Should Old Home Foundation Vents Be Opened or Closed?

How to Recognize a Foundation Vent

• On older homes, foundation vents are typically located 12 to 18 inches above ground level on the concrete or masonry foundation walls. The traditional vent is constructed of galvanized metal and features a sliding louver that allows it to be opened or closed seasonally. A foundation vent for a masonry porch might be as small as a postcard, but a larger home may have foundation vents that measure up to 9 inches wide.

Open Foundation Vents in the Summer

• During summer, the airflow in foundation spaces can be crucial. Moisture can come up from the soil and vapor can also permeate through mortar cracks or the porosity of the masonry material itself. In the short term, that moisture can impart a musty aroma to the home. Humidity allowed to build up on a home's rim board and floor joists can lead to mold and rot, which will damage a home's structural stability and may damage wood flooring as well. Most homeowners will find that opening the foundation vent in summer months allows humidity to escape and reduces these risks.

Close Foundation Vents in the Winter

• When the leaves change colors, it's time to close the foundation vents. Allowing an unrestricted flow of cold air into under-floor spaces will chill the floor above and require the furnace to work harder, leading to higher utility bills. In addition, an open foundation vent may expose plumbing to subfreezing temperatures, which could lead to burst pipes and water damage.

A Different Perspective on Foundation Vents

• Many experts from the field of building science take a different slant on foundation vents and point to some potential drawbacks of venting the space beneath a home's floors to the outdoors. In climates beset by long stretches of high temperatures and high humidity, vapor pressure is likely to force humid air into under-floor areas when foundation vents are open. Individuals who live in those climate areas may want to consult a professional building inspector to determine if it's wise to keep the foundation vents of their home closed year-round.