An Air Barrier Is an Essential Part of the Building Envelope
Bird's Eye View
Air barriers save energy and keep buildings healthy
Uncontrolled air leaks through exterior walls, floors, and the roof can lead to a variety of problems, including significant energy losses, lower air quality, and moisture damage to the structure. Effective air barriers help to minimize these problems, making houses cheaper to heat and cool and less susceptible to moisture damage inside wall and ceiling cavities. In most cases, homes with tight air barriers and adequate ventilation systems have better indoor air quality than leaky homes.
The movement of air through the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. is caused by three factors: wind, fans like those in a kitchen or bathroom, and something called 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.," which is the movement of indoor air due to differences in temperature and density. Although no house is perfectly airtight, carefully installed air barriers minimize the effects of these forces.
Installing an effective air barrier requires the collective efforts of everyone on the construction team. The designer, builder, and every trade on the site should understand its importance and how their work has the potential to affect its integrity.
Air barriers are not a single material
Air barriers are comprised of a number of materials, including concrete, sill seal, sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , housewrap, drywall, caulk, window glass, and spray foam. These materials work in concert to prevent the uncontrolled movement of air through the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials..
Unlike vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. retarders, air barriers can be vapor permeable and still be effective. They stop the bulk flow of air, which carries moisture with it into wall and roof cavities. Even relatively small tears or rips in an air barrier can significantly degrade its performance.
What counts is not just which materials are selected, but how they are used. Even the best quality materials won't work if they are haphazardly installed or installed in the wrong place. Air barriers should be installed next to thermal insulation and be continuous.
Include the air barrier in the design
Air barriers don't happen by chance. A clear roadmap should be available to everyone, and that means that key parts of the air barrier should be clearly marked on construction drawings.
Framers, for example, should know whether an EPDM gasket is to be included between the subfloor and all bottom plates on exterior walls, or whether the rim joist is to be recessed to leave room for rigid foam. If the builder or designer has decided to use the Airtight Drywall Approach as part of the air barrier, drywall installers must know where gaskets or sealants are required.
Although this level of detail is probably unusual on construction drawings, it's always helpful.
Everyone should understand the plan
Someone needs to take responsibility for making sure the 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. is correctly incorporated into the house. That person might be the designer or architect, or the general contractor or lead carpenter. It's likely that one or more tradespeople working on the site won't be familiar with how an air barrier should be detailed, or what their roles are in making sure it's effective.
Start with a briefing for everyone working on the house to explain, first, why an air barrier is important to the overall performance of the house and, second, why their individual efforts count.
Virtually any tradesperson can compromise an air barrier, so a good rule to enforce is that whoever drills a hole seals the hole. Everyone working on the project should be equipped with the materials they need to seal holes they make in the course of their work.
Codes are getting tougher
The national building code in Canada has required an 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. for 25 years. In the U.S., recognition has come more slowly, and state energy codes typically have no such requirement.
However, ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). International organization dedicated to the advancement of heating, ventilation, air conditioning, and refrigeration through research, standards writing, publishing, and continuing education. Membership is open to anyone in the HVAC&R field; the organization has about 50,000 members. 's Energy Efficiency Standard (ASHRAE 90.1) includes an air barrier requirement. The International Energy Conservation Code began requiring testing for airtightness in 2009. In the 2012 IECC International Energy Conservation Code. code, blower-door testing becomes mandatory and airtightness requirements more stringent. The 2009 threshold of 7 air changes per hour at 50 pascals (ach50) has been changed to 5 ach50 for climate zones 1 and 2, and to 3 ach50 for homes in all other zones. The International Residential Code uses the same language.
ABOUT AIR BARRIERS
Save energy and protect moisture damage
Building scientists have learned a great deal about air barriers since the 1970s. They now recognize that air barriers are key to how long a building will last, how much energy it will require to heat and cool, and how comfortable its occupants are going to be. In high-performance houses, few building components are more essential, and building and energy codes have become much more stringent. As a result, houses built to code now must be relatively tight. Building a house to the 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. standard is impossible without a carefully detailed air barrier.
Getting rid of unwanted air leaks accomplishes three essential goals:
- Saving energy (reducing the cost of heating and cooling the house).
- Limiting the amount of moisture that is carried into wall and roof cavities, thereby reducing potential damage caused by moisture.
- Controlling the source of fresh air that's brought into the house, keeping air quality high.
