Crawl space

Published on October 2nd, 2013 | by Henri

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Water condenses in crawl space

Q.    I have a split-level with a crawl space under the main floor of the house. The crawl space has a cement floor, with rigid pink insulation foam board covering the cinderblock walls.

There is R-19 in the rafters with the paper facing the floor of the room above the crawl space and three 6-inch x 9-inch screened openings to the outside for venting. Every year the rafters and the insulation get so damp that water droplets form on them. Water actually drips off of the gas pipes, water lines and insulation in the crawl space.

I put a large fan in the crawl space and after a few days it seems to dry everything out. Why does this happen and what can I do to prevent it from happening again?

A.    The outdoor, humid air of summer gets into your crawl space that is kept cool by dint of its being in contact with deeper, cooler soil. Some building codes still erroneously specify crawl space ventilation, although it is well-documented that in most climates — and particularly in warmer, more humid areas such as the southeast — this ventilation has been responsible for major structural-rot problems.

Newer science recommends not ventilating crawl spaces as long as the floor is protected from soil moisture by a sheet of 6-mil plastic or a concrete slab. Since your crawl space has a concrete floor, there is no need for additional measures. Close all vents and see if it solves the problem. You may need to put a dehumidifier in the crawl space until the moisture level is greatly reduced.

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