A.Henri de Marne, a Waitsfield, Vt., consultant on wood-frame construction problems, responds: The reason that the new mold is at the base of the walls and in the corners is because these are the coldest areas and because the air is stagnant in spite of the fan you installed. There is basically nothing wrong with what you did, and not killing the mold on the masonry walls is not what caused the new mold on the drywall; the mold on the concrete could not grow through the fiberglass insulation and the plastic vapor retarder.

However, I think it was a mistake to install a fan. This obviously requires an air inlet (generally on an opposite wall) and both are most likely in window openings high up on the wall. This has the effect of bringing in a fresh supply of warm, moist air without creating air movement where you need it most — at the base of the walls and in the...

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