Should Foundation Vents Be Covered to Control Humidity?

Foundation Vent Systems

• In older homes, foundation vents were installed as permanent grates that could be opened or closed for moisture control. Many modern homes are equipped with a powered, automated ventilation system that uses two-way fans to move the air in and out of inaccessible under-floor cavities. The newer ventilation systems utilize a larger vent louver to accommodate the incresed air transfer. Fitted covers are available in a variety of sizes to seal and insulate the ventilation grate.

Why Foundation Vents Are Opened During the Summer

• In many regions, building codes have long required new homes be constructed with foundation vents. In older homes, the vents provide a passive exchange of air to inaccessible cavities beneath a home's floor joists. Typically, the floor of these cavities is hard soil rather than concrete, so moisture can seep up from the ground during summer months and cause wooden joists and subfloors to become mildewed and eventually rot. The vents or ventilation fans are designed to allow that moisture to escape the cavity.

Why Foundation Vents Should Be Closed During the Summer

• The theories that prompted homeowners to open foundation vents in the summertime may be out of date, because they predate the era of central air conditioning. In a home where floor temperatures are 15 to 20 degrees cooler than the damp air stagnating in a foundation space, outdoor humidity can condense to the joists, creating a perfect environment for the growth of mold -- and eventually, wood rot. By closing the foundation vents, that risk is minimized. If the foundation cavity has a dirt floor, it should be covered with a vapor barrier to eliminate ground-sourced moisture.

Covering Foundation Vents

• Homeowners who elect to close their foundation vents can take an additional step and cover the vent gratings with fitted covers. The cover prevents the free flow of air and moisture while helping to stabilize the temperature within the cavity. As an additional benefit, the cover may provide some acoustic insulation. Covers are available in a variety of standard sizes and colors through building supply outlets, or you can cut a cover to size from a block of extruded polystyrene foam using an ordinary utility knife.