Betzwood Associates PC

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.

Rigid 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.

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 epa.gov/radon.

For details on radon mitigation, see GreenBuildingAdvisor.com/radon.

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

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

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 (www.todol.com). 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

 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.

START WITH THE SMALL GAPS
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.

MOVE ON TO THE BIG GAP
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.
GASKETS
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 (www.conservationtechnology.com). 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.

TAPE
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 www.smallplanetworkshop.com. 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

4 Options for Shear Bracing Foam-Sheathed Walls

In many parts of the United States, it’s becoming common for builders to install rigid-foam sheathing on the exterior of framed walls. However, this raises a potential problem: Because rigid foam isn’t structural, how are the walls braced to prevent racking? There are at least five ways to brace a foam-sheathed wall, the first of which most builders and inspectors are familiar with.

Using continuous plywood or oriented-strand-board (OSB) sheathing under the foam allows you to use the prescriptive section of the code—a simpler route than an engineered solution—even in high-wind areas. Where rough-cut 1-in. boards are available from local sawmills, diagonal board sheathing is also worth considering. It is a strong, vaporpermeable, code-approved method of bracing walls. If you aren’t building in a high-wind area, however, continuous sheathing is likely to cost more than other bracing solutions. Here are four alternatives.

Plywood or OSB at critical areas

It’s possible to brace a wall with just a few sheets of plywood or OSB. A small house may require only two sheets of OSB per wall, usually located at the corners. Long walls also may require an additional sheet in the middle of the wall. Most builders rely on an engineer to specify the number of required sheathing panels, their placement, and the nailing schedule.

Once the plywood or OSB panels have been installed, the rest of the wall is sheathed with rigid foam. Two thicknesses of rigid foam are necessary to complete the job. For example, if 2-in.-thick foam is used between the OSB sheets, then 1-1/2-in. foam is used to cover the OSB.

1×4 Let-in bracing

The traditional method of bracing unsheathed walls is to use 1×4 let-in bracing. Set in a notch cut into the studs and plates, the 1×4 must extend from the bottom plate to the top plate, and it must be securely nailed to each plate and stud. According to the International Residential Code, “The let-in bracing shall be placed at an angle not more than 60° or less than 45 degrees from the horizontal.”

Diagonal metal strapping

There are at least three kinds of steel strapping used to brace walls: flat strapping, T-profile strapping, and L-profile strapping. All three types need to be installed from plate to plate and must be wrapped around the top or bottom of the plate. The location and frequency of metal strapping depends on many variables and is usually determined by an engineer.

Flat strapping (for example, Simpson WB) works only in tension, not in compression. For that reason, flat strapping must always be installed in pairs that form the shape of a V or an X.

T-profile strapping and L-profile strapping are designed to be installed by inserting one leg of the strapping into a 1/2-in.-deep kerf cut into the studs and plates, and then nailing the strapping to each stud and plate. T-profile and L-profile strapping have an important advantage over flat strapping: They work in both compression and tension. That means they need not be installed in V-shaped or X-shaped pairs.
Not all types of T-profile and L-profile strapping have passed the tests that allow them to be substituted for code-required 1×4 let-in bracing. While Simpson Strong-Tie’s L-profile strapping (RCWB) is made of 20-ga. steel and is considered a prescriptive-code bracing material, Simpson T-profile strapping (TWB) is made of thinner 22-ga. steel and does not meet prescriptive-code requirements. If flat strapping or T-profile strapping is used for wall bracing, it must be part of an engineered design.

L-profile metal strapping is a good bracing solution in many areas; however, the capacity of metal bracing may not be adequate in high-wind or seismic areas.

Inset shear panels

Walls framed with 2x6s can be braced with inset shear panels. These reinforced boxes are framed with 2x4s and sheathed with OSB. Because each panel is 4-6-1/2 in. wide and 4 in. deep,it can slip into a 2×6 wall in a wide bay created by removing one stud. The height of the shear panel allows it to fit between the bottom and top plates of the wall. Because the OSB sheathing on an inset shear panel is flush with the studs, the shear panel does not interfere with the installation of foam sheathing.

