Many builders routinely cover the outside of their foundations with rigid foam insulation. But the exposed part of the foam is easily damaged and can hide termites and carpenter ants as they tunnel their way to the home's framing. Gary Sharp of Post Harvest Developments in Ottawa, Ontario, has developed a foundation system that lets him have his foam and protect it, too.

The wall

Sharp's 15-inch-thick foundation sandwiches 5 inches of rigid foam between 4-inch and 6-inch sections of concrete. The thin section faces the outside and carries the home's brick veneer siding. The thick one supports the building. The foam in the middle acts as a thermal and moisture break.

The footing

A below-grade drainage board directs groundwater to the footing drain and away from the building. The footing was formed with a product called Form-A-Drain, a high-performance, perforated PVC extrusion that stays in place after the pour to serve as a permanent foundation drain.

The forms

The insulation consists of extruded polystyrene with an insulating value of R-5 per inch. Sharp held the foam in place with high-strength nylon ties placed on 1-foot centers. One side of each tie extended 4 inches from the foam, while the other side extended 6 inches. Installing the ties was the most time consuming part of the job. It also added an extra $2000 to the cost. But Sharp believes that the cost was offset by the fact that he didn't need to install and finish exterior foam. (He cautions not to bring in the forming crew until the insulation is ready. Otherwise they'll end up standing around and getting frustrated.)

Erecting the forms was simply a matter of standing up the outer form, then the foam insulation, then the inner form. The steel ties that held the forms in place were then driven as usual through the interior and exterior forms and the foam.

The concrete

The most critical part of the foundation job was the pour as the sheer number of steel and nylon ties made it hard to completely fill the 4-inch section. Sharp solved this problem by adding superplasticizers to the concrete mix, which made the concrete flow more easily around the ties. However, he points out that you could also use a vibrator. (He knows of an architect who tried this technique but didn't use superplasticizers or a vibrator; the result was honeycombing, or voids, in the concrete.)

It's also crucial to fill on both sides of the foam at the same rate; otherwise the concrete will push the insulation towards one side of the form. To help control the placement of the concrete, Sharp made a plywood box with 4-inch and 6-inch compartments. The sides of the box lapped 4 inches over the outside of the forms. One worker directed the concrete into both sides of the box while another dragged it along the top of the form.

For more information on this technique call Gary Sharp at 613-722-4548.