black mold in an attic
Photo: Shawn Tyree/EyeEm/Getty Images
Q: This winter, up in the attic, I found black mold covering the plywood on the underside of the roof. What's causing this? And how do I clean the mold off and keep it from coming back?
Adam Minorczyk, Harwood Heights, Ill.

A: We get that question a lot, especially near the end of winter. That's because all season long, warm moist air has been leaking from the house and into the attic, where it has condensed on—and been absorbed by—the cold underside of the plywood roof sheathing. As temperatures warm up and the icy plywood thaws out, the conditions are perfect for mold to grow on the wet wood.

As the weather gets warmer and the sheathing dries out, that mold will go dormant until conditions allow it to start growing again. Dormant or not, black mold isn't something you want in your house, particularly if anyone has allergies, asthma, or an impaired immune system.

See a demonstration of one pro-level removal technique—blasting the mold away with dry-ice pellets. After the blasting, the exposed wood in that attic was coated with Fiberlock IAQ 6100, a clear, mold-resistant coating.

DIY-friendly spray-on cleaners are also effective for killing mold, as long as they contain 7 to 8 percent hydrogen peroxide. A solution with a lower percentage won't do the job; a higher percentage would require a trained pro. The day after spraying the cleaner, sweep up the residue with a HEPA filter vacuum, then apply a mold-resistant coating.

But whether you get rid of the mold yourself or hire a pro, you can't depend on a coating to keep it at bay indefinitely. The only way to stop mold from coming back is to deprive it of moisture. You do that in two ways: by plugging all the holes that allow air to leak from the house into the attic and by keeping the attic well ventilated, ideally with a vent along the roof ridge and continuous vents at the soffits.

— This Old House general contractor Tom Silva

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