Crawl Space Ventilation

Great strides have been made in recent years regarding the science of crawlspace design dealing with humidity control, durability and pest control. Effective crawlspace design takes into account protecting building materials and extending the life of your home. These spaces kept dry offer the best protection against pest and termites.

Vented crawlspaces are not recommended unless the home is located in a high flood risk zone. Crawlspaces should be sealed and enclosed like any other conditioned space in the home-heated in the winter and cooled in the summer. They should be insulated around the perimeter and not between the crawlspace and the main floor. Good drainage must be in place, and they should not be used for storage unless a concrete slab and conditioning system is in place, and of course, the area must be accessible.

While crawlspace venting has been viewed as good building practice and is still required by some codes, an unvented crawlspace with perimeter insulation provides better moisture control, longer building materials life and improved pest control.

Perimeter insulation rather than floor insulation performs better in all climates. The crawlspace environment becomes an extension of the house, keeping the crawlspace warmer and drier than that of crawlspaces insulated between the crawlspace and the house. Cold surfaces that condense water are minimized.

In addition to no vents, conditioned air and perimeter insulation, these spaces should have a continuous sealed ground cover, like taped polyethylene. Perimeter drainage away from the crawlspace should be in place.

Think of it this way. High humid air enters through the vents in the summer over the cooler ground cover increasing the relative humidity of the space, and condensation forms on cooler surfaces. Summertime ventilation in crawlspaces normally makes them wetter, not drier. Conversely, wintertime ventilation makes them colder and does little to dry them out. Wintertime ventilation also increase home heat loss and can lead to freezing pipes and uncomfortable floors.

The International Building code (ICC) allows the construction of closed (unvented, but conditioned) crawlspaces. You should contact your state's local code officials in the home design phase to determine their requirements.

If you're building your home in a flood zone or along coastal areas-or in a designated "dry climate"-the crawlspace should be designed as an isolated space from the house. In other words, this area is outside the building boundary envelope. Air seal the boundary between the crawlspace and the house by installing a vapor barrier on the underside of the floor. R-Value should be sufficient for summer and winter temperatures-a combination batt and fire-rated rigid insulation, taped and sealed at joints is recommended.

If possible, seal existing crawlspace vents and build new homes without crawlspace vents. If a crawlspace butts with a basement, treat both areas as a conditioned space.