The foundations are an important part of your home, for the obvious reason that they support everything else. But they are also a major source of energy loss unless they are properly insulated, since they are in constant contact with the cold, often damp surrounding earth.

The situation is made worse by the fact that concrete is an excellent heat conductor. Although this is a benefit in a concrete radiant floor slab, for example, it's a real concern in a home's foundations.

The practical result of this is that foundation insulation is absolutely essential, even in normal, non-solar homes. The cost is tiny compared to the returns you will get in heat retention, especially in solar homes and in colder climates. Insulate. It's worth it.

In retrofits and existing homes, insulation is almost always installed internally. This is partly because it's efficient and effective but also because it's so expensive to dig out the foundations and put in external insulation!

Internal insulation offers more choice of materials than external, because it doesn't need to be waterproof and there's no threat of insect attack. However, this means that the foundations themselves are more susceptible to seepage, so you need to apply a good coat of sealant and ensure there is sufficient drainage to keep water away from the concrete.

Internal insulation should always be covered with a fire-resistant material, such as drywall. In the event of fire, it will emit toxic gases, so you need to make sure the danger is minimized.

Thermal mass and radiant floor heating slabs

If you're using a concrete foundation slab as thermal mass for passive solar heating, the insulation should go between the slab and the stemwall. This makes a very effective thermal break between the slab and the wall, reducing heat loss to the exterior.

If you're using a concrete slab for radiant floor heating, you will have to insulate around the thermal storage bed and the slab itself. We cover this elsewhere on the site .