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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