In the past, insulating basements and crawlspaces was often overlooked as a means of improving a home's energy efficiency. Today, more basements and crawlspaces are insulated and many state and local energy codes may require such insulation.

In an unheated or vented crawlspace, insulation should be installed on the underside of the floor over the crawlspace. Fiberglass batts or blankets are the easiest material to use in this type of application. This insulation can be held in place with wire mesh, spring metal supports called "tiger claws", wire or fishing line criss-crossed between floor joists. Be sure to install the insulation so that the vapor retarder faces up toward the warm-in-winter side (this means that you can't staple the vapor retarder to hold the insulation in place).

Crawlspaces with dirt or gravel floors should be covered with a 4 - 6 mil thickness of plastic to prevent moisture from damaging the insulation or structural materials. The six mil thickness is recommended, as it is less likely to be damaged by walking on it. The ground cover should be overlapped 4" to 6" at seams and at the bottom of the foundation wall. The material can be held in place with bricks, small sections of framing lumber, or other objects heavy enough to prevent it from being disturbed if someone walks on it.

Any pipes or ducts that pass through an unheated crawlspace should be insulated to prevent heat loss and freezing of water pipes.

If the crawlspace is unvented and contains a heating system, water heater or significant heat source, you should insulate the crawlspace walls instead to contain heat and warm the floor above. You can use fiberglass batts or blankets or one of the various foam board insulation materials. If using the foam boards, check local fire codes, as they may require covering the insulation with a fire retardant material.

If the basement is unheated and contains no major sources of heat, you should insulate the floor above, as described for crawlspaces.

If the basement is heated or contains any major heat sources, or if you plan to turn the basement into a conditioned space, you should insulate the basement walls instead.

With new construction you can insulate either the interior or exterior of the foundation wall. If insulating the exterior, use several inches of foam board insulation (enough to achieve R-13) down to the footing or below the frost line. If insulating on the inside, use fiberglass batts or foam board on the full height of the wall (R-13 or greater is recommended). There are also some newer construction methods in which the insulation is incorporated into the foundation wall itself.

When insulating a basement wall in an existing home, interior insulation is the only practical alternative. Depending on the use of the basement, you may choose to either finish the walls or use techniques that do not require creating a finished basement.

The majority of the heat loss from a slab on grade foundation occurs at the edges where the slab is exposed to the outside air. In cold climates, insulating around the exposed perimeter of the slab can help reduce this loss. Use rigid foam board insulation and extend it down below the frost line.

Caution:

In locations where termites are a concern, slab perimeter insulation is generally not recommended, as the insulation can provide a path for termites to enter the structure.

In new construction or cases where a sub-floor can be installed, 2 inches of foam insulation can be used either under the slab or as part of the sub-floor. This is generally considered cost-effective only in very cold climates.