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Q.I am building an addition over a 30-inch-high crawlspace. The local inspector has approved my plan to build a sealed crawlspace without ventilation. Should I install rigid foam insulation on the interior of the block walls, or should I insulate between the floor joists?

A.Howard Ferree, a licensed general contractor in the Asheville, N.C., area, responds: In almost all cases, I’d suggest installing the insulation between the floor joists. Unless you are located in a climate where the crawlspace needs to be heated to keep the pipes from freezing, you don’t want to heat and cool more space than necessary.

Although there is still some controversy about the best way to prevent crawlspace moisture problems, I’m now convinced that the most effective approach is to seal crawlspaces completely. Polyethylene should be installed across the crawlspace floor and should continue up the interior of the perimeter foundation walls, to form as complete a seal as possible.