What’s the difference between an air and vapor barrier?
Provided by the Cold Climate Housing Research Center (CCHRC)
Air and vapor barriers protect your home from moisture and air flow through the building envelope, a critical part of keeping your home warm and dry. If selected and installed properly, these products can help you conserve energy, prevent mold growth, and maintain the structural integrity of your home. On the flip side, not using these products or using them incorrectly can wreak havoc.
All building envelope assemblies need an air barrier. In a climate like Fairbanks, air barriers help to slow heat loss through the walls for most of the year. They also prevent moisture-laden air from leaking into the building envelope from outside, where it could condense and lead to mold growth.
An air barrier can be used anywhere in the envelope assembly (on the interior, exterior, or both). Their main job is to control the movement of air in and out of the conditioned space. By doing so, they improve the energy efficiency of a building, as well as its comfort and sound transmission.
They also prevent the movement of water vapor, which is a component of air, through the envelope. In a cold climate, the air barrier is typically located on the interior (warm-in-winter) side of the wall. This air barrier separates the inside environment from the outdoor environment and makes the house airtight. The effectiveness of the air barrier can be measured using a blower door test.
Often, a second air barrier, or at least a weather resistive barrier, is then located on the exterior side. This air barrier keeps wind and other elements out of the building envelope.
Most air barriers are actually a system of several different materials sealed at the joints, working together to control airflow - for example, gypsum wallboard with a sealed transition to the windows. Other materials used in air barriers include Tyvek house wrap with taped joints, concrete, rigid foam insulation or plywood with taped joints. Air barriers need to be rigid and durable in order to stop airflow caused by wind, stack effect, or a mechanical system.
While air barriers serve a variety of functions, vapor barriers are used solely to limit water vapor from getting into a building envelope. It should be noted that vapor barriers also reduce the rate of water vapor diffusion out of a building envelope, so they need to be installed in the right place so they’re not trapping moisture in the envelope. Installing more than one vapor barrier can also trap moisture in the walls (such as plastic sheeting on the inside and impermeable foam on the outside).
Vapor barriers are generally installed on the warm side of the insulation – in Fairbanks, that means the inside. The most commonly used vapor barrier is a 6 mil polyethylene sheet.
What’s the difference?
To summarize, air and vapor barriers block the flow of water vapor in different ways: air barriers prevent it from being carried into the envelope by air, while vapor barriers prevent it from diffusing through the wall. Because more water vapor is carried by air than diffusion, builders should first focus on the air barrier.
In some cases, a material can double as the air and vapor barrier, as long as it’s properly air-sealed around all openings: for instance, a rubber membrane or polyethylene sheeting (in sufficient thickness).
However, many air barriers do not prevent water diffusion - water can diffuse relatively easily through plywood and gypsum wallboard. In these cases, a separate vapor barrier would need to be installed if the design called for one.
Both air and vapor barriers should be installed carefully with properly taped seams and no holes or tears. This is especially important for the air barrier.
While a small tear or hole will not substantially affect the performance of a vapor barrier, the same tear in an air barrier will allow much more air (and water vapor) through. In other words, one flaw can make an entire air barrier system ineffective.
It is important to understand the function of both air and vapor barriers before you install them.
For more questions about wall systems, as well as details on commonly used wall systems in Alaska, visit the Your Northern Home Website: http://cchrc.org/yourhouse/walls.htm.
This video shows how to install a vapor barrier: https://youtu.be/R8RMR-e-LlE?list=PLBtw7qpC24vLes3TTm-6m7Zz7gWf1QgVo
The Cold Climate Housing Research Center is an industry-based nonprofit based in Fairbanks, Alaska that develops and tests energy efficient building technologies for the north. The articles and videos included in this guide are part of its mission to promote healthy, sustainable, affordable housing in Alaska and beyond. Find more at cchrc.org.