If mold forms on the ceiling under a duct or register and there’s no sign of a roof leak, badly insulated ductwork may be the cause. Warm, moist air condenses and forms water on ducts carrying cold air through the attic or crawl space. The condensation is a sign that the duct is uninsulated or missing a vapor barrier. Eventually the water saturates the insulation and drywall and mold spores (which are everywhere) take root. In cold weather, the reverse happens. Moisture forms anywhere warm air escapes—for instance, at unsealed joints between duct sections. Inspect your ductwork as a test for mold

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