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Should Crawl-Space Vents be Open or Closed?

Should we block our crawl-space vents or keep them open? This Old House’s Tom Silva answers.

Tom Silva wearing Silva Brothers Construction Shirt

Should we block our crawl-space vents or keep them open?

Tom Silva replies: I think working vents in crawl spaces are a good idea, and so do the building codes, which generally require them. These vents allow outside air to circulate under the floor in summer to prevent the moisture buildup that encourages mildew and rot. In winter, when the air is drier, the vents are closed to reduce the chance that the pipes in the crawl space might freeze.

How to Close Foundation Vents in the Winter

The simplest way to close foundation vents for the winter is to plug them from the outside with foam blocks made specifically for this purpose. Just remember to remove the plugs when the weather turns mild in the spring.

As you do that chore, check to make sure that the vent screens are intact so that insects and other critters don’t make a home under your house. Automatic vents are less fuss. Air Vent makes ones that open at about 70 degrees F, close about 40, and don’t require any electricity.

You do not say whether your crawl- space floor is made of dirt or concrete, but if it is dirt, spread 6-millimeter plastic sheeting over it to help block that source of moisture.

Are Foundation Vents Necessary?

Before there were building scientists and houses were just built by carpenters and contractors (with few if any outsiders telling them how to do their jobs), 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.

However, 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 crawl spaces should be sealed, insulated, and installed with a vapor barrier; 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, many foundation vents continue to be installed. Some building departments are also slow to change and still require foundation vents as part of their building code.

Advanced Energy is a highly respected North Carolina think tank of building scientists and researchers who want to help you build homes that last longer and use less energy. It’s hard to argue with that goal, isn’t it? In December 2009, they released an exhaustive report about crawl space ventilation as it pertains to a variety of different climates across the country.

Researchers monitored two sets of new homes in different climates: a 15-home modular housing development in hot, humid Baton Rouge, La.; and 12 stick-framed homes in cold, dry Flagstaff, Ariz. The houses used a variety of heating systems and ductwork configurations and different locations for their thermal barrier.

The report concluded that closed, unvented crawl spaces stay considerably drier than vented ones, regardless of the climates. On the other hand, some of the energy-use results were surprising. I don’t want to spoil the surprise here, but you can read a summary and the full report here: www.crawlspaces.org