Inset shear panels obtain much of their strength from threaded rods that join the top and bottom plates; these rods are sometimes connected to carefully placed anchor bolts by means of threaded couplings.

The work of designing inset shear panels, specifying how many to use, and deciding where to place them in a wall is best left to an engineer. If designed correctly, inset shear panels can provide enough lateral load resistance to work in all wind and seismic zones.

What’s the Difference: Panel Ratings
Decoding the APA Stamp

About 70% of North America’s structural sheet-goods manufacturers voluntarily participate in a third-party auditing and certification program through a nonprofit organization called APA—The Engineered Wood Association.

More than likely, when you buy a sheet of structural plywood or OSB, it will have a prominent stamp on one side that details the panel’s properties and intended uses. This information is used by architects and structural engineers when specifying products and by the building official during site inspection. It also is helpful to builders faced with various panel options. APA maintains a website (www.apawood.org) where this and other information is available for no charge. Assistance is also available through the APA help desk at 253-620-7400.

Panel Grade

Sheathing: Intended mainly for roofs and walls, these panels also can be installed as subflooring when used in conjunction with structural wood flooring (such as conventional plank  looring) or beneath underlayment-grade plywood or OSB.

Single-layer floor: The APA labels single-layer subfloor sheathing as Sturd-I-Floor. These panels are used under nonstructural finish flooring such as wall-to-wall carpeting. Sturd-I-Floor panels also have a relatively smooth surface. Plywood is C-p (C-plugged), and OSB is touch-sanded.

Underlayment: Designed to be used over subfloor sheathing, underlayment panels provide a base for tile and other nonstructural finish-flooring materials.

Siding: Although they are designed to be used as siding—T-111 is one example—these panels are also commonly used for fences, soffits, and other exterior applications.

Span Rating

This is the maximum distance between supports—joists, studs, or rafters—that the panel is designed to span safely. These ratings assume that the panel is oriented with the long axis across at least three framing members, and it should be read in the context of the sheet’s panel grade, which is printed directly above it on the label.

For example, a Sturd-I-Floor panel is intended for use only as flooring, so it lists only one span rating. Sheathing panels have two numbers separated by a slash, as shown at right. The left-hand number is the maximum on-center distance, in inches, between supports when the sheathing is used on a roof. The right-hand number is the maximum on-center distance, also in inches, between supports when the sheathing is used on a floor.

Bond Classification

This classification dictates what environmental conditions the panel can withstand. It used to have two additional classes, Interior and Exposure 2, but has recently been limited.

Exterior: where full cycles of wetting and drying will occur or where the moisture content of the panel will often be at 19% or higher. Common uses range from siding panels to fences and soffits.

Exposure 1: Although they are bonded with the same water-resistant glue used to make exterior panels, these panels don’t necessarily have the same composition and are intended to survive the elements only during construction, about three to six months, with subsequent prolonged moisture content no higher than 16% to 19%. One common example of this class is C-D Exposure 1 sheathing, which is commonly referred to as CDX.

Fine Homebuilding 220, pp. 36
by Justin Fink

What’s the Difference: Tile thinset mortar
Meeting the standard

The most common adhesive for setting tiles is thinset mortar—that is, mortar designed to be applied in a layer no more than 3⁄16 in. thick. Most of these mortars are available in either gray or white. If you intend to use a dark-colored grout, choose a gray mortar; choose white if your grout will be a light color. Achieving a white color requires modifications in the manufacturing process, so expect to pay a few more dollars for white mortar than gray.

Not all thinsets are the same, and the one to use depends on the location of the surface to be tiled and the nature of the substrate. When deciding which thinset to use, best practice is to look for the American National Standards Institute’s (ANSI) approval for each product. Within the three common categories shown here, most manufacturers offer a range of products that differ in grade, curing time, composition, and price.

A118.1: Mortars meeting this standard contain only portland cement, sand, and a water-retention compound. After being mixed with water, they can be used on basic tile jobs over concrete, drywall, or cement backerboard. They don’t adhere to wood when dry, so they should not be used over plywood. These thinset mortars are called nonmodified.

