By Mark LaLiberte
High-performance builders have long understood the importance of good air sealing, but the topic is now attracting interest from conventional builders as well. With codes mandating confirmed air leakage numbers of 3 ACH 50 or better, those builders are realizing that they need help.
Along with my business partners, Justin Wilson and Gord Cooke, I teach air sealing as part of the Applied Building Science classes at Construction Instruction. Again and again, we've seen builders of all types challenged by this important step.
But the truth is that effective air sealing isn't mysterious, even if some builders seem intent on over-complicating it. Some complain that the amount of information on the topic is overwhelming and contradictory. Others point out that they can't know for sure before starting a house where the air leaks will be. While both of these objections may be true, they're really just signs that the builder lacks a defined air sealing protocol.
This article outlines the basic framework for devising such a protocol. Once you understand air sealing best practices and how to prioritize them, you can be pretty confident of the likely results.
You're Halfway There
A great starting point is something Gord likes to say: "The home is already made of air barriers; all you have to do is connect them." Air barrier materials consist of sheet materials like housewrap, as well as foam board, sheathing and drywall.
Regardless of which one you choose, the basic job is as follows:
- Designate one of these components as the home's primary air barrier.
- Seal any holes or gaps in and around the sheets or panels with sealants; tapes or sealants that the manufacturer recommends or lists as compatible.
- Connect the air barriers on adjacent assemblies to create a seamless whole. Again— air sealing is about connecting. That means paying close attention to common problem areas like roof/wall intersections as well as walls or ceilings that separate the garage from the living space.
Innie or Outie?
What you designate as an air barrier material will depend in large part on whether you want the air barrier to be on the inside or outside the conditioned space. In most cases, I find that interior air barriers are good, but that exterior ones tend to be better.
When carefully detailed, an interior air barrier can get you down to 3 ACH, which is good enough for most codes. When using an interior approach, drywall typically serves as the primary air barrier material.
Interior air sealing includes details like:
- Adding a bead of sealant to the underside of bottom plates at exterior walls.
- Stapling drywall gaskets to the exterior wall top plates
- Foaming wire and pipe penetrations
- Specifying LED ceiling can lights, which are significantly easier to seal than conventional recessed lights.
- Using electrical boxes with built-in drywall gaskets. (These will cost you about a dollar apiece rather than the 50 cents you pay for a conventional box.)
Interior detailing includes problem areas that don't come into play on the outside of the house. For instance, you need to remember to insulate and air seal behind showers on exterior walls before installing the shower unit. There's a good video on the project here.
I always advise people to consider prioritizing exterior air sealing when possible. (Though I realize this may be difficult in a cold climate in winter.) That’s because, in my experience, the exterior is a lot simpler to detail because it's often part of the water management layer, which all builders are really focused on.
With exterior air sealing it's also easier to get down to 2 ACH, though reaching that goal means carefully taping, gasketing or applying sealant to seams between sheets or panels, as well as sealing around plumbing, electrical and other penetrations. It also means paying attention to things you may not have given much thought to in the past. For instance, if you're using housewrap as the air barrier you need to tape the tops and bottoms of the sheets, something not all builders do.
Note, too, that if the attic is not part of the conditioned space then your exterior air barrier will have an interior component: the ceiling plane on the home's uppermost floor. The edges of that ceiling plane need some sort of bridge to connect it to the exterior air barrier.
Another advantage of most exterior air barrier materials is that (unlike most people), they're good at multitasking. That's because every building has four control layers—Rain Control, Air Control, Vapor Control and Thermal Control—and one product can usually address two or more of them.
A good example of this is a coated sheathing, which has a moisture barrier built into the surface and can, when properly taped, serve as a rain and air barrier, as well as offering structural strength. It costs more than regular OSB but serves multiple functions without the need for multiple trades.
An Even Better Approach
As I mentioned, you can get down to 3 ACH or even 2 ACH with careful detailing. But while that may be sufficient for code-minimum construction, high performance builders generally aim for 1.5 ACH or less, and getting there with tapes and sealants is a challenge, to say the least. The crew can't know for sure if they sealed everything until the blower door test. If the test shows that they missed some holes, finding those holes can be extremely time-consuming.
This is why so many high-performance builders use spray foam, whether as a flash-and-batt option or as the home's primary insulation. It's an excellent material for filling gaps and voids in wall cavities.
The problem, of course, is that many air leakage points aren't within wall cavities and thus don't get sealed by the foam. That's why (at the risk of sounding promotional) I like to recommend AeroBarrier as a final step.
To understand this product think "Fix-a-Flat." The applicator tapes off doors, windows and duct grilles, then pressurizes the house with a blower door. Molecular-sized particles of an acrylic latex are sprayed into the air and forced by that pressure into any unsealed gaps in the building envelope. The particles, which are similar to an aerosolized white glue, coagulate on the edges of a hole and slowly seal it. It's possible to plug holes as big as 5/8 inch diameter.
People worry about the spray coating the walls but that's not a concern; it only seeks out actual holes. Any excess just dries out and falls to the floor where you can sweep it up.
The AeroBarrier process also lets you fine-tune the results. The blower door constantly monitors air leakage during application, so the applicator can just keep adding more spray until reaching the target number.
With an exterior air barrier, you can do this work after rough-in, when the windows, doors and attic hatch have been installed. If you're using drywall as an interior air barrier, then do it right after first mud. Many builders in the far north will choose the drywall as their primary air barrier, as AeroBarrier requires that home's interior be warmed to at least 40 degF. That's a lot easier to do with portable heaters after the home has been insulated.
Gord was so impressed with the product that he partnered with his son to open an AeroBarrier installation company in Canada. They have completed more than 200 homes, with averages costs of about $1.50 per square foot, or $3,000 for a 2000 square-foot house. That's in line with what builders in other markets tell me they are paying.
Pricing will no doubt come down as more builders get on board. Meanwhile, if you're using flash-and-batt for air sealing, you may be able to get better results from this approach without the foam.
Mark LaLiberte is a partner in Construction Instruction, which offers hands-on building science training at its location in Denver.
This house illustrates best practice approaches for interior and exterior air sealing, with drywall gaskets, sealant applied to the bottom of exterior wall plates and housewrap taped on all edges, including the top and bottom of the wall.