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Q.How can I get rid of mildew growing on joists in an insulated crawlspace? The mildew is only on the bottom edges and sides of the joists where the batt insulation doesn’t cover them. I suspect it’s from warm, humid air wafting through the basement in the summer. The soil is basically dry.

A.Paul Fisette responds: Mildew needs a local relative humidity of around 70% to grow. Even though the earth feels dry to the touch, it isn’t. That’s probably where the moisture is coming from. (I’m assuming there are no other obvious sources of moisture, such as a reverse grade outside, or gutter downspouts dumping water against the foundation.)

As a first measure, you should install a polyethylene ground cover throughout the crawlspace. There is quite a bit of research showing that ground cover eliminates up to 75% of the moisture that ends up in the air as humidity in a crawlspace. The mildew itself will probably not cause any great problem in the crawlspace, but this moisture eventually moves up into the walls and living spaces through wiring and plumbing holes, and other bypasses.

Take some humidity readings in the basement before you put down the poly and after, when the basement has had a chance to dry out. If this doesn’t make a big difference, you will need to search harder for the source of the moisture.