by Pacific Northwest National Laboratory

The foundation of a house is a somewhat invisible and sometimes ignored component of the building. It is increasingly evident, however, that attention to good foundation design and construction has significant benefits to the homeowner and the builder, and can avoid some serious future problems.

Good foundation design and construction practice means not only insulating to save energy, but also providing effective structural design as well as moisture, termite, and radon control techniques where appropriate.

From the Builder's Foundation Handbook

A substantial amount of heat is lost through an uninsulated slab, resulting in cold, uncomfortable floors. Even if the foundation wall is insulated vertically under the slab (Figure 1), significant heat is still lost from the slab edge that is closest to the cold outside air.

Where and how should perimeter slab insulation be applied?

The insulation can be applied outside or inside the foundation wall. Exterior applications (Figure 2) require a metal flashing or durable finish for protection.

The insulation can also be placed vertically along the foundation wall (Figure 3) or horizontally under the slab (Figure 4). Perimeter slab insulation can give termites access, so be sure to provide a termite shield (Figure 8). Some jurisdictions do not allow external insulation because the foundation must be visible for termite inspection.

Is there a way to avoid bringing the insulation in Figures 3 and 4 to the top of the slab edge so a carpet tack strip can be attached?

Significant heat can be lost through the slab edge. Image: Pacific Northwest National Laboratory Insulation can be applied outside the foundation wall. Image: Pacific Northwest National Laboratory   Click on thumbnail images to view full-size pictures.