Insulate a slab floor with rigid panels of polystyrene and tuck them up tightly under the siding. Insulate a crawl space with fiberglass batts or blankets. Fit batts or blankets between joists, foil side up.
How you go about insulating a cold floor depends largely on what's under it. With a slab floor you insulate around the perimeter or build up a wood floor on top of the slab, with insulation under it. If the floor is over a crawl space, you're best off insulating the crawl space walls. To insulate floors located over porches, garages, or other unheated areas, fit insulation between the joists, up against the floor. Fill the full joist depth with insulation.

One way to warm up a chilly slab floor is to dig around it and glue rigid 1-inch insulation panels to the foundation and footings. Insulating footing only above the frost line. To avoid digging attach sleepers to the slab, install insulation between them, and lay a new wood subfloor.

Covering crawl-space walls with insulation turns the crawl space into a snug, sealed air chamber that brings further insulting value to the floor. Begin by nailing 1 x 2 strips across the ends of unfaced batts or blankets into the band joists. Next, unroll the insulation down the wall about two feet onto the ground under your house. Slip a polyethylene vapor-retarding barrier under the insulation and weight down everything with bricks or stones. Don't put the vapor barrier down first, you could puncture or tear it as you crawl around installing the insulation.

If the joists are covered by a ceiling on their underside, hire a contractor to blow in insulation. With open joists you have two choices. Fit batts or blankets between joists or if you have allot of obstacles such as bridging, glue planks or panels of rigid foam insulation to the joist bottoms with adhesive.

Whether you use batts, blankets, or planks, get materials with the foil facing and install the insulation with foil side up. The foil is a vapor-retarding barrier that helps prevent warm, moist air from penetrating the insulation and reflects heat back into the living space.

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