Masonry Magazine September 2006 Page. 34
AIR BARRIERS
The problem is that in climates with hot summers and cold winters, the vapor barrier will be on the wrong side of the insulation at least part of the year. In summer, hot humid air from outside may condense on a vapor barrier placed for a cold climate, but cooled by air conditioning. And warm, moist air from living spaces can condense on vapor barriers placed for a hot climate but cooled by winter temperatures. Both conditions contribute to water inside the wall assembly.
Recent studies have shown that comparatively little water actually gets into the walls through diffusion. It is mostly carried in through air leaks in the wall assembly. So a vapor barrier also has to stop those leaks.
Since stopping air movement turns out to be what's important, some systems have dispensed with the problematic vapor-impermeability of vapor barriers. Air barriers just concentrate on stopping air movement.
Air Barriers
As we've discovered, much more water gets into walls through air leakage than by diffusion. While diffusion may deposit several ounces of water into the cavity over the course of a year, moisture carried into walls by humid air leaking through small holes, seams and penetrations can deposit several gallons.
Effective air barriers stop air from leaking through the sheathing or CMU backup by swathing it 100 percent, from top to bottom. If air, which always carries water, can leak in or out anywhere, the entire installation is no good.
That's the key to air barriers: they don't have to be impermeable. Many will let a little vapor diffuse through them as part of stopping water buildup in the cavity. In fact, letting water vapor diffuse out of a cavity contributes to keeping the cavity dry.
But air barriers must stop air movement - period.
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