- Q. How can the floor over a 4- to 5-inch crawlspace be insulated?
A. There is no good way to insulate such a space. If there is no vapor retarder over the soil surface, it should not be insulated. Rather, it should be vented, at least during the summer. If there is a vapor barrier over the soil, the best solution is to insulate the walls of the crawlspace, rather than the floor. When this is an existing structure with no access around the perimeter, it is necessary to insulate the foundation from the outside. The backfill should be dug out to expose at least a foot of the foundation wall below grade, and extruded polystyrene insulation attached to the outside of the foundation, extending from the bottom of the trench to the bottom of the siding. The exposed foam should be protected with a trowelled-on coating, FDN-grade plywood, or some other suitable protective material.