In most cases, crawl spaces should be vented. Therefore, the walls or the floor above should be insulated. If ducts and/or plumbing are in the crawl spaces, it may be preferable to insulate the crawl space walls.

If you are insulating the floor above the crawl space, insulate as detailed in the section on Floors (page 9). A vapor retarder should be installed covering the ground to prevent moisture from entering the crawlspace. The vapor retarder ground cover should be a minimum of 6 mil polyethylene sheeting with the joints lapped at least 6 inches and sealed. The edges should extend a minimum of 6 inches up the wall and be sealed to the wall using adhesive caulking.

Masonry Walls

Furring members to hold the insulation in place are unnecessary. If vapor-retarder-faced insulations are required, use only special low flame-spread vapor retarders recommended for exposed applications. The vapor retarder should, in most cases, face toward the crawl space, the warm-in-winter side. Consult individual manufacturers' instructions. Two methods of installing insulation are recommended.

First,after insulating the band joist separately,nail the insulation to the sill plate with 1/2 x 1-1/2 nailer strips. (See Figure 23.)The insulation should be snug against the piece next to it. The insulation should hang down to the bottom of the wall and extend out onto the ground cover (usually polyethylene) about 2 feet. Where the insulation bends onto the vapor retarder on the ground,the insulation should be held in place by using weights such as bricks or rocks placed on the edge of the insulation.

An alternate method of insulating masonry walls is to run the insulation up the wall, past the sill plate to the subfloor. Again, insulate and air-seal the band joist separately. (See Figure 24.)

All-Weather Wood Foundation

Insulate the band joist separately. Where the crawl space floor is concrete, install the insulation as recommended for wood-frame walls. If the crawl space floor is soil, apply the insulation and the vapor retarder ground cover. (More information on ground cover is detailed on page 15.)