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Published on December 24th, 2013 | by Henri


Covering attic vents causes mold

Q.    We have a two-story frame house about 100 years old. I went into the attic, which is just used for storage, and saw white powder on the rafters. I assumed it was some sort of mold.

I always cover the vents in the wintertime with plastic, but this is the first time I’ve seen this substance. The deposits are not pervasive; they are spotty — only on the north side of the peaked roof. My insurance adjuster says it’s from moisture, and made a suggestion about covering the whole-house fan in the attic and leave the vents open.

Once again, this is the first time I’ve seen this in more than 50 years in the house. I decided to do the clean-up myself, so I mixed a half and half solution of bleach and water. I used a mask and rubber gloves in the process, using a stiff brush to apply the solution. During the application, I noticed a certain amount of foaming activity. There is a lot of residue in the bucket, and for the most part, the wood is clean, with no white residue. I’m going to wait a few days to see if the powder re-appears.

What should I do next? Should I continue the process? How often should I replenish the bleach/water solution? Is my bleach/water solution correct? Is there a sealer of some sort that I can use to prevent recurrence? Should I leave the vents open as the adjuster suggested?

A.    By covering the attic vents but not covering the whole-house fan (I assume that it is a large fan in the floor of the attic, used to cool the house in summer by exhausting inside air through the attic), you have trapped the warm, moist air convecting from the living quarters into the attic through the attic fan’s housing.

Your insurance adjuster is correct to suggest covering the fan’s housing — and do so tightly — and leaving the attic’s vents open. You did use the correct solution, and it may not be necessary to treat the wood again if you have eliminated the cause of the problem.

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