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Field reports from colder climates such as Alaska, Minnesota, Ohio, etc. indicate that wall moisture is a major problem, if there is no vapor retarder. A polyethylene vapor retarder is recommended in Alaska. Studies of retrofitted walls in older residences from warmer climates such as Oregon intimates that moisture in the wall may not be a problem even though there is no vapor retarder. However, the Oregon study still recommends a polyethylene vapor retarder for new construction.
There are two potential vapor retarders, which are designated in this paper as the primary and secondary vapor retarders. The primary vapor retarder for heated buildings, in cold climates such as Alaska, should consist of a polyethylene membrane placed at the inner skin directly under the gypsum wallboard (drywall). If the primary vapor retarder does not function as intended, then the outer skin consisting of a combination of plywood sheathing, siding, wind barrier, etc., may function as a secondary vapor retarder, which may trap moisture in the wall cavity.
Computer spreadsheet simulations of various wall sections indicate that wall moisture problems are dependent on inside and outside psychrometric conditions. The vapor pressure differential of the inside and outside air determines the driving force of the vapor. The vapor pressure and permeability of the primary and secondary vapor retarders determines how much water vapor may diffuse into the wall cavity. If the temperature gradients at the vapor retarder remains below dew point the moisture will condense in the cavity. If the temperature in the cavity remains below 32°F for prolonged periods the condensate will freeze and continue to build up as frost.
Condensation, vapor retarder, ambient temperature, dew-point temperature, relative humidity, absolute humidity, humidity ratio, vapor pressure, temperature gradient, permeability
Professor Emeritus, Extension Engineer, Cooperative Extension Service, University of Alaska, Fairbanks, Alaska