Basement and Crawlspace Insulation

While most homeowners understand the importance of insulation, they often overlook one of the greatest areas of winter heat loss - the basement or crawlspace underneath their home. This is due in part to the mistaken belief that "heat rises". While heated air does tend to rise, heat itself always moves from a warmer environment to a colder one. The driving force behind this heat movement is temperature difference - the greater the temperature difference, the faster the movement of heat. In cold weather, the temperature in a basement or crawlspace can be twenty to thirty degrees colder than that in the living space. This results in a significant amount of heat being lost downward through the floors. Insulating the basement or crawlspace will reduce this heat loss, saving space heating energy and improving comfort by keeping the floors warmer.

Unconditioned Basements

An unconditioned basement is defined as an unheated basement with no significant heat-producing appliances or devices. The only heat comes through the floors of the living space above. If there is no need for this heat, the basement ceiling should be insulated to keep the heat upstairs in the living space (if some heat is desired in the basement, see "Conditioned Basements" below).

Basement ceilings can easily be insulated with unfaced fiberglass batts or blankets installed between the floor joists and held in place with wire, fishing line or spring metal supports called "tiger claws".

Insulating the basement ceiling will keep most of the heat upstairs in the living space. As a result, the basement will be colder in the winter making it important to also insulate any water pipes or heating/cooling ducts in the basement. Pay particular attention to any water pipes around or near the perimeter, as they can potentially freeze in very cold weather.

If you have an unheated garage with living space above, the garage ceiling should also be insulated in the same manner.

Floor Insulation - Recommended R-Values by Climate*

Warm with cooling and minimal heating requirements R-11 to R-13
Mixed with moderate heating and cooling requirements R-13 to R-25
Cold R-25
* U.S. Department of Energy recommendations

Conditioned Basements

A conditioned basement is one that is heated, either intentionally or due to the presence of a heating system, water heater, or other heat-producing appliance. If the basement is intentionally heated, the temperature will be the same as, or close to, that in the living space. In an unintentionally heated basement, the temperature tends to be somewhere between that in the living space and that outdoors. In either case, the objective should be to contain that heat, by insulating the basement walls - not the basement ceiling.

Any frame walls in the basement (as with a "walk-out" basement above ground level) should be insulated with fiberglass batts installed between the studs. If the insulation has a facing or vapor retarder, it should face inward (see "Exterior Wall Insulation").

For unfinished concrete walls, there are several options. If you plan to finish the basement (or that section), you can install wood framing and insulate with fiberglass batts. Otherwise, you can install several inches of rigid foam insulation board directly against the inside of the concrete wall. The insulation should extend all the way down to the basement floor, or at least to below the "frost line" (the depth to which the ground freezes in winter).

When insulating basement walls, don't forget the band joist (the piece of wood at the top of the wall that extends around the perimeter and encloses the ends of the floor joists). This two-inch thick piece of wood and the siding covering it are all that separates the interior of the basement from the outdoors. Make sure that any water pipes are kept on the warm side of the insulation to prevent freezing in winter.

Basement Insulation - Recommended R-Values by Climate*

Warm with cooling and minimal heating requirements R-11 to R-15
Mixed with moderate heating and cooling requirements R-11 to R-22
Cold R-11 to R-22
* U.S. Department of Energy recommendations


Crawlspaces that are vented to the outside (the majority) should be insulated in the same way as an unconditioned basement. Insulate the floor from below, and be sure to insulate any water pipes or heating/cooling ducts in the crawlspace, especially any water pipes at or near the perimeter.

Unvented crawlspaces can be treated like an unintentionally heated basement, by insulating the exterior foundation walls. If the crawlspace contains a heating system or un-insulated heating/cooling ducts, this may be the more practical alternative, as it can be less expensive and easier than insulating both the floor above and the heating/cooling ducts. Be sure to insulate the band joist as well (see "Conditioned Basements" above).

Slab at Grade Foundations

Most of the heat loss from a slab at grade foundation occurs at the edges where the slab is exposed to cold outside air. While this is a relatively small surface area, in cold climates the heat loss can be significant, increasing space heating energy costs and making the interior floors colder in winter. Rigid foam board insulation can be installed around the perimeter of the slab, extending down to the frost line. Because of the cost and difficulty of trenching around the perimeter, this method is usually cost-effective only in very cold climates. This method is not recommended in locations where termites are a concern, as the insulation can provide a path for termites to enter the structure.