It is possible to insulate between and under the joists, and create an unheated crawl space. However, this can lead to problems of freezing pipes, frozen ground and possible rot at the joist ends. For these reasons, use floor insulation only when combined with foundation-wall insulation to create a partially heated crawl space.

Points of general importance

• The air and vapour barrier must be applied on the warm (top) side of the insulation. If the floor above the crawl space is already covered with an impermeable material (e.g. vinyl flooring, linoleum or plywood), you already have an air and vapour barrier where you want it. The solid materials of the floor can serve as the air and vapour barrier, but be sure to locate and seal any air leaks (e.g. piping and wiring holes). The air tightness at the rim joist area is critical and can be sealed with polyurethane foam.
• Batt insulation may be held in place with breathable building wrap stapled to the joists, chicken wire, sheets of polystyrene bead board (Type I or II) or a commercially available insulation-support system.
• Place the insulation tightly against the underside of the floor above, filling the joist cavity to the insulation support system.
• Tape the seams in any heating ducts and insulate all ducts and water pipes in the crawl space. Remember, even insulated water pipes may freeze if the temperature of the crawl space is allowed to fall below freezing. Consider installing energy-efficient, self-regulating electric heat trace to protect the plumbing pipes.
• Make sure that the crawl space is adequately ventilated in the spring. Vents should be installed at a ratio of 1 to 500 (vent area to floor area). Do not ventilate in winter; the vents should be plugged and insulated. Inspect and monitor the crawl space humidity levels and condition every month, at a minimum.
• There must be a moisture barrier on the crawl space floor. For full basements with a section of crawl space where a part of the floor above has been insulated, insulate the wall separating the basement from the crawl space.
• If the ground level inside the crawl space is lower than the ground level outside, there is a risk that frost heave can damage the walls by pushing them inward. Make every effort to keep water away from the foundation walls.
• As an added safety precaution, you may want to install a thermostat attached to a small heater in the crawl space. This unit can automatically prevent the crawl space from freezing.