Crawlspace ventilation

The Great Debate…. Crawl Spaces, To Vent or Not to Vent!

Photo Credit: Basement Systems


During the 20th century, before there were building scientists. Homes were built by carpenters and contractors and a quality construction job included foundation vents into the crawlspace. The thinking was that the vents allowed air to circulate under the house, keeping it dry.

In fact, what happens in many locations is that moist outside air coming through the vents condensed or collected, causing corrosion, mold, mildew and rot.
More recent thinking from building scientists dictated that crawlspace should be sealed and insulated; they should be treated as partially conditioned space to control moisture and decrease energy use.

A debate ensued, and because some builders are slow to change. (if you are still using a flip phone, you probably are one of these builders) many foundation vents continue to be installed. Some building departments are also slow to change and still require foundation vents.”What do you mean, take out the vents? We have to have venting – it’s the only way the moisture has to escape!” This is typically what we hear when we tell a fellow contractor about getting rid of crawl space vents. Yet a ton of new research indicates that rather than removing crawl space moisture, venting makes the problem worse. Building scientists have found consistently that when warm, moist outside air enters a crawl space, it instantly cools and drastically increases the relative humidity of the crawl space. When the relative humidity goes over 100 percent, the moisture is released into the crawl space atmosphere, with condensation accumulating on the walls, floors and building components.

Should you eliminate vents from your additions and new construction? It depends on what codes you must follow and what exceptions they include. One word of caution here – if the crawl space houses an appliance such as a furnace or hot water heater, the appliance in most cases needs make-up air for combustion. Consult your HVAC professional.

There is just enough information here to get the juices flowing (no pun intended). There are plenty of articles on crawlspace science on the web.

Kevin Michaels is owner of Hudson Valley Property Inspections, llc, and a licensed NYS Home Inspector & Mold Assessor based in the Ulster County town of Saugerties, NY.

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