For example, adding 1 inch of expanded polystyrene (E.P.S.) foam to the exterior of a standard concrete basement wall can raise the wall’s resistance to heat flow from R-2 (about the same as insulated glass) to almost R-7. Adding another inch of foam brings the R-value to R-12, similar to an insulated 2x4 frame wall.
   If your builder uses any type of exterior foam insulation, make sure he covers all above grade material with a rigid fiberglass board or a dramatic facing—such as brick or stone veneer—as quickly as possible to protect the foam from becoming damaged by sunlight.
   During initial meetings with your builder or architect, make clear whether you intend to insulate the slab or foundation wall. Also clarify how you want the above grade portion finished. If you decide to add insulation after the walls have been framed, the extra thickness of insulation and facing may jut out beyond your home’s exterior.