During the course of home inspections, we come across dark and dirty spaces under many homes. These spaces are known as crawl spaces. Probably so named because they are often so short that crawling on your hands and knees is all you can do there. These spaces are sold abroad 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 in the foundation. These openings allow air to pass through so that moisture can be removed by air movement.
This ventilation 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 from county to county. Required or not at the time of construction does not mean that it is absolutely the best thing to do for the home.
This is what can happen in a ventilated space under a house. During the hot and humid months, the outside air is warm and therefore can hold more water vapor. Once moist air enters the crawlspace, 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 hot enough to “carry” as much water vapor. When this temperature is reached, condensation begins to form and will settle on the joists, pipes, and ductwork. This now high humidity environment is a conducive condition for mold, mildew and decay of the home’s structural elements.
Wood normally needs a moisture content of twenty percent to be at risk of deterioration. The formation of condensation can easily bring the wood to that level of humidity and higher.
So what should one do? All water ingress into the crawl space must first be stopped. The exterior of the house should slope to direct liquid water away from the house. 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 most good in keeping crawl spaces dry. Even if these things have been done, moisture can still enter the crawl space. Moisture can enter the basement and the house as moisture vapor by passing through the ground under the house. To reduce this, it is important to have a vapor barrier in the ground that traps the vapor below it before it can gain access to the structure of the house. The vapor barrier is usually a simple sheet of thick plastic that is laid on the ground. Ideally it is sealed at the edges and the plastic sections overlap and are also sealed.
When all outside water is prevented from entering the crawl space and moisture vapor is prevented from entering, it should not be necessary to vent the crawl space to the outside. It is still important to periodically inspect these areas in case the water somehow leaks or the pipes have a leak. With all the tight spaces, regular inspection is important.