©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
UPDATED on December 12, 2014
Builders of a certain age — say, those older than about 55 or 60 — started their careers at a time when no one talked about air leakage or air barriers. Back in the early 1970s, even engineers were ignorant about air leakage in buildings, because the basic research hadn’t been done yet.
Times have changed, and most residential building codes now require builders to include details designed to reduce air leakage. Today’s young carpenters are working on job sites where air barriers matter.
A. A wide variety of materials make good air barriers, including poured concrete, glass, drywall, rigid foam insulation, plywood, and peel-and-stick rubber membrane. (Note that evidence is increasing that OSB is not an air barrier; for more information on this issue, see Is OSB Airtight? )
Although air can’t leak through these materials, it can definitely leak at the edges or seams of these materials. When these materials are used to form an air barrier for your home, additional materials such as tape, gaskets, or caulk may be required to be sure seams and edges don’t leak.
To make a good air barrier, a material not only needs to stop air flow; it also needs to be relatively rigid and durable. If you want to determine whether a material is an air barrier, hold a piece of the material up to your mouth and blow. If you can blow air through it, it’s not an air barrier.
Is OSB Airtight? 
Blower Door Basics 
Engineers distinguish between air barrier materials (drywall, for example), air barrier assemblies (for example, plywood with taped seams attached to wall framing), and air barrier systems (all of the materials and assemblies that make up a building's air barrier).
A. Not necessarily. Although Tyvek and other brands of plastic housewrap are sometimes marketed as air barrier materials, the primary function of housewrap is to act as a water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. (WRB). In other words, the Tyvek is there to protect the wall sheathing from any wind-driven rain that gets past the siding.
Some builders have experimented with using Tyvek as part of an air barrier system. If the seams of the Tyvek are taped, and if the gaps between the Tyvek and window openings are carefully sealed, and if the transitions between the Tyvek and other materials at the bottom of the wall and the top of the wall are detailed in an airtight manner, then Tyvek can work as part of an air-barrier assembly. But siding contractors often rip holes in the housewrap with their ladders; they also penetrate the housewrap with hundreds of nails and staples. Builders interested in achieving a tight air barrier have found that other air sealing methods are more effective than an approach that depends on housewrap to be a wall’s primary air barrier material.
A. Probably not. During the 1980s, interior polyethylene was widely promoted as an interior vapor barrier. Its use in new homes is now relatively rare, except in very cold locations (for example, Minnesota, Canada, and Alaska).
Most polyethylene installations leak a lot of air — especially at the seams between adjacent sheets of poly, at penetrations, and around electrical boxes. That’s not usually a problem, since polyethylene is an effective vapor barrier even when it is not installed in an airtight manner.
Some cold-climate builders have successfully used polyethylene as part of an air barrier system. To act as an effective air barrier, however, polyethylene needs to be installed with careful attention to a long list of fussy details, including the use of acoustical sealant (non-hardening caulk) at all seams and the use of airtight electrical boxes. This type of polyethylene installation is relatively rare.
A. Most air leaks happen at the seams or cracks between different materials: for example, where the mudsill framing meets the foundation, where floors meet walls, and where walls meet ceilings.
Although gaps around windows and doors — the first areas of concern for many homeowners — occasionally contribute to air leakage problems, the most significant air leaks are usually in hidden areas. Because such hidden leaks — called “thermal bypasses” by weatherization contractors — usually don’t cause obvious drafts, homeowners are often unaware of their existence.
Here’s a list of some of areas that are often poorly sealed, and therefore responsible for significant air leakage:
A. Tracking down air leaks can be tricky, especially for builders or homeowners who are unfamiliar with the devious paths that air can take. For example, builders are often surprised to learn that significant air leakage paths can occur through interior partitions located far from exterior walls.
The best way to test the integrity of a home’s air barrier is to perform 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.. To learn more about blower-door testing, see “Blower Door Basics.” 
A. If you have punctuated your insulated cathedral ceiling with recessed can lights, it’s very difficult to keep the ceiling airtight. (Unfortunately, even so-called “airtight” can lights are actually fairly leaky.)
In a ceiling like yours, warm humid air enters the insulated rafter bays through cracks around the can-light trim. The air is drawn into the rafter bays by the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season.; the air usually exits the rafter bays through cracks near the ridge.
Since fiberglass batts are air-permeable, they do little to slow air movement. Regardless of whether the rafter bays are vented or unvented, there are usually plenty of cracks that allow humid indoor air to find its way to the underside of cold roof sheathing, where the moisture condenses. On cold nights, a layer of frost can build up on the underside of the roof sheathing. When the weather warms up, the frost melts, leading to dripping can lights.
The solution is to create a tight air barrier at the ceiling plane. The best way to achieve this goal is to remove the can lights, patch the drywall, and substitute surface-mounted light fixtures like track lighting.
A. An air barrier system is a three-dimensional balloon surrounding the conditioned area of a home. Assuming that your basement is part of your conditioned area — and I think it makes sense to include it — then there are several areas of concern.
