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Despite continuing debate about the need for, and efficacy of, vapor retarders as part of a roofing system, little information has been published on the specifics governing their use. This paper proposes criteria and computation methods to determine the need for vapor retarders.
We evaluate vapor diffusion as a source of condensation. We present a procedure to calculate moisture migration by diffusion, using typical interior and exterior temperature and moisture parameters, to determine the vapor migration into a roofing system. Our methods include provisions for calculating moisture accumulation in the roofing system during winter conditions, and its drying out during the summer when vapor pressures reverse direction to the interior. We propose allowable limits for moisture accumulation at any time, which are designed to prevent loss of insulating value and degradation of the roofing system from excessive moisture. This analysis, when coupled with design limits, provides a rational basis for the incorporation of vapor retarders into the roofing system.
We also consider the problem of air leakage through the building envelope as the more frequent cause of condensation problems. We discuss the mechanisms by which condensation problems arise, and provide guidelines to minimize this problem through the use of vapor retarders and other air seals. Condensation through air leakage is best controlled by proper detailing of the roofing components to prevent air leakage. Calculations are unnecessary.
air leakage, buildings, condensation, limit state, moisture content, partial vapor pressure, permeability, permeance, roofing, safety factor, thermal gradient, vapor barrier, vapor migration, vapor pressure, vapor resistance, vapor retarder
Senior staff engineer, Simpson Gumpertz and Heger Inc., Consulting Engineers, Cambridge, Mass.