One Air Barrier or Two?
If your home has an air barrier at the exterior sheathing, do you also need an interior air barrier?
Although building scientists have understood the advantages of airtight construction details for years, few residential plans include air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. details. That’s nuts.
Do the blueprints show where the air barrier goes?
Ideally, construction documents should show the location of a building’s air barrier, and should explain how the builder is expected to maintain air-barrier continuity at penetrations and important intersections. In a typical house, these intersections might include:
- where the basement slab meets the basement wall;
- where the basement wall meets the mudsill;
- where the mudsill meets the rim joist;
- where the rim joist meets the subfloor;
- where the subfloor meets the bottom plate;
- where the top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate. meets the vertical drywall; and
- where the top plate meets the ceiling drywall.
A designer who doesn’t know how to make these areas airtight can hardly fault a builder who fails to intuit details that aren’t even mentioned on the plan. (For more information on air barriers, check out these three resources: Questions and Answers About Air Barriers, the GBA Encyclopedia, and a useful Web page from Oikos, "Advanced Air Sealing.")
The road ahead is steep
If the day ever comes when most new homes include an air barrier that addresses typical penetrations and the intersections listed above, energy efficiency experts will cheer. That day is a long way off, however.
In the meantime, some progressive builders insist that every home needs not one but two air barriers: an exterior air barrier and an interior air barrier. Although this belt-and-suspenders approach is controversial, it has many strong advocates.
To understand the controversies surrounding double air barriers, it’s important to explain the two positions.
One good air barrier is enough
The basic rule of infiltration is “air out always equals air in.” If you have a good air barrier on one side of the insulation, you have stopped both infiltration and exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration. — so you’re done.
According to this philosophy, it doesn’t matter where the air barrier is located — as long as the insulation is in contact with the air barrier:
If you set a particular airtightness goal — for example, 1.5 ac/h @ 50 Pascals — you can achieve it with any of these methods, as long as you pay attention to penetrations and intersections. Once a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. verifies that you have achieved your airtightness goal, you have a tight house.
You need two air barriers
In the U.S., the most important proponent of the two-air-barriers approach is the Energy Star HomesA U.S. Environmental Protection Agency (EPA) program to promote the construction of new homes that are at least 15% more energy-efficient than homes that minimally comply with the 2004 International Residential Code. Energy Star Home requirements vary by climate. program. Since July 1, 2006, Energy Star has required builders to comply with a Thermal Bypass Checklist. The Thermal Bypass Checklist Guide notes, “Generally, the Thermal Bypass Inspection Checklist requires a sealed air-barrier on all six sides of insulation.” The checklist notes that “insulation shall be installed in full contact with [a] sealed interior and exterior air barrier.”
The Energy Star program allows three main exceptions to this two-air-barrier mandate:
For the most part, these exceptions are due to political compromises rather than important building science principles, so I won’t be discussing the exceptions here.
The classic problems that this checklist attempts to address include:
The rationale behind the two-air-barrier principle is that many of these areas — especially ventilated unconditioned attics — are subject to wind-washing or convection. If the builder installs air-permeable insulation like fiberglass in these areas, the performance of the insulation will be seriously degraded unless the insulation is protected by air barriers on all sides.
Of course, there is an inherent flaw in the Energy Star Homes mandate for both interior and exterior air barriers — the fact that no technology exists to test the air tightness of each individual barrier. A blower-door test can reveal a home's air leakage rate, but it can't reveal whether a home has one or two air barriers. If an Energy Star builder provides a good exterior air barrier, the interior air barrier can be as leaky as a colander — and building inspectors and blower-door technicians will never notice the flaws.
Double air barriers in Europe
In Europe, meanwhile, there is a significant effort underway to train builders to install two air barriers — an exterior air barrier and an interior air barrier — on all new buildings. One strong proponent of this construction approach is SIGA, a Swiss manufacturer of membranes, tapes, and gaskets.
In its English-language literature, SIGA explains that the interior barrier provides “airtightness” while the exterior barrier provides “windtightness.” (Presumably, these terms are translated from the German; the distinction between “airtightness” and “windtightness” escapes me.)
