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Questions and answers about moisture issues in buildings, both in vapor form and liquid moisture.
(NOTE: Please consult the applicable building codes and code officials before proceeding with any building project.) Vapor barriers are designed to stop moisture-laden warm air from escaping into cold areas and condensing. Condensation is a real concern in New Hampshire's cold climate, but some "vapor barriers" cause more problems than they solve. We at Building Energy Technologies, LLC are not fans of polyethylene plastic vapor barriers. They have their place in certain situations, but other materials are often better.
As the U.S. Dept. of Energy graphic shows, over 98% of moisture movement comes from moisture-laden air movement (e.g., leaky buildings) rather than vapor diffusion. The graphic shows that even a 1/2 inch hole in a building can leak 50 pints of water vapor during a heating season, compared to 2/3 pint of water vapor diffusing through a wall. This is why Building Energy Technologies always air seal attics and walls before we add insulation, why we densepack walls with cellulose, and why we test air leakage with a blower door. Some people fear cellulose insulation because it can absorb moisture. If we install cellulose in a wall or attic, the reasoning goes that water vapor will pass through it and make it soggy. We at B.E.T. don't want your walls to be soggy, and we use building science to use the most appropriate materials, such as dense-pack cellulose that keeps moisture-laden air from flowing through it. Plastic vapor barriers get a further knock with air conditioning. With a central air conditioning systems being installed increasing number of New Hampshire homes, a plastic vapor barrier behind the drywall is now on the wrong side for summer conditions where the moist warm air is outside. Again the air barrier is more important than a vapor barrier. See also the bath fan attic venting Q&A B.E.T. -- Solving your building energy problems |
In our opinion, a wall or ceiling insulated with dense-pack cellulose, carefully drywalled, and painted with vapor barrier paint is far superior to a figerglass-plastic sheeting-drywall assembly. The key factor here is air movement. Densely packed cellulose will block air movement while fiberglass does not. Even with a continuous layer of plastic, there are cutouts for outlets and switches, small holes where the plastic is stapled, and various other avenues for moisture-laded warm air to leak into a fiberglass insulated wall or ceiling cavity and potentially condense.  |  |