A118.4: To meet this standard, thinsets must include a latex polymer. You can either buy a mortar that includes this polymer, or you can buy a nonmodified mortar and stir in a liquid latex additive in place of the water. The latex allows a small degree of movement in the substrate, and mortars that include it are ideal in exterior locations, in wet areas, in high-traffic areas, and over vinyl flooring. These modified thinset mortars also meet A118.1. or unmodified

A118.11: Mortars with the ability to adhere tiles to exterior-grade plywood meet this standard, as well as A118.1 and A118.4. Again, you can either buy a thinset that meets this standard, or you can buy a nonmodified thinset and mix in a latex additive. A118.4: To meet this standard, thinsets must include a latex polymer. You can either buy a mortar that includes this polymer, or you can buy a nonmodified mortar and stir in a liquid latex additive in place of the water. The latex allows a small degree of movement in the substrate, and mortars that include it are ideal in exterior locations, in wet areas, in high-traffic areas, and over vinyl flooring. These modified thinset mortars also meet A118.1.

Fine Homebuilding 220, pp. 37
by Don Burgard

Can Houses be “Over Insulated ” or “Too Tight”?

No. Green homes are insulated well, tightly built, and well ventilated

Let’s take these issues one at a time. The “too tight” theory holds that houses need to breathe. Traditionalists can point to old houses and claim the only reason they’re still standing is because air leaks amount to natural ventilation that dries everything out and keeps the house healthy.

In reality, air leaks mean you’ve lost control of air movement. Air and moisture can be forced into wall and ceiling cavities where water vapor condenses and fosters the growth of mold. Warm air exiting the top of the house can draw in cold air to replace it, wasting heat and energy. In many ways, uncontrolled air movement wastes energy and increases the risk of long-term damage to building components.

Effective air and moisture barriers reduce those problems, but they come with a few caveats: Tight houses need mechanical ventilation to ensure a supply of fresh air to keep people healthy; and existing houses should not be tightened without assessing whether the existing combustion appliances have an adequate source of combustion makeup air.

As far as insulation goes, there may be a theoretical point of “too much,” but in most cases buildings have too little. At the very minimum, insulation should meet recommendations of the Department of Energy, but adding more is always a good thing. Properly insulated buildings are cheaper to heat and cool.

Where insulation is added can be as important to how much is added. Walls and roofs with an extra layer of rigid insulation outside the framing help cut energy losses due to thermal bridging. What’s more, some types of insulation are inherently more effective than others. But using too much should be the least of our worries.

Fiber cement, cellular PVC, or wood composite

While solid wood continues to be the most popular material for exterior trim, its vulnerabilities leave big openings for alternative products. Builders who choose not to use wood typically look instead at trim manufactured from either cellular PVC, wood composite, or fiber cement. All three materials are less responsive to humidity variations than wood, which means that they hold paint for longer than wood. Each is of uniform quality and density, so there is little waste. Finally, these materials are more resistant to water, fungi, and insects than solid wood.

From left to right: fiber cement, cellular PVC, wood composite     

FIBER CEMENT
Cement, sand, and cellulose are the basic ingredients in fiber cement, and trim boards come with a smooth or wood­ grain finish. Cutting them can involve a special sawblade or dedicated shears. Care must be taken in handling long fiber-cement boards, which are heavy and can break if carried by one person. Once the trim is in place, however, it performs well: It is noncombustible, and its rate of expansion and contraction is so low that gaps between boards are unnecessary. (CertainTeed recommends that adjacent boards be butted with moderate con­tact.) Boards come primed or finished; all field-cut edges must be primed and/or painted before installation. Check with manufacturers on which fasteners to use. Some manufacturers replace much of the sand in their boards with fly ash, a by-product of coal combustion. This not only reduces the risk from inhaling the dust, which can lead to silicosis (a lung disease), but it also reduces the amount of fly ash that ends up in landfills. James Hardie uses no fly ash and claims that the material has a negative affect on durabil­ity. On the other side, CertainTeed and Nichiha claim to have developed stronger fiber-cement products with fly ash. Postconsumer recycling of fiber cement is virtually nonexistent, but because the material is inert, its presence in a landfill poses no health or environmental risks.

