Investigation of the performance of gypsum sheathing.

The computer program EMPTIED was used to assess the moisture performance of representative brick veneer and exterior insulation finish system (EIFS) walls under the climate conditions for fifteen locations in Canada The indoor conditions were assumed to be those expected in a one-bedroom apartment occupied by two persons which was ventilated with outdoor air at a constant rate of 0.3 air changes per hour.

The results obtained suggest that if insulation is installed in the stud space of either of these wall types, the temperature of the gypsum sheathing is likely to fall below the indoor  dewpoint temperature in all locations unless an excessive thickness of exterior insulation is applied. If the stud space is left uninsulated, however, only modest thicknesses of exterior insulation should be able to maintain the sheathing temperature above the room dewpoint temperature. For walls with insulation in the stud space and 25 mm and 50 mm of exterior insulation, the limiting leakage areas that would restrict the moisture content of the gypsum sheathing to below that conducive to mold and mildew growth, and to that for saturation and possible structural deterioration, were determined for each city.

An experimental study in which gypsum sheathing was exposed to condensation conditions under a temperature gradient helped to establish the limiting moisture content conditions for these assessments but raised questions regarding the conventional assumption that condensation will occur on all surfaces which fall below the dewpoint temperature of the air in contact with them. When considered with the analyses and measurements of the some other researchers, the observations made suggest that condensation will form on non-absorptive surfaces that are below the dewpoint temperature but may not occur on absorptive materials until they reach a moisture content in equilibrium with 100% relative humidity. These observations further suggest a number of possibilities for the protection of gypsum sheathing from condensation and for the development of improved building envelope design details.