with canned, one-component spray foam. Low-expansion foam is usually best, because other  types of foam can exert enough  pressure to bow window jambs inward and cause a window to bind. How­ ever, as long as you use a light touch and don’t install too much  foam, almost any type of canned  foam will work.

European air-sealing materials may be better than caulk or foam

European builders say that either EPDM (synthetic rubber) gaskets or special air-sealing tapes do a better job of sealing cracks than caulk or spray foam because these products are more flexible, stay flex­ible longer, and create a better air seal.
The EPDM gaskets that European build­ ers use to seal around windows resemble backer rod. One problem with gaskets is that shims interrupt the continuity of the gaskets. To address this problem, gaskets can be trimmed to fit between the shims, or the window can be installed with special European screws called shim screws ­instead of wooden shims.

A shim screw has a threaded outer sleeve holding a captured free-spinning lag screw. A special installation tool grabs both parts simultaneously so that the screw can be installed in a hole drilled in the window frame. The tool is then used to screw the lag into the rough framing while the sleeve remains fixed in the window frame. Then the lag screw can be adjusted to make the window plumb and level.

The best source for shim screws and EPDM gaskets is Conservation Technology ( The company offers three sizes of gaskets that can be used to seal the gap between win­dow frames and rough openings: BG44 for gaps 1/4 in. to 1/2 in., BG46 for gaps 3/8 in. to 3/4 in., and BG48 for gaps 1/2 in. to 1 in.

Many energy-conscious European builders use air-sealing tapes to seal around widows Siga Corvum 12/48 tape is one such product available in the United States; it can be ordered from This prefolded Siga tape has ½- and 2-in, legs. The narrow leg is stuck to the window, while the wider leg (which has a paper backing for easier placement) is stuck to the window’s rough opening. The maker claims the tape is stronger and more reliable than caulk.

Fine Homebuilding
August/September 2011
Posted in Construction Tips | Comments Off on Airtight Window Installations