Cost per lin. ft. of 1×4: 75¢ to $1.35
Manufacturers: CertainTeed, James Hardie, Nichiha, Plycem

CELLULAR PVC
Cellular PVC trim can be cut and shaped just like wood, with regular woodworking tools. Because it has a uni­form consistency, PVC requires no special treatment of field-cut edges. As long as it was manufactured with a UV-inhibitor, it doesn’t even need to be painted. If you choose to paint it, keep in mind that the drying time will be longer than with wood. This is because PVC does not absorb moisture, a characteristic that also makes PVC the only trim product that can be installed in areas where standing water may develop. Boards are available with smooth finishes on both sides, or with a smooth finish on one side and a wood-grain finish on the other. Most manufacturers recommend that scarf joints and PVC cement be used on long runs. If installing on a cold day, you’ll need to leave gaps of up to ⅛ in. on each end to allow for expansion. PVC expands and contracts more than wood, but following the manufacturer’s installation instructions will limit potential problems. Only exterior fasteners should be used. Whether PVC is a green product is a matter of debate. On the one hand, on many homes, it may never need to be replaced. On the other hand, it contains petroleum and has a high level of embodied energy.

Cost per lin. ft. of 1×4: $1.40 to $1.75
Manufacturers: Azek, CertainTeed, Fypon, Kleer

WOOD COMPOSITE
Composed of wood fibers, phenolic resins, and wax, wood-composite trim can also be cut and shaped like solid wood. Field-cut edges need to be primed, and one manufacturer recommends a coat of paint as well. Boards come factory-primed and with a smooth finish on one side and a wood-grain finish on the other. Look for composite trim that includes zinc borate, a wood pre­servative. Butt joints are generally preferred on long runs, with gaps to allow for expansion and contraction (see manufacturers’ instructions for recommended gap sizes), but scarf joints may be used as well. Gaps should be filled with flexible caulk. Composite trim is typically made from recycled, leftover, and/or sus­tainably harvested wood.

Cost per lin. ft. of 1×4: 70¢ to 90¢
Manufacturers: Collins Products, LP

Fine Homebuilding – May 2011
by Don Burgard, copy/production editor

Dimmers for CFL and LED Lighting Fixtures

Compact fluorescent (CFL) bulbs have come a long way in the past few years. New versions resemble incandescent bulbs and put out a nice, warm, white light. Despite all their pluses, though, these energy-saving light sources have a serious downside: Most, even those labeled “dimmable,” can’t be dimmed very low with standard dimmer switches. Similarly, many LED bulbs, even those labeled “dimmable,” won’t dim much below half brightness with a standard dimmer.

But electronics manufacturers have stepped up to the plate with a new generation of dimmers that cater specifically to high-efficacy lighting. One example is Lutron’s new C•L family of dimmers, which are available in four switch styles, including one for table lamps.

• Manufactured by Lutron                
• 888-588-7661; www.lutron.com
• Cost: $15 to $40                                 

The reason high-efficacy bulbs are so much harder to dim has to do with differences in how they and incandescents generate light. In an incandescent lamp, a tungsten filament turns white hot at full operating voltage. A standard dimmer switches the power on and off over 100 times a second, effectively reducing the voltage so that the filament doesn’t get as hot and the lamp produces less light.

A CFL bulb works in a very different way: A high voltage from an electronic ballast ballast creates an arc inside a spiral glass tube that contains a tiny bit of mercury vapor. The vapor turns into plasma that emits UV-light, which is converted to white light by a phosphor coating on the inside of the tube. If you try to dim a standard CFL by lowering the voltage, it’ll work normally to a point, then just shut down because there’s not enough energy to sustain the plasma. “Dimmable” CFLs have ballasts designed to handle the rapidly pulsing voltage, but they don’t dim to low light levels, either, when paired with standard dimmers.

Lutron’s C•L dimmers allow you to finetune the low end of the dimming range, enabling a low light output from a dimmable CFL or LED bulb. This is done by adjusting a little wheel on the dimmer, which essentially establishes the lowest possible threshold to which you can dim the CFL before it goes out or flickers. You can’t dim a CFL or LED to zero light output as you can an incandescent, but the difference in the light level that the C•L dimmer produces is pronounced.

