During the course of home inspections, we found dark and dirty spaces under many houses. These spaces are known as crawl spaces. It’s probably called that because they are often so short that all you can do there is crawl on your hands and knees. These spaces are ventilated to the outside and sometimes not. This article will discuss the conditions in which it is recommended to ventilate or not to ventilate.

These spaces are often vented to the outside with the use of small vents at various points on the base. These openings allow air to pass through to remove moisture with air movement.

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

This is what can happen in a ventilated space under a house. During hot and humid months, the outside air is warm and can therefore retain more water vapor. Once that moist air enters the crawl space, the air temperature drops and that air temperature sometimes reaches the dew point. The dew point is the temperature at which the air is no longer warm enough to “handle” so much water vapor. When this temperature is reached, condensation begins to form and settles on beams, pipes, and ducts. This now high humidity environment is a condition conducive to mold, mildew and rotting of the structural elements of the house.

Wood generally needs a moisture content of twenty percent to be at risk of deterioration. The formation of condensation can easily cause the wood to reach that level of humidity and more.

So what should one do? First, any entry of water into the crawl space must be stopped. The exterior of the home should be sloped to divert liquid water from the home. Gutters and downspouts should be kept clean and have extensions that carry water as far away from the house as possible. These two things will do the best to keep crawl spaces dry. Even if these things have been done, there may still be moisture entering the access space. Moisture can enter the crawl space and home as moisture vapor as it passes through the ground under the home. To reduce this, it is important to have a vapor barrier in the ground that traps the vapor underneath before it can access the structure of the house. The vapor barrier is usually a simple sheet of thick plastic that is placed on the ground. Ideally, it is edge sealed and the plastic sections overlap and seal as well.

When all outside water is prevented from entering the access space and moisture vapor entry is prevented, it should not be necessary to ventilate an access space to the outside. It’s still important to periodically inspect these areas in case water leaks in some way or your plumbing leaks. Regular inspection is important in all underground access spaces.