Crawl space

Published on April 29th, 2018 | by Henri

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Crawlspace vents

Q. I just recently had cement poured in my crawlspace. There are four vents to the outside. There are also heating and air conditioning pipes there. I am confused now as to whether I am supposed to keep the vents permanently closed or if I should open them again when the warm weather comes. Thank you.

A. If the crawlspace soil moisture is thoroughly controlled, be it by covering the soil with plastic or an effective concrete slab, there is no need for any ventilation as long as the crawlspace air smells “sweet,” i.e., it does not smell damp or musty.

To determine if your new concrete slab is effective at controlling the soil’s moisture, tape all sides of a 2-foot by 2-foot piece of plastic on the slab. If no moisture collects under the plastic, it should be safe to close all vents year around.

But I suggest you wait a full year before making the test, as new concrete will take time to cure and stop generating its own moisture in such a confined space.

The effectiveness of the concrete at controlling the soil’s moisture also depends on how it was poured. If directly on the soil, concrete being porous, it may not thoroughly control it. But if it was poured over XPS rigid insulation or a stone bed covered with plastic, it should be fine. Meanwhile, keep the vents open to help dissipate the moisture emanating from the concrete.

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