Insulating beneath a slab
Standard preparation underneath a residential slab these days seems to include the use of rigid insulation. But I’ve heard that with the perimeter wall insulated to below the frost line, the temperature of the earth below the slab is about 50° F, so how important is it to insulate against such mild temperatures?
Brent Harold, Wellfleet, MA
William A. Randall, an architect in Eugene, Oregon, replies: You are right that the earth below the frost line does maintain a relatively constant 50°. The primary goal of insulating floor slabs is to prevent the escape of heat at the perimeter, not at the center of the slab. Generally, heat loss through the center of the slab will merely warm the earth slightly and thereby reduce heat loss until equilibrium is reached. The net effect is virtually no heat loss through the center of the slab.
Most codes allow the option of insulating the perimeter of the slab to below frost line (i.e., insulating the foundation wall to the bottom of the footing) or returning insulation under the slab a specified distance (usually 24 in.). I generally choose the former to allow a sound base under the slab for bearing. This effectively prevents heat loss from the slab perimeter and maintains energy-conscious design.