To Close or Not to Close Home's Foundation Vents?

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By Paul Bianchina

Q: Several people have said to never close our foundation vents, including the home inspector, the Realtor, and even the gardener! However, I notice in previous columns that you recommend closing them during the winter. Can you shed some light on this for me? -- Dawn G.

A: The foundation vents are designed to provide a cross flow of air under the house that assists with the removal of any moisture accumulation in the crawl space before it can build up and cause mold, mildew, or damage to structural components. On the downside, the vents also allow freezing air to pass through an area of the home that typically contains a number of plumbing pipes, which increases the risk of having a pipe freeze. These frigid breezes can also aggravate air leaks into the house, causing uncomfortable drafts and robbing the home of expensive heated air.

During the coldest part of the winter, humidity levels are at their lowest, so the risk of moisture-related damage in most areas is minimal. On the other hand, the risk of damage from frozen pipes is at its highest at this time. For that reason, I always recommend to people that their foundation vents be closed or blocked whenever freezing temperatures are a possibility, and that they remain open the rest of the year.

How long you leave the vents open or covered each year is dependant on where you live. In Hawaii, for example, where moisture levels are very high and the chance of freezing is very low, vents can typically stay open all year. In other regions where the opposite conditions exist -- low moisture and very low winter temperatures -- the vents may need to remain covered for several months. If you have a severe moisture buildup in your crawlspace during the short period that the vents are closed, then you probably have a moisture problem in the home that is due to poorly graded soil, plumbing leaks, unvented fans, or some other source not related to ambient air-moisture conditions.

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