A.Marc Rosenbaum responds: To control mildew growth, we must first look at the relationship between air, water vapor, and relative humidity (RH). Water vapor is water in its gas form, and is present in air. The warmer the air, the more water vapor it can hold; the colder the air, the less water vapor it can hold. RH is a measure of how much water vapor is in the air compared to the maximum amount of water vapor the air can hold at that temperature. As air cools, without changing its water vapor content, its RH goes up. As the air continues to cool, it reaches the point where the water vapor it contains is all that it can hold — this is 100% RH. Cooling the air any further will result in condensation as some of the water vapor changes to liquid.

Mildew can only grow on surfaces where the RH exceeds 70%. Closet surfaces tend to be colder than adjacent rooms, because of poor air circulation from the heated room to the closet, and because, relative to their size, they often have more exterior surface area for heat to escape. A corner or cold wall section lacking proper insulation is...

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