Air Barriers: Getting it Right
There has been a lot of information over the years regarding air and vapor barriers. The information over these barriers has evolved quite a bit. I recently came across this article from Buildingscience.com covering a few of them and I thought I would add a few things to this thought process.
Joseph Lstiburek usually does a great job of breaking down technical engineer jargon, but I thought I would go a step or two farther and highlight what I think are some of the most important parts of this article. At the basic level this article talks about two different things: air barriers and vapor barriers.
Air barriers are important because most insulations are fibrous (meaning they allow air to move through them), thus we need to add some kind of air barrier to stop the air flow. Think of it this way: during the winter you put on a nice wool coat and if you go outside it will keep you warm. But if a gust of wind sweeps by the air will pass through the wool coat and make you cold. The same idea is transferred to fiberglass insulation and cellulose insulation. This is why they need an air barrier to help them work properly.
Vapor barriers are designed to stop the vapor from travelling through the wall cavity and soaking the insulation. Let’s go back to the wool coat idea again. Take that wool coat and soak it in a bucket of water then put it on. You will find very quickly that wet wool or wet fiberglass and cellulose conducts temperature very quickly. As Joseph Lstiburek points out, “Vapor is principally transported by air flow not by vapor diffusion. We needed air barriers not vapor barriers to control vapor flow. It took decades for that distinction to be appreciated.”
It was widely thought that vapor diffusion was causing insulation to become saturated with moisture when it was really air flow or air movement. He covers many different ways that have been tried or are currently being used to add an air or vapor barrier. While most of these will work if perfectly installed, in reality most of these do not work because they are not perfectly installed. Joseph points out that in the real world applications are just not matching the designed standards. Talk about labor intensive, check out this fluid applied air barrier:
What Really Works
So let’s cover what does work in the real world: spray foam insulation. Pretty much all of the wall assemblies mentioned here could easily be fixed installing open or closed cell spray foam. Both open and closed cell spray foam are an air seal or air barrier. While some people may think there is a lower cost way to stop air movement in a wall cavity, I would argue that most of these ways require great attention to detail which in turn equals great cost. Spray foam insulation quickly sprays into place filling cracks, holes, gaps, etc. air sealing the wall or attic assembly.
You may try to reinvent the wheel when it comes to air barriers, but you really need a good spray foam contractor!