We build crawlspaces because they are cheaper to build than basements, or because ground conditions make building a basement impractical. A crawlspace can also provide service access for plumbing, electrical, and heating-and-cooling systems.
In the past, Building America and others have done a lot of research around the country on crawlspaces. By looking at this research, we can learn a lot about which crawlspace configuration works where.
Historically, most crawlspaces have been vented to the exterior. In some climates-especially with the introduction of central A/C-this can cause problems. In climates with extended periods of hot-humid weather-this includes the Southeast and most of the Northeast and Midwest-closed crawlspaces are the best option. Under hot-humid conditions, warm, moist air enters the vented crawlspace from outside and can condense on the cooler surfaces. These cooler surfaces are created within the crawlspace by the shade provided by the building and the moderating effect of the contact between the crawlspace and the ground. Even when condensation doesn't take place, relative humidity (RH) above 80% for an extended period can support mold growth and eventually rot wooden structural materials. Add A/C to the house, and the floor above the crawlspace becomes even cooler-and any ductwork in the crawlspace creates an added risk of condensation.
In closed conditioned crawlspaces, insulation is normally placed around the perimeter and not in the floor. With perimeter insulation, the house is coupled to the cooler temperatures of the ground, and this can reduce the overall amount of A/C needed to cool the house. When closed conditioned crawls are used in heating-dominated climates, this same ground coupling can actually increase the heating load of a house, but that may cost very little compared to the cost of damage done by moisture brought in by venting.Continue reading on the BPA Journal