According to Patrick Haacke, SIGA’s head of product management, most new Swiss buildings include two air barriers installed with great attention to seam sealing. SIGA brochures (and SIGA training sessions for builders) recommend a meticulous series of steps to seal exterior housewrap and roof underlayment seams and penetrations.
SIGA sells a variety of air-barrier products, including housewrap, roof underlayments, underlayment seam tape, tapes for sealing underlayment to skylights and plumbing vents, and even gasketing material (“nail sealing tape”) for use under furring strips installed on top of roofing underlayment. (By the way, the quality of these tapes and gaskets appears to be excellent. I've played around with samples of the products; I'm impressed.)
Once the roof underlayment has been made airtight, SIGA recommends similar air-sealing measures at housewrap seams and wall penetrations. With the exterior air-sealing work (including details to seal the intersection between the roof and walls) complete, one might assume that the Swiss builder would happily call up his blower-door contractor and enjoy his excellent test results. But no — now the builder moves indoors, and repeats all of these steps on the interior.
SIGA brochures show a happy Swiss worker (dressed, of course, in a white collared shirt and a black vest) installing an interior membrane (similar to polyethylene, but more permeable). The builder uses special tape (sticky on both sides) along every stud so that the membrane can be attached to the framing without any staples.
Then the membrane seams are sealed with a special tape. A different tape is used to seal between the ceiling membrane and plumbing vents; still another product is used to seal the seam where a ceiling membrane meets a wall membrane. SIGA also sells tape to seal around floor joists or exposed rafters that penetrate the wall membrane. Of course, tape is used to seal the membrane to windows and to exposed ridge beams.
Are you tired yet?
Not all European builders comply with the SIGA approach, however. Although Haacke reports that it’s now standard practice for Swiss builders to install two air barriers, some PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. builders disagree with the technique. With more than one air barrier, it's always possible that builders will fail to be meticulous; in other words, redundancy can lead to imperfect execution of details. According to Hans Porschitz, a building systems associate at Bensonwood Homes in New Hampshire, German Passivhaus consultants usually advise that “one effective air barrier is better than redundant ineffective layers.”
So, are two air barriers necessary?
There are two possible reasons for installing two air barriers:
- The two barriers are a belt-and-suspenders solution to the problem of builder sloppiness.
- The two barriers are an attempt to prevent degradation in the performance of air-permeable insulation due to wind-washing and convection.
The first reason is certainly defensible. For example, some manufacturers of structural insulated panels (SIPs) now insist that panel seams be sealed twice: once with spray foam, and then later with interior tape. This belt-and-suspenders approach is a direct result of a cluster of SIP failures in Juneau, Alaska, where poorly sealed SIP seams allowed exfiltration, condensation, and rot. The air leakage problem in Juneau was caused by job-site difficulties encountered during freezing weather or rain; difficulties in accessing certain SIP seams; and worker sloppiness. The manufacturers’ response — “we need two air barriers” — makes sense.
However, some building techniques don’t need such redundancy. Marc Rosenbaum’s use of the Huber Zip System for roofs and walls, along with early blower-door testing, results in high levels of thermal performance without the need for two air barriers.
If you have a good air barrier, how much air moves through your insulation?
The second reason — addressing wind-washing and convection currents — is subject to debate. Obviously, wind-washing is a concern when fiberglass batts are exposed at soffits or the back of attic kneewalls. Elsewhere, however — for example, in walls with Huber Zip System sheathing and no interior air barrier — it’s hard to imagine that wind-washing or convection currents are significant.
To a large extent, double air barriers are an attempt to respond to the inherent performance problems of fiberglass batts. As more builders choose other insulation materials, there will be fewer justifications for the expense and hassle of double air barriers.
An academic discussion
For many energy consultants in the field — at least on this side of the Atlantic — the discussion about interior and exterior air barriers resembles the debate over how many angels can dance on the head of a pin. Here, most energy experts lament that “American homes don’t yet have a single air barrier; two air barriers are a far-off dream.”
Last week’s blog: “Energy and Construction Photos from Greece.”
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