October 4, 2023

Home Architecture

The Extraordinary Design

Crawl Space Ventilation – Vents Below Grade

2 min read

With an unconditioned crawl space (usually dirt covered with a plastic vapor barrier and open foundation vents) a home inspector does not want to see the vents below the level of the soil. When vents are below grade, rain and runoff water are likely to enter the crawl space — which can attract wood destroying organisms or lead to fungal issues such as rot or mold.

If vents are far enough below grade, the dirt can block the openings so air will not circulate. The vents will not ventilate! When home inspectors see this condition, if vents cannot be fixed by removing or grading soil, the inspector will probably recommend that the client put in vent wells. The most simple vent wells consist of frames made from pressure-treated lumber. However, metal or plastic vent wells, specially made for the purpose, can be purchased at building supply stores. The well is dug into the soil, in front of the vent and deeper than the vent, so there is an opening in front of and below the vent. Personally, I like to see the well dug a few inches deeper than the vent, then the space filled with pea gravel up until about 2″ below the vent.

Another issue is that often people block the vents in an unconditioned crawl space. Although this is sometimes justified in winter, when there is a danger to pipes from severe temperatures, in Washington State blocked vents in an unconditioned crawl space are defined by the Washington State Department of Agriculture as conducive to attracting wood destroying organisms. A state licensed structural pest inspector should, in most cases, site blocked vents as a conducive condition. In the Pacific Northwest, if pipes are wrapped, foundation vents can be left open year-round most years. This basic information might not apply in severe climates or in partially heated or “conditioned” crawl spaces.

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