Since the 1970s, building scientists have discovered a great deal about home air barriers. They now understand that air barriers are crucial to a home’s durability, the energy needed to heat and cool it, and the comfort of its residents.
No other building element is as critical in high-performance homes, and building and energy requirements have recognized this and become much stricter on air-sealing.
Houses constructed per building code must therefore be “fairly tight.” While building a home to the Passivhaus standard is impossible without a meticulously planned air barrier.
A home’s air barrier should continuously encompass the entire building envelope because the barrier separates the inside and outside air, along with air pressure.
You can design an air barrier for a home’s interior or exterior. In either location, the air barrier must be:
- Resilient for the duration of the home’s anticipated lifespan (a great goal to aim for is 100 years)
- Impermeable to air movement (including where pipes and wires penetrate it)
- Able to withstand pressures acting on them during and after construction
- Continuous across the whole building enclosure
Example: A car wheel is an excellent example:
- The rim is metal, and the tire is of thick rubber. Both materials are impervious to air.
- The tire has a continuously sealed connection to the metal rim.
- A valve stem penetrates the two but is also sealed (think of this like an electrical conduit).
Three crucial objectives are achieved when undesirable air leaks are stopped:
- Energy efficiency (reducing the cost of heating and cooling the house).
- Limiting the quantity of moisture entering wall and roof cavities minimizes possible moisture damage.
- Maintaining excellent air quality by managing the supply of fresh air coming into the home.
An efficient air barrier cannot be created using a single technique or material. Instead, air barriers are a combination of several distinct materials.
The mixture includes caulk and spray foam to rubber gaskets, drywall, house wrap, and sheathing. How these materials are installed is what matters.
The ability of various tradespeople to comprehend how an air barrier functions and their part in preserving its integrity will significantly determine the success or failure of the air barrier.
There are many ways to accomplish the same thing, but two general guidelines should be followed:
- The air barrier should be close to the insulating layer
Common Materials Used to Create An Air Barrier
Here are four of the most common ways to create an air barrier:
- Housewrap to create an exterior air barrier (Tyvek is popular)
- Polyethylene as an internal air barrier
- Exterior sheathing for an external air barrier (Zip sheathing is the leading manufacturer)
- Air-sealing framing and drywall for an interior air barrier
Sometimes spray-foam insulation is used for an air barrier as well. Although, most high-level building scientists don’t recommend it since building materials can expand, contract, and break the seal.
Your home’s climate zone and design will determine the best method.
Interior Air Barriers (Challenging to Install)
Warm air holds more moisture (PDF).
A home will have more moisture inside during winter in cold climates than outdoors. Since heat moves from hot to cold, warm moisture-rich air will move into the wall cavity on its way outdoors. An interior air barrier stops this from happening, helping prevent the walls from moisture/rotting.
The downfall of interior air barriers in most assemblies is that they do not prevent wind-washing, degrading the insulation’s performance.
Also, interior air barriers can be challenging to install continuously due to partition walls and penetrations like electrical boxes.
Exterior Air Barriers (Remember Moisture)
Installing continously is much easier on the exterior of a home. Although, there are still challenges, such as where the exterior wall meets the roof framing.
The solution is wrapping an air barrier material over the top plate of an exterior wall, then sealing to the ceiling air barrier.
An exterior air barrier prevents outside wind penetration into the insulation but does not prevent the interior air from getting into the wall cavity.
The Solution: Use Both
In theory, you only need one air barrier; installing two makes sense. Two air barriers are common in Europe (where they’re ahead of the U.S. regarding home energy efficiency).
You get the best of both methods, and each weakness is solved by each other.
Important Caveat: Know When it Needs to Dry
In the early days of high-performance homes, interior air barriers used polyethylene film. The film was inexpensive and was air-tight if sealed properly using staples, caulk, and tape.
The problem with this application was air-conditioning.
During summertime, the polyethylene film does not allow drying from the interior (which is needed) when air-conditioning.
Rule of Thumb: Even in cold climates, never install an interior polyethylene air/vapor barrier in an air-conditioned home. Also, remember the future homeowners who may want to install AC, even if you can live without it.
Air Barriers Reduce Sound Transmission
If you want a quiet home, then an air barrier will be your best friend. An air barrier reduces sound transmission, benefiting areas like basements, hallways, bathrooms, and other common areas.
The fewer air leaks through an air barrier, the quieter your house will be.
Air Barriers Are Not the Same as Vapor Barriers (& More Important)
Air barriers and vapor barriers have two distinct purposes. Vapor barriers impede (or at least delay) the diffusion-driven movement of moisture. On the other side, air barriers halt the flow of air. Vapor permeable or vapor-impermeable air barriers are both possible.
Air barriers are far more crucial than vapor retarders when preventing unwelcome moisture from entering wall and roof cavities.
Diffusion transports far less moisture than an air leak. For example, a vapor barrier with 10% of its surface covered in rips and tears is nevertheless 90% effective. However, even a very little flaw in an air barrier can allow a significant amount of air and moisture.
History of Air Barriers
One of the first forms of home air barriers was tar paper. Installed behind shingles, also known as “asphalt-impregnated felt paper.”
Home builders began using felt paper in the 1900s, serving as a weather resistive barrier (WRB) and air barrier.