There is no single method, and no single material, that makes an effective air barrier. Instead, air barriers are really a number of different materials that work together. Everything from caulk and spray foam to rubber gaskets, drywall, housewrap and sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. can be part of the mix. What counts is how these materials are installed. The success or failure of the air barrier depends largely on a number of tradespeople understanding how an air barrier works, and their role in ensuring its integrity.
Although there are lots of ways of getting to the same goal, there are two rules of thumb that apply: the air barrier should be next to the insulation layer, and it should be continuous.
Air barriers are not vapor barriers
Vapor barriers and air barriers serve two different functions. Vapor barriers stop (or at least slow down) moisture that moves by means of diffusion. Air barriers, on the other hand, stop the movement of air. Air barriers can be vapor-permeable or vapor-impermeable.
When it comes to keeping unwanted moisture out of wall and roof cavities, air barriers are much more important than vapor retarders. Far less moisture is transported by diffusion than by an air leak. A vapor barrier with rips and tears accounting for 10% of its surface area is still 90% effective. But even a very small fault in an air barrier can let a lot of air, and a lot of moisture, pass through.
For more information on vapor barriers, see:
- Vapor Retarders and Vapor Barriers
- Forget Vapor Diffusion — Stop the Air Leaks!
- Do I Need a Vapor Retarder?
Research on air barriers continues
Air barriers remains the focus of ongoing research. At the Building Science Corp., research funded by eight insulation manufacturers is measuring the thermal performance of different test walls based in part on how much air leaks through them. The work is connected with an effort to replace R-values with a different metric for measuring thermal performance, similar to the transformation that window labeling underwent. (Their work is the subject of an article by Green Building Advisor senior editor Martin Holladay.)
MORE ABOUT AIR BARRIERS
Understanding why buildings are leaky
Writing in Fine Homebuilding magazine, John Straube of Building Science Corp. says that air leaks can be responsible for a third or more of energy losses in a typical house. Air leaks also have a powerful effect on indoor air quality.
“The only way you can know for sure that the air coming into a house is clean is to know where it's coming from,” Straube writes. “People who say, ‘I want my house to breathe’ are really saying, ‘I want to rely on the mistakes that were made by the plumber and the electrician to provide me with fresh air.’ That's exceptionally dangerous. Any air that enters a house through leaks in the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. may be loaded with pollutants. The dead squirrel in your attic and the SUV idling in your garage are not going to provide you and your family with fresh indoor air.”
Leaks allow the passage of air, moisture, and pollutants into the building envelope. Wind, fans and 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. provide the push.
The side of the house facing the wind is under positive pressure, Straube explains, which blows air (and rain) into windows, walls, and roof penetrations. On the leeward side of the building, negative pressure pulls air out of the house. That can pull moisture into wall and roof cavities at the same time.
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. also can force a lot of air through the building envelope. In winter, warm air inside the house is buoyant, so it rises. When it gets to the top of the building and exits through leaks in the building envelope, cold air is drawn in at the bottom of the building. In summer, the situation is reversed. Air cooled by HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment drops to the bottom of the building and as it does, warm, moist air is drawn in through cracks in the upper part of the house.
Air pressure is also affected by bathroom exhaust fans, range hood fans, and other mechanical equipment. As these fans push air from the inside to the outside, they create negative pressure inside the building. Without a source of clean makeup air, what's pulled into the house can include pollutants like radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles. from the soil around the house, carbon dioxide (or worse, carbon monoxide) from backdraftingIndoor air quality problem in which potentially dangerous combustion gases escape into the house instead of going up the chimney. appliances, or exhaust or other harmful fumes from the garage.
Some fans move a tremendous amount of air. A big range hood or a gas fireplace, for example, can exhaust as much as 1,200 cubic feet of air per minute.
An effective air barrier is the first step in controlling the effects of these forces.
Trouble spots for air leaks
Many materials in combination create an air barrier. Failures usually occur where different materials come together. Here are some typical trouble spots:
- Where the foundation meets the floor framing.
- Where the floor meets the wall.
- Where walls meet the roof.
- At interior kitchen soffits and plumbing chases.
- At cantilivered floors, and the floors of bonus rooms over a garage.
- Around metal fireplaces.
- Gaps between windows and the framing.
- Holes in the subfloor for bathtub traps, and holes into attics for plumbing vents.
- Around recessed lighting fixtures.
