Analyzing the Differences Between Air Barriers and Vapor Barriers

Editor’s Note: Applied Building Technology Group has developed a technical library of performance and installation guidance on air and vapor barriers, click here for more information.

What's the difference between air barriers and vapor barriers?

Air barriers are an extremely important component of high-performance buildings. For the most part, these systems are non-maintainable, which means that repairs can only be made by removing the exterior cladding. Sure, older buildings can be retrofitted to improve air sealing, but the truth is: You only get one chance to get it right.

The same cannot be said about vapor barriers. Although the two are often confused, each has a very different function to perform in the building assembly. Understanding the core differences are paramount to building the high performing homes of the future.

What Is An Air Barrier?

Air barriers are systems of materials designed and constructed to control airflow between a conditioned (indoor) space and an unconditioned (outdoor) space. Air barriers can be mechanically fastened building wraps, adhesive membranes, fluid-applied materials, insulating board stock, non-insulating board stock, spray polyurethane foam, poured concrete, metal, glass, and a host of other materials. But no matter what material you choose, all air barriers should be:

• impermeable to air flow;
• continuous over the entire building enclosure or continuous over the enclosure of any given unit;
• able to withstand the forces that may act on them during and after construction;
• durable over the expected lifetime of the building.

What Is A Vapor Barrier?

Vapor barriers are materials used to slow or reduce the movement of water vapor through a material. Vapor barrier materials are installed on the warm side of the insulation in a building assembly, as determined by climatic conditions. In warm climates, it will be on the exterior and in cold climates, it will be on the interior.

A vapor barrier can be a mechanically fastened sheet-material, adhesive membranes (depending on composition), fluid-applied materials, insulating board stock or medium density spray polyurethane foam. The thickness of the material will impact whether it is a vapor barrier or not.

But Wait… There’s More

Here’s where things can get confusing. Water vapor may be transported by air leakage, but you address this issue by installing a proper air barrier not by a vapor barrier. Vapor barriers are intended to control the rate of diffusion into a building assembly. Therefore, the vapor barrier does not have to be continuous, does not have to be free of holes, does not have to be lapped, does not have to be sealed, etc. A hole for example in a vapor barrier will simply mean that there will be more vapor diffusion in that area compared to the other areas of the vapor barrier.

To simplify, consider this wool sweater analogy: A wool sweater is insulation. It will keep you warm when there is no air movement, but it still allows the wind to move right through it. A wool sweater with a raincoat will keep you warm, but hold moisture inside and soak your insulation. A wool sweater with a windbreaker will keep you warm, stop the wind from stealing your heat, yet allow moisture to diffuse through it.

So think of a windbreaker as an air barrier, and a raincoat as a vapor barrier.

In high-performance buildings, air barriers and vapor barriers, as well as water-resistive barriers, can be combined. There are also vapor permeable air barriers, and there are water resistive barriers which are not air barriers. So it’s important to understand the separate functions and then determine whether the material provides more than one function. As an example, you can have two, three or even four air barrier materials in a wall assembly, but its effectiveness will depend on which material you have chosen and how you have connected the air barrier materials together.