The problem with tar paper as an air barrier is that it is not air-tight. This is because it was usually overlapped and not taped.
After tar paper came Tyvek air barriers. Its advantages were:
- Available in larger sheets (fewer seams that have the chance to leak air)
- More durable than tar paper (resisted damage during installation and weather during construction)
Some contractors have used Tyvek as an air barrier, but it must be carefully detailed with taped seams and connections. Also, remember that ladders, staples, and nails usually puncture this material during construction.
Most applications best use this material as a weather barrier and do not rely on it for air tightness.
The arrival of Tyvek created a wave of new products labeled “house wrap.” Many products function very similarly to Tyvek, just made by different manufacturers. Functioning slightly different.
Fully Adhered Membranes
The next evolution of air barriers is fully adhered membranes. The advantage of these materials is that the entire material is coated with adhesive, so there is no risk of the material being torn from wind (common around windows).
Current Air Barrier: Wall Sheathing
Home builders have recently switched to wall sheathing as air barriers. The sheathing comes with a factory-applied coating that resists air and water (although it does let water vapor through).
To function correctly, the panels must be put together in an air and water-tight manner- using a special tape that the manufacturer has designed for the sheathing.
Building Codes Recognize Importance of Air Barriers
Canada’s national building code has mandated an air barrier for 25 years. However, acceptance has taken longer to spread in the United States, and such requirements are often absent from state energy regulations.
However, an air barrier requirement are included in ASHRAE’s Energy Efficiency Standard (ASHRAE 90.1).
In 2009, the International Energy Conservation Code mandated airtightness testing. As a result, blower-door testing is made mandatory, and the air-tight standards are stricter in the updated IECC code.
For residences in climatic zones 1 and 2, the 2009 threshold of 7 air changes per hour at 50 pascals (ach50) has been altered to 5 ach50. Homes in all other zones have been raised to 3.
How Do You Know You Have a Good Air Barrier? (Test it)
Perform a blower door test to know if your home’s air barrier was installed correctly and working as it should.
Blower doors allow contractors to discover and address air leaks before the house is finished by measuring the severity of the leaks. The term “blower-door-directed air sealing” refers to this procedure.
Simply feeling for drafts with your hands can reveal several air leaks. However, with a smoke pencil, smoke bottle, fog machine, or incense, leaks that are more difficult to find are frequently easy to locate.
Workers can use caulk, spray foam, or other materials to stop leaks while the blower door is running. In addition, they can monitor their progress by regularly testing the airflow with the blower door.
Adding an Air Barrier to an Existing Home
While installing an air barrier is best done during construction, it is possible to add one to an existing home.
Often best done during replacing siding (as it will need to be removed).
The process typically involves removing the siding, installing an exterior air barrier, adding external insulation, then new siding (make sure to include a rainscreen for additional venting and drying potential).
Frequently Asked Questions
How do you create a basement air barrier?
An air barrier system encloses a home’s conditioned space in a three-dimensional balloon. There are various areas of worry if your basement is included in your conditioned area, which most of the time it should be.
It’s crucial to remember that the stack effect causes basements to become less pressurized throughout the winter. In actuality, air can be drawn through the earth beneath the basement slab through stack-effect depressurization. (Contrary to popular belief, most soils are sufficiently permeable to permit a link between the air within the soil and the air above grade, even at a depth of 7 feet.)
An efficient air barrier is a slab in the basement. However, the perimeter seam where the footing or foundation wall meets the basement slab is a source of air leakage and has to be caulked.
Large amounts of air can enter a home through basement sumps. Installing a sump with an airtight lid is the answer.
The majority of contractors are aware of how crucial it is to install sill-seal between the mudsill/sill plate and the top of the foundation wall. This connection may need to be caulked or sealed with spray foam from the inside of the sill-seal is inadequate.
Your basement’s rim joists should be insulated if they aren’t already. The ideal insulation for this place is closed-cell spray polyurethane foam, which thankfully also aids in sealing air leaks in the rim-joist location.
If you’re on a budget you can also cut out squares of rigid foam (2″ is usually thick enough) around 3/4″ smaller than the rim joist area, then, seal the gap with canned spray foam. Once complete you can place bat insulation on top of the foam board for higher r-value.
You might want to think about weatherstripping the door at the top of your basement steps to reduce the stack effect.
What is the “pencil test” for air barriers?
Although it’s not often done, the air barrier placement should be shown on a section drawing of a house design.
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 taking your pen off the paper because this air barrier must be continuous, without any interruptions where the walls meet the ceiling or at other transitions.
Is spray foam the best air barrier?
Several builders have achieved extremely low amounts of air leakage without using any spray foam. In addition, a lot of homes with spray foam insulation still have a lot of air leakage.
How is it even possible? The answer is straightforward: most walls and ceilings don’t leak in the middle. Instead, at penetrations, transitions, and edges, they leak. So you still need to pay attention to the details even when spray foam is used to insulate a wall.
Most air sealing solutions don’t need expensive materials. A few tubes of quality caulk and a few rolls of gasket material are frequently all that is required. High-tech tools or equipment rarely matter more than attention to detail.
The blower door results, or testing, prove air sealing works without using spray foam.
Resources + References
Defining Airtight – PDF