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CREIA Explains How to Avoid Moisture Problems Under Your Home

April 2004 – Many homeowners make the mistake of closing sub-area vents around the foundation of their homes — sometimes for cosmetic reasons or because they erroneously think it will make the house more energy efficient. The California Real Estate Inspection Association warns homeowners and homebuyers to avoid this common error that can have costly consequences.

The purpose of foundation vents is not to adjust the energy efficiency of the building. It is to prevent humidity caused by ground moisture from condensing on the structure. Condensation can severely damage the wood framing and rust the structural hardware. Therefore, open vents should be maintained at all times (especially during the rainy season where there is an increase in ground moisture).

The building code requires that sub-areas be cross-ventilated. Minimum ventilation is defined as one square foot of vent opening for each 150 square feet of floor area. In some cases, this is not sufficient to prevent condensation, and additional vents may be needed. If condensation beneath your home is occurring mainly at the corners, the addition of corner vents would be advisable. If the moisture problem persists, have a plastic membrane installed on the ground surfaces beneath the building. This will prevent evaporation, the source of humidity.

Make sure to have your home inspected to determine if excess moisture below the building is causing symptoms in the living area of the home, moisture damage to wood members could be extensive in the sub-area, especially if there is inadequate ventilation. All of these conditions warrant immediate evaluation. If any flooding has occurred beneath the home, the foundations, sub-floor framing, and other building components should be carefully examined for possible moisture-related damage.

A professional home inspector has an obligation to inspect the crawlspace beneath the dwelling, unless that portion of the home is inaccessible. In that case, lack of access should have been specifically noted in the inspection report, with a recommendation for further evaluation as soon as access could be provided. The California Real Estate Inspection Association (CREIA) includes inspection of the crawlspace as part of their Standards of Practice for all member inspectors.