The new Lutron dimmer can be used with both dimmable CFLs and dimmable LED bulbs as well as incandescent or halogen bulbs. I tried it with a 23w dimmable CFL (equivalent to 100w incandescent) and a 7w dimmable LED (equivalent to 40w incandescent). They both dimmed to a low level I’d estimate over 90% with the Lutron dimmer. That’s a big difference compared to a standard dimmer.

When set to the dimmest setting, both switched on fast (though not instantly). When dimmed too much, the CFL flickered something that will kill the lamp fairly quickly, and why you need to adjust it.

There are a couple of things to keep in mind with this new dimmer. First, the color of the light doesn’t change as it’s dimmed. We’re used to a really warm light from incandescent lamps that are dimmed way down, and you don’t get that shift in color temperature with CFL or LED bulbs. Second, if you replace the CFL or LED bulb (especially with a different wattage or manufacturer’s bulb), you should readjust the low-end dimming setting. It’s not difficult; you just take off the wall plate and adjust the little wheel until the bulb is as dim as it can be without flickering. The light will turn back on after being turned off.

This dimmer doesn’t need a neutral wire connection like some dimmers, which is an advantage in the common situation where there isn’t a neutral in the switch box. As with any dimmer, it’s important to make sure the dimmer wattage rating is equal to or higher than the total wattage of the lights it’s controlling. Because the dimmer is bulky, be prepared for a tight squeeze if you’re retrofitting one into a small switch box.

Fine Homebuilding 219, pp. 26
by Clifford A. Popejoy

What’s the Difference: Programmable Thermostats

Basic vs. full feature
The advantage of a programmable thermostat is that it can keep you comfortable and save on energy costs—significantly, if programmed with aggressive temperature setbacks when you’re asleep or away. At big-box home-improvement stores, you can choose among 20 or more thermostats that range in price from $20 to $100. Add in a handful of models available through HVAC contractors, with top-end thermostats running from $250 to $700, and the choices can be bewildering. If your day-to-day routine doesn’t vary and you have a relatively simple heating and cooling system, a basic $20 thermostat will do everything you need. As the cost goes up, you’re purchasing more flexible programming options, improved user interfaces, and proved the ability to control more sophisticated HVAC systems.

Basic Programmable $20 TO $30
Basic programmable thermostats are sold through retail stores and divide the week into workdays and weekends. You can program one schedule for Monday through Friday, and then a separate schedule for the weekend. (The 5-1-1-day variety allows separate Saturday and Sunday schedules, while the 5-2-day type uses the same program for both days of the weekend.) They run most heating types but won’t control a multistage system, and in some cases, they aren’t compatible with heat pumps that have backup heating.

Quality thermostats at this price point offer compressor protection, a feature found on their more expensive cousins. Compressor protection locks out the compressor for several minutes (usually five)after shutoff to keep the system from cycling on and off. These thermostats also keep the temperature within +/-1ºF of the setpoint using an algorithm that reduces short cycles, which are a stress on equipment, by strategically overshooting and undershooting the target. Unlike full-feature thermostats, they don’t have smart-response software that brings the temperature to the target by the beginning of the program period. (Example: Honeywell RTH2300B; $25)

Full-Feature Programmable $50 AND UP
Full-feature programmable thermostats have large, backlit touchscreens with crisp displays, sometimes in full color, and the flexibility to store different heating and cooling programs for every day of the week. Programming is fairly intuitive; some models use a question-and-answer setup wizard.

Smart-response software allows the thermostat to learn how long it takes to bring the house to a programmed temperature and will call for heating or cooling early enough that the house is at the target temperature when the program period begins.

Full-feature thermostats come in two varieties: contractor-sold-and-installed models and homeownerinstalled versions marketed through retail stores. HVAC systems with multistage heating and cooling, mechanical ventilation, humidifiers, and hard-wired controls require thermostats sold through contractors. Remote sensors are unique to the pro-installed category. An outdoor sensor communicates weather conditions to the thermostat, which can use that information to help control heat-pump systems. Wireless indoor remote sensors can control the HVAC system, too.

