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Crawl Spaces: How to insulate a crawl space depends on whether you vent it. Traditionally, crawl spaces have been vented to prevent problems with moisture. However, this often did not work well. Today, building researchers are moving towards treating the crawl space the same as any other basement. This section will address both options.

If the crawl space is to be vented, carefully seal any and all holes in the floor above ( "ceiling" of the crawl space) to prevent air from blowing up into the house. Insulate between the floor joists with rolled fiberglass. Install it tight against the sub floor. Cover the insulation with a house-wrap or face the fiberglass vapor barrier down. Seal all of the seams carefully to keep wind from blowing into the insulation. Also, adequately support the insulation with mechanical fasteners so that it will not fall out of the joist spaces in the years to come. DO NOT just rely on the friction between the fiberglass and wood joists to secure it in place.

Figure 85: A prototype panelized foundation being installed in a test home.

Install a polyethylene vapor retarder, or equivalent material, over the dirt floor. Tape and seal all seams carefully. You may also cover the polyethylene with a thin layer of sand or concrete to protect it from damage. Do not cover the plastic with anything that could make holes in it, such as crushed gravel. Be sure the headroom of the crawl space meets local code regulations if you decide to pour a concrete slab.

If the crawl space is to be unvented, seal all holes in the foundation where outside air can enter. Install the plastic ground cover as described above for an unvented crawl space. Run the plastic up the walls and attach it to the first piece of wood (the mudsill). Install rigid insulation foam board against the foundation from the sub floor to the plastic (or concrete slab) on the floor of the crawl space. Do this all the way around the perimeter of the foundation. An alternative to foam board is to drape fiberglass roll insulation down the foundation walls with the edges butted tightly together. This is an acceptable alternative to foam board insulation as long as the crawl space stays dry.

Installation Cost and Performance: Although you can achieve considerable savings in space conditioning costs by insulating the foundation, the installation costs can become relatively high, especially for retrofit projects. The type of materials used, the application method, and the extent of work all affect the overall cost. Simple payback is typically in the range of 6 months for a simple do-it-yourself installation to 20 years for "professionally" installed and more involved work. Adding foundation insulation during new construction is usually less expensive.

Field studies have found that foundation insulation for new houses (in the United States) have good economic outlooks, except for the warmest climates. The Builder's Foundation Handbook and Building Foundation Design Handbook are excellent references.

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