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Q.If I use foam insulation board on the exterior of a wood-framed building, will it cause condensation within the walls?

A.Joseph Lstiburek responds: The biggest factor causing moisture problems in walls is excessive indoor humidity. If your building operates at between 20% and 30% relative humidity (a comfortable level for most people), the walls should be fine with any kind of sheathing.

If indoor humidity rises above 40%, moisture in wall assemblies isn’t your only worry. Condensation on windows will probably cause trouble as well, and the mold that starts growing on cold spots can become a health problem for the home’s occupants. So your first strategy should always be to reduce indoor humidity.

To be on the safe side, though, it’s best to allow for interior humidity approaching 40% or even higher. In that case, exterior insulating foam helps by keeping temperatures within the wall assembly above the dewpoint — and the higher the R-value, the better. For moderate indoor humidities, an R-5 to R-7 layer of foam should be okay, but if you’re looking at a hot tub room or similar situation, go for R-10 or higher.

To prevent air from forcing its way between sheets of foam, tape the seams with a gap-sealing builder’s tape. These tapes hold well to foil-faced foams or extruded polystyrene, but we haven’t found a tape that sticks well to expanded polystyrene (beadboard). If you don’t tape the seams, cover the building with housewrap.

In any case, a layer of poly or some vapor-retarding paint on the side of the wall facing the occupied room is a good idea. You should also take care to seal electrical outlets and other penetrations.

Building consultant Joseph Lstiburek is the author of the Department of Energy’s Moisture Control Handbook.