First of all, it’s important to note that the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. depressurizes basements during the winter. In fact, stack-effect depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. is strong enough to pull air through soil under the basement slab. (Believe it or not, most soils are porous enough to allow a connection between the air in the soil — even 7 feet below grade — and exterior air above grade.)
A basement slab is an effective air barrier. However, the perimeter crack where the basement slab meets the footing or foundation wall is a source of air leakage and should therefore be caulked.
Basement sumps can allow significant volumes of air to enter a house. The solution is to install a sump with an airtight lid; these are available from Jackel, Inc.  in Mishawaka, Indiana (574-256-5635).
Most builders know the importance of installing sill-seal between the top of the foundation wall and the mudsill. If the sill-seal is ineffective, this joint may need to be caulked or sealed from the inside with spray foam.
If the rim joist in your basement isn’t insulated, it should be. Closed-cell spray polyurethane foam is the best insulation for this location; fortunately, spray foam also helps seal air leaks in the rim-joist area.
To limit the stack effect, you may want to consider weatherstripping the door at the top of your basement stairs.
A. Several different driving forces affect air leakage rates. Some of these driving forces — including wind, exhaust appliances like bathroom fans, and unbalanced or leaky ductwork used for forced-air heating or cooling systems — are intermittent. However, one driving force — the stack effect — acts continuously on a house, as long as the exterior air temperature is significantly below the interior air temperature.
Because of the stack effect, it’s fairly easy to predict infiltration and exfiltrationAirflow outward through a wall or building envelope; the opposite of infiltration. patterns during the winter. Warm air usually exits the house through cracks near the top-floor ceiling. The stream of departing air pulls air into the house through cracks in its lowest level — for example, through the crack between the basement wall and the mudsill.
In other words, the air at the top of the house is pressurized with respect to the outdoors, while the air in the basement is depressurized.
In the center of your house, maybe somewhere in the vicinity of your living-room windows, the indoor air is neither pressurized nor depressurized. It’s at about the same pressure as the air outdoors. This is called your home’s “neutral pressure plane.” Even if your living-room window has all kinds of cracks, not much air will leak through — as long as the wind isn’t blowing. That’s because there is no driving force. The outdoor air and the indoor air are at the same pressure.
What happens to your home’s neutral pressure plane when you turn on an exhaust fan in an upstairs bathroom? It moves upward a few feet, that’s what. More air is now leaving your house, so more air needs to enter the house to replace it. As more air enters — perhaps through those cracks around your living-room window — the neutral pressure plane moves a few feet higher, up near the ceiling.
Understanding these pressure dynamics helps guide the efforts of weatherization contractors when they perform air-sealing work. The most important cracks to seal are those under negative pressure — that is, cracks in a basement or crawl space — and those under positive pressure — that is, cracks in the attic floor. Air leaks in the center of the house — in the vicinity of the neutral pressure plane — are less important.
A. Probably not. Unfortunately, most house plans don’t indicate the location of the air barrier. As a result, a home's air barrier details — if such details even exist — were probably made up by the builders on the job site.
A wall’s air barrier can be located at the exterior sheathing (by taping the sheathing seams), at the interior drywall (by following the Airtight Drywall Approach ), or in the middle of the wall (by using spray polyurethane foam). If the details are done correctly, any of these three methods can result in a very tight air barrier.
A. It means you have chosen a good designer. Although it’s rarely done, a good section drawing of a house design should indicate the location of the air barrier. Because this air barrier must be continuous, without any interruptions where the walls meet the ceiling or at other transitions, it should be possible to trace the air barrier on the section drawing — from the basement slab, up the walls, over the ceiling, and down the other side — without lifting your pen from the paper.
A. Yes. If your home lacks a decent air barrier, lots of cool indoor air can escape through cracks in your walls and ceiling. That departing cool air is replaced by hot outdoor air that sneaks in other cracks, forcing your air conditioner to work overtime.
If your summers are humid, these air leaks carry a double penalty. When outdoor air enters your home, your air conditioner struggles not only to cool the air, but also to wring the moisture out of the air. In states with humid summers, a significant portion of your air-conditioner run time is actually devoted to dehumidification. The tighter your home, the easier it is is to keep your indoor air cool and dry.
A. Absolutely. Many builders have achieved very low levels of air leakage without any spray foam at all. Moreover, many homes that have been insulated with spray foam still have high levels of air leakage.
How is this possible? The reason is simple: most walls and ceilings don’t leak in the middle of the wall or ceiling. They leak at edges, penetrations, and transitions. So even when a wall is insulated with spray foam, you still need to worry about the details.
The lowest levels of air leakage are achieved by builders who have studied airtight construction techniques, who think through potential air leakage paths during each phase of construction, who are conscientious and methodical, and who have already built a few homes that were tested by a blower door.
Most air sealing details don’t require fancy materials. In many cases, a few rolls of gasketing material and some tubes of high-quality caulk are all that’s necessary. Attention to detail usually matters more than high-tech tools or equipment.
When it comes to air sealing, the proof is in the pudding — in other words, the blower door results.
Last week’s blog: “Net-Zero-Energy Versus Passivhaus.”