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Q.I was hired to refinish a basement game room built in 1936 in a house on the shores of Lake Michigan. The T&G paneling in the room was badly warped and the bottom 3 feet were moldy and partly rotted. I removed the paneling to find that it had been installed directly over the poured foundation on furring strips embedded in the concrete. On hot, humid days the bare concrete sweated so much that water trickled down the wall. I checked during a rainy period to establish that there were no foundation leaks. Then I proceeded to insulate the wall and the floor, installing 2x2 pressure-treated furring to the concrete with 1 1/2-inch rigid foam placed tightly in between. I sprayed foam in all cracks and at wall-to-wall and wall-to-floor corners. I stapled up a 6-mil plastic vapor barrier, then installed drywall. I also installed a high-volume exhaust fan controlled by a humidistat.?
Now, three years later, there’s a new problem: mold growing on the drywall at the base of the walls and in the corners of the room. What should I do? The owners want a 4-foot-tall pony wall with a display shelf on top. I plan to stud-frame this and fill the stud cavities with insulation, hoping that will help. I also wonder if I should have killed the preexisiting mold with bleach before insulating the room. I would appreciate any ideas on how to cure this problem.

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 corners of the room.

It would be better to close all windows and doors to the basement during the hot, muggy summer months and run a dehumidifier with a humidistat. This will remove unwanted moisture from the air. There is no harm in circulating air within the room with a fan, and the owners can still open the windows for natural ventilation on dry, breezy days if they wish. You should kill the mold either by spraying the moldy areas lightly with a mixture of equal parts water and fresh Clorox bleach or by rubbing them gently with a sponge dampened with the same solution.

It would have been better to adhere rigid insulation over the entire concrete wall surface, then install the furring strips, to avoid the thermal short circuits now happening through the wood strips.

For the pony wall, instead of using studs and fiberglass, why not fasten a layer of 2-inch rigid insulation over the existing drywall as a wainscoting, then cover it with drywall or paneling and top it off with a display shelf?