Double 2x4 stud walls insulated with dense-packed cellulose are about the simplest and cheapest super-insulated wall system you can build. But cellulose is vapor-permeable. So in cold climates, there's a risk that humidity inside the building will penetrate the wall, condense on the cold side of the sheathing, wet the wall, and support the growth of mold or rot.With backing from the Department of Energy's Building America program, Kohta Ueno, a researcher with Building Science Corp., has been taking a closer look at that risk. Using test walls built into a real zero-energy house in Devens, Mass., as his laboratory,...

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