Q If I’m building in a heating climate on frost walls, with an uninsulated crawlspace and a dirt floor, how deep do I need to place my footings for the posts supporting the center beam?
A Steve Baczek, a residential architect from Reading, Mass., who specializes in building science, replies: Typically the footings for the center support posts are placed at an elevation similar to that of the perimeter frost-wall footings. But this placement is not necessarily because of frost protection; rather, it’s done because the hole for the crawlspace is usually excavated to that depth. If the depth of the frost-wall footings meets the code requirements for frost protection (Section R403.1.4.1 in the IRC), then the footings for the support posts at the center of the crawlspace—being at the same elevation—will meet those requirements by default. Even when the foundation walls are trenched and the crawlspace floor is not excavated to the same depth as the footings, most building officials are likely to require the footings for center supports to be at frost depth as well.
With a vented crawlspace, there is a chance that the crawlspace air could get very cold. But the risk of the ground freezing in the center of the crawlspace is still minimal. In most cases heat loss from the floor above would keep the crawlspace temperature above freezing. Only if you went to great lengths to air seal and insulate the framed floor above—without insulating the crawlspace—would the air in the crawlspace be completely unconditioned and the risk of freezing increase. But insulating a floor, with all the pipes, wiring, and penetrations, is very difficult to do well. It is much easier, and uses less material, to create a sealed crawlspace by insulating the foundation walls and air sealing, so the entire crawlspace is brought inside the conditioned volume of the house. This is the strategy I recommend for most crawlspaces. It also requires covering the dirt floor, and of course, closing any crawlspace vents. By conditioning the air in a crawlspace—whether the heat source is the house above or the ambient temperature of the earth below—the floor of the crawlspace should not be at risk of freezing, and the soil supporting the footings in the center of the crawlspace (at any depth) should not be in danger of freezing.
Regardless of the climate you build in, you should never leave the dirt floor of a crawlspace open and uncovered. At a minimum the dirt floor should be covered with a 6- to 10-mil polyethylene sheeting ballasted in place with stone or covered with a 2-inch to 3-inch concrete “rat” slab.