A question about crawl space foundation vents

Q. My house has four crawl space foundation vents that have flaps that can be opened or closed. The furnace people said to keep the vents closed all the time to prevent condensation from forming on the metal ducts in the crawl. The exterminators who do annual inspections say the vents should be open in the summer to keep moisture out of the crawl. There is plastic covering the ground in the crawl space. They say the area is dry. What is the right thing to do?

A. In the mild weather of the Midwest, the vents should be open in the winter and closed in the summer.

When the outside air is warm and moist, the vents are left open so the moisture will migrate to the cooler crawl space and condense on the cold surface of the ductwork.

When the outside air is cold and dry, the crawl vents should be open to allow the warmer, moisture-laden air from the crawl space to migrate to the outside, helping to lower the humidity of the crawl space.

If you want to leave the vents closed year-round and the area is dry, leave the vents closed. In colder climates, the vents should be open and closed, depending on the outside temperature, to protect water-supply pipes in the crawl space from freezing.

In warmer climates, vents should be left closed or eliminated. Most of the newly constructed homes I see do not have foundation vents. The crawl space is sealed, insulated and an opening on the ductwork conditions the crawl.

When a crawl space is properly conditioned and drained, the area is dry. When the crawl space is dry, the insect and other pest population is minimal.

Ÿ Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home improvement questions at d.Barnett@insightbb.com.

Scripps Howard News Service