- Q. What is the best way to insulate the footing, foundation, and slab on the walk-out side of a basement?
A.Probably the easiest way is to pour the footing and insulate the inside of the stem wall with 2-inch-thick extruded polystyrene foam (see illustration). The insulation should extend at least 2 feet below grade, and the top can be tapered to a thin edge so it is concealed by the interior trim. This works particularly well if there are heating ducts in the slab near the perimeter.
An alternative is to insulate the slab perimeter, extending the insulation at least 2 feet from the edge. Again, where the insulation turns up along the perimeter, it should be tapered so it can be concealed.
A third option, of course, is to insulate the outside of the foundation with extruded polystyrene. The portion above grade should be protected with stucco or foundation-grade pressure-treated plywood. However, this system should not be used if the walls have a masonry veneer.