Chris Laumer-Giddens and his wife Jodi are Atlanta architects specializing in high-performance houses. In this JLC article, he discusses the widespread but mistaken belief that a house in the Southern U.S. climate doesn’t need much insulation under the slab. That belief is supported by the attitude among some builders that building code requirements, which represent the bare minimum, are already too strict.
He says that view is based on misinformation about Atlanta's climate, which is actually not much different from that in Baltimore. So for an energy-efficient house, insulation against the cold is just as important as insulation against the heat.
For the “High-Performance Bungalow,” we isolated the reinforced concrete slab from the earth and from the footing with 2 inches of XPS foam. We turned the foam up at the edge of the slab to break the slab thermally from the footing and the outdoors. In our climate, the ground isn’t cold enough to freeze. Year-round ground temperatures range between 55°F and 65°F. So the heat loss through the slab is never as extreme as it can be through the walls in winter. On the other hand, in a 2,400-square-foot home, the area of contact between the slab and the ground is large, so the total heat loss can add up.