Crawl Space or Basement Insulation

Crawl Spaces

In the past, insulation of basements and crawl spaces was often overlooked as a means of energy efficiency. Today, more basements are insulated and many state and local energy codes may require such insulation.

In an unheated or vented crawl space, insulation should be installed on the underside of the floor over the crawl space. 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," or 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).

Crawl spaces with dirt or gravel floors should have a 4 - 6 mil plastic ground cover to prevent moisture from damaging the insulation or structural materials. Six mil 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 crawl space should be insulated, to prevent heat loss and freezing of water pipes.

If the crawl space is unvented and contains a heating system, water heater or significant heat source, you may want to insulate the crawl space walls instead to contain the 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.

Basements

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

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

With new construction you can insulate 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 new methods in which the insulation is incorporated into the foundation wall itself.

When insulating a basement wall in an existing home, insulation is mostly limited to the interior of the wall. 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.

Slab on Grade Foundations

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. Insulating around the exposed perimeter of the slab will help reduce this loss. Use rigid foam board insulation and extend it down below the frost line.

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.