To Vent Or Not To Vent? That Is A Crawl Space Question

During the course of doing home inspections we come across dark and dirty spaces underneath many homes. These spaces are known as crawl spaces. Likely called this because they are often so short that crawling on ones hands and knees are all you can do in there. These spaces are vented to the exterior and sometimes not. This article will discuss conditions in which venting or not venting is advised.

These spaces are often vented to the exterior with the use of small screened vents at various points on the foundation. These openings allow air to pass through so that any dampness can be removed with the air movement.

This venting is often required by the local authority in charge of building codes. Home inspectors are not code inspectors. Codes change and vary from city to city and between the various counties. Required or not at time of building does not mean that it is absolutely the best thing to do for the home.

Here is what can happen in a vented space under a home. During the hot and humid months the exterior air is warm and therefore able to hold more water vapor. Once that humid air enters the crawl space the air temperature falls and that air temperature at times reaches the dew point. The dew point is the temperature where the air is not longer warm enough to “support” as much water vapor. When this temperature is reached condensation begins to form and will settle on the joists, pipes and the ductwork. This now high moisture level environment is a conducive condition for molds, fungus, and rotting of structural elements of the home.

Wood is typically needs a moisture content of twenty percent to be at risk of deterioration. The formation of condensation easily can get the wood to that level of moisture and higher.

So what should one do? First all water entry in the crawl space must be stopped. The exterior of the home should slope so as to direct liquid water away from the home. Gutters and downspouts should be kept clean and have extensions that take the water as far as reasonable away from the home. These two things will do the most good in keeping crawl spaces dry. Even if these things have been done there can still be moisture entry in to the crawl space. Moisture can enter the crawl space and home as moisture vapor by passing through the soil under the home. To reduce this it is important to have a vapor barrier on the soil trapping the vapor under it before it can get access to the home structure. The vapor barrier is typically a simple sheet of thick plastic that lays on the soil. Ideally it is sealed at the edges and the sections of plastic overlap and are sealed as well.

When all exterior water is kept from entering the crawl space and moisture vapor is prevented from entering there should no need to vent a crawl space to the exterior. It is still important to periodically inspect these areas in the event that water does somehow sneak in or your plumbing gets a leak. With all crawl spaces regular inspection is important.