A.Ned Nisson responds: Exterior rigid foam sheathing is basically a high R-value vapor retarder on the outside of a wood-framed wall. Theoretically, it can cause two types of moisture-related problems: wet walls and wood siding failures.

In nearly 20 years’ experience, however, there have been no documented cases in which exterior foam sheathing was solely responsible for damage due to moisture accumulation within walls — whether in warm or cold climates. In fact, a controlled experiment at the U.S. Forest Products Laboratory, in Madison, Wis., showed that walls insulated with...

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