Vapor Barriers

"A minimum of 1 in. of closed-cell foam, the “flash” layer, is used to seal the framing cavity and raise the temperature of the condensing surface (the inner face of the nearly impermeable foam, in heating climates) to prevent moisture- laden indoor air from condensing on the exterior sheathing. Open-cell foam’s lower cost may be tempting, but its relatively low R-value, vapor permeance, and high expansion rate make it a poor choice for flash-and-batt applications." Prepared by Michael Maines

"Closed-cell (about 2 pounds per cubic foot density or more) spray foam applied in thicknesses of over 2” (50 mm) will control vapor diffusion to safe levels in all climates up to 10000 HDD and interior winter-time relative humidities of up to an over 50%RH. As thickness increases the level of diffusion control increases. The diffusion control is equivalent to walls with the traditional fiberglass batt and polyethylene." Prepared by John Straube

"To manage moisture effectively, we need to control both air movement and moisture flow. The two are always intertwined. Sometimes one product, such as spray foam, addresses both issues." Prepared by Mark LaLiberte

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