Crawl Space or Slab On Grade Insulation

Floor or Basement Insulation

Almost everyone is aware of the importance of ceiling and wall insulation, yet very often nothing is done to insulate floors or basements. While the heat loss through floors or foundation walls is usually less than through ceilings or exterior walls, it can be significant. Insulating your floors or foundation walls can save money and improve comfort as well.

If you are fairly "handy" and have basic hand tools, and if there is sufficient space to work in, you may be able to do the work yourself. If not, contact several qualified insulation contractors for estimates. When comparing estimates, be sure the estimates are based on the same amount (R-value) of insulation.

Complete coverage, with no gaps, is important except that floor joists may be left exposed. Take care to custom fit the insulation as required.

Crawl Spaces

In an unheated or vented crawlspace, 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. The 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 barrier faces up toward the warm-in-winter side (this means that you can't staple the vapor barrier to hold the insulation in place).

Crawl spaces with dirt or gravel floors should have a 4 - 6 ml thickness of plastic for ground cover to prevent moisture from the ground from damaging the insulation or structural materials.

Any pipes or ducts that pass through an unheated crawl space should be insulated as well, to prevent heat loss and freezing of water pipes.

If you have a crawl space that is unvented and contains a heating system, water heater or other 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 finish material.


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 conditioned space, insulate the basement walls to below the frost line (usually several feet below grade in the northern half of the United States). Use either fiberglass batts or blankets or rigid foam board insulation.

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 perimeter of the slab will help reduce this loss. Use rigid foam board insulation and extend it down to just below the frost line if possible. The insulation should extend up to the siding and be equiped with a special drip cap for moisture protection and to provide a more finished appearance. Exposed insulation should have a protective covering and this is available either pre-applied or as a separate finish coat.