Placing the sensor in a room that’s consistently cooler or warmer than the rest of the house overcomes a poor thermostat location and improves comfort. (Example: Emerson Big Blue; $200 to $250)

Full-feature programmable thermostats sold for the homeowner market are universal thermostats. They control a variety of different heating and cooling systems, and can run on battery power if there’s no compatible hard-wiring. Although they can’t support ventilation and humidification equipment, they share most of the other features of the contractor-installed variety, including the ability to switch automatically between heating and cooling. (Example: Honeywell RTH7600D; $99)

Fine Homebuilding 219, pp. 100
by Sean Groom

Preventing Ice Dams

Ice dams form when a home’s escaping heat warms the roof sheathing and melts the underside of the snow layer on the roof. Water trickles down the roof until it reaches the cold roofing over the eaves, where it freezes. The ice buildup at the eaves forms a dam and creates a reservoir of water that can back up under the roof shingles and leak into the house. There are four steps to preventing ice dams and the damage they cause:
1. Seal air leaks between the warm interior and the attic or cathedral ceiling.
2. Add more ceiling insulation.
3. Improve ventilation between the top of the insulation and the roof sheathing.
4. Install self-adhering, waterproof roof underlayment.

While the first two steps can reduce or eliminate the chances of ice-dam forma­tion, ventilation is a less likely fix, and a waterproof membrane will only minimize the damage. It won’t prevent the ice dams from forming.

STEP 1: SEAL AIR LEAKS
Most ice dams are caused by flaws in a home’s air barrier. If you don’t have access to a blower door, the only way to diagnose air leakage to attics is to crawl up there and look around. Common points of air leak­ age through a ceiling are along top plates, at mechanical penetrations, and around recessed lights. Many of these trouble spots can be sealed with spray foam. Some areas around a chimney, for example may require other measures.

STEP 2: CHECK INSULATION LEVEL
Once you’ve plugged all the air leaks, check the insulation levels. The latest version of the International Residential Code requires R-49 ceiling insulationin climate zones 6, 7, and 8. In these areas, ceilings need a mini­mum of 14 in. of fiberglass batts, cellulose, or open-cell spray foam. If you’re using blown-in fiberglass, you’ll need about 20 in. to achieve R-49. In climate zones 4 and 5, you’ll need a minimum of R-38 insulation  in the ceiling. That means at least 11 in. of fiberglass  batts, cellulose, or open-cell spray foam, or about 15 in. of blown-in  fiberglass.

In houses where there isn’t enough room to get R-38 or R-49 at the perimeter of the attic, the best thing to do is to install as much closed-cell spray polyurethane foam as the space permits. In some cases, it may be necessary to install additional rigid-foam insulation on top of the existing roof sheath­ing. Of course, many energy smart builders choose to exceed these code minimum requirements for insulation.

STEP 3: IMPROVE VENTILATION
In the past, code minimum insulation requirements were woefully inadequate, and almost every cold climate home leaked enough heat to generate ice dams. Many building inspectors, noting that attic insula­tion was code compliant, falsely concluded that the only way to stop ice dams was by improving ventilation. These days, if your ceiling is airtight  and is insulated to the lat­est building code requirements, ice dams are far less likely.

Although so called hot roofs (roofs with­ out ventilation) can work well, ice dams can still form on a hot roof if snow is deep enough because snow is an insulator. If your roof is covered with 2 ft. of fluffy snow, the bottom of the snowpack is insulated from cold outdoor temperatures. That raises the chance that meltingwill occur. If you live in an area with very snowy winters, you prob­ably want to stick with a ventilated roof. Again, ventilation should always be the third step in fixing ice dams. If ridge vents are added without proper air-sealing, escaping attic air candepressurize the attic, increasing air leakage through the ceil­ing. The best ventilation channels include a balance of soffit vents and ridge vents. Also, remember to install insulation dams at the perimeter of the attic to keep  from spilling into the soffits and to prevent unconditioned air from flowingthrough the insulation and lowering its per­ formance. Dams are provided with some brands of ventilation baffles, or they can be cut from pieces of rigid foam. Ideally, insu­lation dams should be sealed at the edges with caulk or canned foam. Cathedral ceil­ings need an air barrier between the top of the insulation and the ventilation channel.

