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 below grade vents. When vents are below grade, rain and runoff are likely to enter the crawl space, which can attract wood-destroying organisms or cause fungal problems such as rot or mold.

If the vents are far enough below ground level, dirt can block the openings so air can’t circulate. The vents will not vent! When home inspectors see this condition, if the vents cannot be repaired by removing or leveling the floor, the inspector will likely recommend that the customer install vents. The simplest ventilation shafts consist of frames made of pressure-treated lumber. However, metal or plastic ventilation shafts, made especially for that purpose, can be purchased at building supply stores. The pit is dug into the ground, in front of the vent and deeper than the vent, so there is an opening in front of and below the vent. I personally like to see the pit dug a few inches deeper than the vent, then gravel filled space to about 2″ below the vent.

Another problem is that people often block the vents in an unconditioned crawl space. Although this is sometimes warranted in the winter, when there is danger to pipes due to extreme temperatures, in Washington State, the Washington State Department of Agriculture defines blocked vents in an unconditioned crawl space as they are conducive to attracting organisms that destroy wood. In most cases, a state licensed structural pest inspector must locate blocked vents as a conductive condition. In the Pacific Northwest, if pipes are wrapped, foundation vents can be left open year-round in most years. This basic information may not apply in severe climates or partially heated or “conditioned” crawl spaces.