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Moisture in a concrete-block crawlspace

Q: We are putting an addition on our Wisconsin home and elected to go with a concrete-block crawlspace. The floor joists rest on open block atop a layer of foam sill sealer. I capped off the first floor and tarped it for the winter, intending to resume construction in the spring. A week after capping off, I noticed condensation building up underneath the subfloor, especially around the perimeter of the addition. Should I let air into the crawlspace? Do you have any suggestions?

Richard Richter, Mequon, WI

A: Fred Lugano, a weatherization expert from Vermont, replies: It’s important to attack any sign of excessive moisture at the source. Flushing the crawlspace with cold air will simply freeze condensation in place.

You can get an enormous amount of water vapor flowing out of open block cores. There are also unlimited amounts of ground moisture ready to evaporate from the crawlspace floor. So first cover the floor with a moisture barrier such as heavy-duty plastic, and then plug those block cores.

You can carefully cut and seal pieces of expanded polystyrene foam over the masonry between each joist. Or you can stuff plastic bags down each core. Either method should suppress evaporation inside the concrete block, and the condensation should stop.

From Fine Homebuilding 132, pp. 26 July 1, 2000