Many materials can be used to seal leaks
A variety of products have been developed to seal leaks. Spray foam is often used to fill the gap between door and window frames and the rough opening, or the gaps between structural insulated panels. But some builders prefer gaskets made from EPDM, a synthetic rubber. Gaskets can be used between the subfloor and the bottom plates of exterior walls, between the rough opening of a window and the window frame, and on the seams between SIPs.
In other areas, small gaps can be filled with non-hardening caulk. Housewrap tape closes the potential leak where two pieces of housewrap overlap.
In Europe, builders are being encouraged to make two air barriers, one outside and one inside (read more about this campaign in Martin Holladay's blog One Air Barrier or Two). The two best-known European manufacturers of air-sealing tapes are Siga and Pro Clima. Siga tapes are available in the U.S. from Small Planet Workshop, while Pro Clima tapes are available from Four Seven Five.
Using the Airtight Drywall Approach
One effective technique for creating an interior air barrier is called the Airtight Drywall Approach, or ADA, which was developed in Canada in the early 1980s. Although drywall is a vapor permeable material, it makes an excellent air barrier when detailed correctly, and when painted with a vapor-retarding paint it also can become a vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. retarder.
Air won't leak through the seams that are covered with paper tape and drywall compound, but leaks are possible at the crack between the bottom of wall panels and the subfloor, at electrical boxes, and at other similar transition points. ADA employs a variety of products to seal these leaks, including gaskets, sealants, expanding foam, and airtight electrical boxes.
For sealing drywall to the framing, caulk can work (experts find polyurethane is the most effective caulk), but gaskets work better. They can be made of open-cell foam, EPDM, or can be made by ripping lengths of foam sill seal.
Airtight electrical boxes, which prevent interior air from getting behind the drywall, often include a flange that seals against the drywall, and some means of sealing holes in the back of the box where wires come in. The boxes are manufactured by Airfoil, Allied Moulded Products, Lessco, and Thomas & Betts.
Moving the insulation outside the building
A construction technique called PERSIST (an acronym for Pressure-Equalized Rain-Screen Insulated Structure Technique), developed in Canada in the 1960s, is another way of achieving a virtually airtight building.
Walls are framed with 2x4s, without eaves or rake overhangs. Once the roof and walls have been sheathed with plywood or OSB, they are covered with peel-and-stick membrane and then at least two layers of rigid foam insulation. The thickness of the insulation varies with climate, ranging from 2 in. in Florida to as much as 8 in. in extremely cold climates.
Vertical strapping covers the insulation, creating a rain screen, and siding is installed over the strapping. On the roof, 2x4 sleepers are installed over the rafters, which are cantilevered over the wall line to create eaves. Ladder-framing is used to make rake overhangs. A second layer of sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. is followed by the roofing.
The peel-and-stick membrane is a highly effective air and vapor barrier.
Although there are several disadvantages to the PERSIST approach (higher costs and added labor among them), it also offers exceptionally low air-leakage rates along with improved thermal performance. It also works in all climate zones. A variation on this approach, called REMOTE, has been promoted in recent years by the Cold-Climate Housing Research Center in Fairbanks, Alaska.
Adding service cavities to exterior walls
In buildings constructed with PERSIST or REMOTE techniques, wall cavities are typically left empty — that is, not filled with insulation. This creates a void where wiring, pipes and other utilities can be routed without affecting the integrity of the air barrier.
Other builders have achieved the same result by creating service cavities inside exterior walls. These are secondary walls, made with 2x strapping, where utilities can run. The two wall systems are separated by an air barrier, which can be a flexible membrane or sheets of plywood or OSB with the seams taped.
Tedd Benson of Bensonwood Homes is one proponent of this approach. "Wires and fixtures should be separated from high-performance wall and roof systems whenever possible," he told Martin Holladay in a blog. "We add a mechanical chase layer to our walls always, and to our roof panels when there is a heavy mechanical demand in that area. Otherwise, we have dedicated chases at the building peak, at the eave and other strategic locations when necessary. The point is that after sealing a building ... we also need to make changes and upgrades easy and provide a path other than one that violates the integrity of the envelope."
This approach also has a few disadvantages. It uses up some exterior space, and it's more expensive and labor-intensive than conventional construction. But it also yields a superior air barrier that is unlikely to be damaged by alterations to the building and is easier to implement than the Airtight Drywall Approach.
Some builders, however, continue to favor ADA because the air barrier can be repaired if it's breached.
- Fine Homebuilding