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 foam
sheathing had moisture accumulation than similar walls
sheathed with plywood. Evidently, the high R-value foam reduces
the condensation potential by keeping the stud cavity warm.
With R-11 batts in the wall, use a minimum of 1 inch of foam in
cold climates; two inches is safer. Combined with proper air
and vapor barriers, foam sheathing actually appears to be
effective in controlling condensation.
Wood siding over foam is another issue. There have been
plenty of reported failures of wood siding installed over foam,
but not much agreement over whether the cause was the
non-absorbent foam or poor quality siding. The answer may be
With vertical-grained, all heartwood siding that is properly
preserved and nailed, the sheathing type probably makes no
difference. The siding should have little tendency to deform
and the nails should resist minor warping force.
But with poor quality, flat-grained siding that is poorly
preserved and installed, the situation is different. The siding
may have a greater tendency to warp, particularly if up against
non-absorbent foam sheathing.
The "official" solution to this issue is to select "good
quality" siding (if you can get it) and install it properly.
But the unofficial and probably safest recommendation is to
install 1x3-inch vertical nailers over the foam at each stud.
Not only does this create a 3/4-inch cavity to allow backside
drying of the siding, but it also provides a much better nail
J. D. Ned Nisson is editor
of Energy Design Update
of Arlington, Mass., a monthly
technical newsletter on energy-efficient building design and