How to Insulate a New Home's Foundations

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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.

Insulating foundation blocks

If you built your home on a foundation of concrete blocks, rather than a poured solution, your first step should be to insulate internally. Whether you chose concrete, fly-ash concrete, or Faswall or Rastra blocks, you can fill their internal air space with insulation.

Almost any kind of insulation will do. Some builders use high-pressure foam – which works very well – or polystyrene beads or vermiculite. There are even specially-designed foam inserts which fit snugly into the holes in the blocks to reduce heat flow.

Whatever method you choose, bear in mind that it is not a complete solution. Tests show that heat still transfers uncontrolled through the mortar used in the foundation, so you still need to add another barrier. You can do this in the same way as you would for a poured foundation.

Poured foundations

Comparing internal and external foundation insulationThe vast majority of foundation insulation uses rigid foam, though basement walls can be internally insulated with the new bubble insulation. This consists of plastic bubbles squished between reflective aluminum layers.

The foundation itself can be insulated either internally or externally. For new construction, it's generally cheaper and easier to put the foam on the outside. However, external insulation is less effective than an internal rigid foam wall because it doesn't stop the heat escaping in difficult-to-reach locations (such as out of the bottom, underneath the footing). Internal insulation prevents heat from ever reaching the concrete of the foundation, which is a better solution. The diagram shows how internal insulation is more effective.

Don't forget to insulate the basement walls, too. You should cover them externally down to four feet below the grade level, using insulation at least two to four inches thick. Below the four-foot level, you can reduce the insulation by half. In cold climates, you will want thicker insulation.

The inside of the basement should also be insulated, filling in the spaces between the framing and covering with drywall or paneling to finish.

Insect barriers for external insulation

External insulation faces additional difficulties from insects, specifically termites and carpenter ants. Although rigid foam insulation isn't at all nutritious, eating through it gives them access to the rest of the foundation and the house, most of which is tasty and filling!

Some foam manufacturers impregnate their insulation with a chemical insect repellant, usually