Q: I have a client with an older home on a slab with an uninsulated foundation. In our northern climate, the perimeter of the floor gets very cold in winter. Would insulating the foundation from the outside make a significant difference in how cold the slab gets?
A: Steve Baczek, a residential architect from Reading, Mass., who specializes in building science, responds: Because the edge of the slab links directly to the cold outside air (via the uninsulated foundation), the surface temperatures of the floor and wall materials at the perimeter of the house will be cold as a result. Preventing warmth from escaping along the edge of the slab would greatly improve the floor temperature at the perimeter of the house, and applying a layer of insulation to the outside of the foundation is an excellent way to do this.
How much insulation? The more the better. I'd recommend 2-inch XPS (extruded polystyrene) rigid foam, which has an R-value of 10. But depending on the details of your client's home, the outside plane of the foam board may stick out past the siding, creating an aesthetic problem. And finding a visually acceptable protective covering for the rigid insulation might also be a challenge. Though 1-inch board would not give you as high an R-value, it would still provide a thermal break, and it may be easier to blend into the home's appearance with a protective covering.
As far as depth is concerned, the above-grade portion of the slab and foundation has the largest temperature difference between the inside and outside, so that area benefits the most from being insulated. Below grade, the difference in temperature diminishes as you go deeper into the ground. Cover the entire exposed area of the foundation and extend the insulation at least 18 inches into the ground.
If you apply insulation to the outside of a foundation, please note that rigid insulation and its protective covering can provide concealed access for insect infestation. Cover the top of the insulation board with a material such as metal flashing and seal that against the foundation to create an impenetrable barrier.