Published: Jan 1991
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For some time the impact of the use of rigid foam exterior insulating sheathing on moisture levels in residential wood-frame walls has been uncertain. Recently a field study of 86 newly-constructed Washington and Montana homes was completed that provides clear evidence. Measurements were made of the moisture content of wood members within the wall cavities, almost one-third of which had exterior insulating sheathing. Unacceptably high moisture levels were measured in the walls of a large percentage of the test homes. However, the use of exterior insulating sheathing was found to greatly reduce wall moisture levels. Furthermore, the higher the R-value of the sheathing, the lower were the wall moisture levels. Those walls were found to be drier because the insulating sheathing keeps the wall cavity wood members warmer and also because the sheathing is an excellent exterior moisture barrier or break that keeps wet siding from transmitting or wicking moisture into the wall cavity better than other exterior moisture barriers. Moisture entry from the outside, called “splashback,” is an extremely important and common wall cavity wetting mechanism that heretofore has not been widely recognized. The results of this study indicate that the use of exterior insulating sheathing is one way of providing additional wall insulation while at the same time reducing the potential for wall moisture problems.
moisture, walls, residential, insulating sheathing, field study, insulation
Professorprivate consulting engineer, Portland State University, Portland, OR