STEP 4: INSTALL SELF-ADHERING ROOF UNDERLAYMENT
No roofing underlayment will prevent ice dams. What a waterproof underlaymentwill do is limit the damage caused by any ice dams that form by minimizing the chance of water leakingthrough the roof. In other words, it is relatively cheap insur­ance and is the best practice in most roofing installations. When reroofing, install a self-adhering underlayment extending from the eaves to a point 3 ft. higher than the plane of the exterior wall.

Fine Homebuilding - May 2011

Keeping the Groundwater and Contaminants Out

Buildings used to be constructed over cellars. Cellars were dank, dark places where coal was stored. People never intended to live in cellars. Now we have things called basements that have pool tables, media centers and play rooms.

Cellars were easy to construct – rubble, stone, bricks and sometimes block. If they got wet or were damp so what? Basements are different. They are not easy to construct if we intend to live in them. They need to be dry, comfortable and keep contaminants out.

Over the last 50 years there has been a notable expansion of living space. The useful conditioned space of building enclosures is expanding to the outer edge of the building skin. Attics, crawlspaces, garages and basements are valuable real estate that are being used to live in or used for storage or places to locate mechanical systems. Basements are viewed by many as cheap space that can easily be incorporated into a home. Keeping basements dry, comfortable and contaminant free is proving to be anything but simple.

The fundamentals of groundwater control date back to the time of the Romans: drain the site and drain the ground. Today that means collecting the run off from roofs and building surfaces using gutters and draining the water away from foundation perimeters. Roof and façade water should not saturate the ground beside foundations. Grade should slope away from building perimeters and an impermeable layer should cover the ground adjacent to buildings.

A free draining layer of backfill material or some other provision for drainage such as a drainage board or drainage mat should be used to direct penetrating groundwater downward to a perimeter drain. The perimeter drain should be located exterior to the foundation and wrapped completely in a geotextile (“filter fabric”). A crushed stone drainage layer under the basement slab should be connected through the footings to the perimeter drain to provide drainage redundancy and to provide a temporary reservoir for high groundwater loading during downpours if sump pumps fail during electrical outages (if gravity drainage to daylight is not possible).

Groundwater exists in more than the free-flowing liquid state. Water from wet soil canal so wick (capillary flow) and move by diffusion through the soil and the materials used to make basements. Therefore the basement wall should be damp-proofed and vapor-proofed on the exterior and a capillary break installed over the top of the footing to control “rising damp”. Damp-proofing and vapor-proofing in these locations is often provided by a fluid applied coating of bitumen. In the past, capillary breaks over footings were not common. They were not needed when basement perimeter walls were uninsulated and unfinished on the interior, because these conditions permitted inward drying of the migrating moisture. For finished basements they are an important control mechanism. Without them, moisture constantly migrates through the foundation, and then into the interior insulation layer and interior gypsum board lining.

A capillary break and vapor barrier should be located under concrete basement floor slabs. Crushed stone or coarse gravel acts as an effective capillary break and sheet polyethylene in direct contract with a concrete floor slab acts as an effective vapor barrier. The concrete slab should be sealed to the perimeter basement wall with sealant (the concrete slab becomes the “air barrier” that controls the flow of soil gas into the basement).

The drainage layer under the basement concrete slab should be vented to the atmosphere to control soil gas. Atmospheric air pressure changes are on the order of several hundred Pascal’s (an inch of water column) so that the soil gas vent stack is in essence a “pressure relief vent” or “soil gas bypass” to the atmosphere. Perforated pipe should be attached to the vent stack to extend the pressure field under the slab to the foundation perimeter and to the drainage layer outside the walls. Pipe connections through the footing extend the pressure field further to the exterior perimeter drain (as well as providing drainage redundancy as previously noted).

