Home & Garden

Home Fix: Newer homes with crawl space foundation have vent opening on ductwork

Q: I read your suggestions for replacing the homes ducts when replacing the furnace. I have a newer home and decided to check the ductwork in the crawl space. I found a small opening on the ductwork that the builder told me was to keep the crawl space dry. Is this true?

A: Newer homes with a crawl space foundation are being constructed without the familiar foundation wall vents to provide ventilation. Now the crawl areas are being conditioned by sealing and insulating the crawl space perimeter walls and rim joists and adding a vent opening on the ductwork to condition the crawl area.

This will heat and cool the crawl space and help dehumidify the area. The environment in the crawl should be similar to the environment inside the home. How does this help? Dehumidification helps in the prevention of mold and decay. A dryer environment will also discourage pest infestation except subterranean termites.

The only problems I have found so far are the size and the placement of the vent opening. First is the size of the vent opening – in most cases the vent is no more than a 4-inch diameter opening with a clothes dryer vent cover. Is this the proper size for the area to be conditioned and has the installer performed the math required to determine the size of the opening? Probably not.

Second, where is the opening located? I found one on the top of the ducts that was blowing air up and against the wood flooring of a new home. The wood above was condensing from the direct air supply and that area of the flooring was starting to buckle. I have also found openings on the end of the main supply duct, which will lower the pressures inside the ducts. Low duct pressure can result in comfort issues in one or more rooms that are farther from the furnace.

If you think you are having comfort issues or if you notice higher humidity and sweating metal ducts in the crawl space, call a licensed and professional heating contractor for a duct inspection. To maintain a proper airflow make sure the furnace filter is clean and is changed on a regular schedule determined by the heating contractor.

C. Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Reach him at d.Barnett@insightbb.com.

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