The traditional approach to basement water control has been to place the barrier and control layers on the outside and then allow drying to the inside. Drainage, dampproofing or water-proofing and vapor control layers have historically been located on the outside of basement perimeter walls and crushed stone layers and plastic vapor barriers have been located under concrete slabs. The operative principle has been to keep the liquid, vapor, and capillary water out of the structure and locate vapor barriers on the outside – and allow inward drying to the basement space where moisture can be removed by ventilation or dehumidification.

The approach to basement soil gas control should be to allow pressure relief by creating pressure fields under and around basement foundations that are coupled to the atmosphere – intercepting the soil gas before it can enter the structure and providing a bypass or a pathway away from the conditioned space.

by Joseph Lstiburek

Closed-cell spray foam and fiberglass batts can be used together to maximize performance and minimize cost

Closed cell spray polyurethane foam may be the best performing insulation available today. It seals against air infiltration, it boasts more R-value per inch than other forms of insulation, and  it blocks the passage of water vapor. It’s also the most expensive option, and it requires dedicated equipment for large scale installations. Fiberglass batts, on the other hand, are by far the most commonly installed form of insulation. Batts are widely available, are easy to install, and insulate fairly well provided no air moves through them. The problem is that most homes, new and old, have a lot of air moving through their walls and roofs. Like a knit sweater on a windy day, fiberglass batts do nothing to stop air movement. Even when there are no air leaks, a great enough difference in temperature between indoors and out creates convection loops, currents of air ferrying Btu from warm drywall to cold sheathing. Fiberglass batts are cheap, though, making them a tempting choice.

What if you could balance the performance of spray foam with the cost effectiveness of fiberglass batts? Enter flash and batt. This hybrid system relies on a thin flash coat of foam sprayed against the inside of the sheath­ing, with the remainder of the framing cavity filled with fiberglass. Flash and batt costs as much as $2 per sq. ft. less than meeting R-value requirements with foam alone.

Like any element of building, however, there’s more to a successful flash­ and batt installation than many people realize.  If the ratio of fiberglass to foam isn’t carefully considered, the performance and financial benefits of this hybrid approach will be diminished quickly.

Fine Homebuilding – February/March 2011

Airtight Drywall, and Simple Caulk and for Air Movement Seal Control

Leaky homes waste a lot of energy, so most building codes now include requirements calling for floors, walls, and roofs to be sealed against air leakage.

Builders can detail a house’s air barrier in several ways. By taping sheathing seams, the air barrier can be established at the exterior wall and  roof sheathing. Using spray foam, the air barrier can be established inside the wall and ceiling cavities. Or the air barrier can be located on the house’s interior by using polyethylene sheeting or by implementing the airtight-drywall approach. Building scientists now realize that in all but the coldest climates, interior polyethylene causes more problems than it solves, so builders who favor interior air barriers usually choose the airtight drywall approach.

Developed in Canada during the early 1980s, the airtight-drywall approach uses ordinary drywall as the chief air barrier. To achieve a high level of airtightness, you must pay close attention to potential leakage points at the perimeter of the drywall. Although air can’t leak through drywall seams sealed with paper tape and drywall compound, it can easily leak through cracks wherever drywall is screwed  to framing lumber. The airtight-drywall approach relies on caulk and gaskets to seal these cracks.

If you climb into the attic of an average  house during the winter and pull aside the insulation above a partition, you’ll discover warm air rising through the visible crack between the drywall and the top plates. Air usually enters partition walls through cracks atelectrical boxes. The airtight-drywall approach addresses these cracks, as well as others.

A wide variety of caulks can be used for airtight drywall, but most experts recommend poly­urethane for sealing drywall to framing. Gaskets, however, outperform caulk in this application. At least three types of gaskets have been used successfully for airtight drywall sealing; open-cell foam gaskets, EPDM gaskets or gaskets that are made from ripped lengths of foam sill seal. You’ll also need airtight electrical  boxes. Most include a flange that helps to seal the gap between the box and the drywall. Some airtight boxes have flanges with an integral gasket, while others have flanges that are designed to be caulked. In addition to sealing the crack between the box and the drywall, it’s also important to use caulk or spray foam to seal the leaks that exist around the holes in back of the box where the electrical cables enter. It’s important to keep in mind that filling in electrical box full of spray foam is a code violation.

Fine Homebuilding – October/November 2010