Formulas For A Warmer Floor

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With the foundation protected from water entry, there are a number of insulation choices. A common practice in vented spaces is to wire-support fiberglass batts-paper toward the warm side-between the floor joists. Thicknesses of 3 1/2" or 6" (or a combination) can be used to meet code. Drawbacks include the difficulty of working around wires, pipes and braces, the necessity of seasonal vent control-with occasional checks for condensation-and the added expense of insulating pipes and ductwork separately.

Unvented spaces allow more leeway. It's typical to insulate the perimeter wall, rather than the crawlspace ceiling, and is more economical besides. Duct and pipe insulation becomes unnecessary. Without getting into a scientific analysis, it's safe to say that, though placing the insulation outside the wall offers more protection to the structure, putting it inside is easier and less costly. The rate of heat loss is about the same in both situations.

For inside work, rigid polystyrene (molded or expanded) is less expensive and therefore preferable to the more costly extruded styrene board, which is dense and highly water resistant for exterior use. Polystyrene may not need an additional vapor barrier, and it can be caulked in place or fastened with stick clips or an adhesive sealant. Fiberglass batting can be used in the cavity above the sill plate, against the band joist, but should be covered with polyethylene or fitted sections of rigid insulation board.

Another approach would be to hang flexible insulation, such as 2" foil-backed duct wrap or fiberglass batting, over the vapor sealed face of the wall on concrete nails, then carry it 1' or 2' inward so it rests flat on the crawlspace floor. The batting's foil facing should be toward the warm surface and may need to conform to fire code.

It's ideal, though not always practical, to have some type of horizontal insulation at least along the inside perimeter of the wall. Unless the soil is dry and graded level, a rough concrete "mud slab" makes a good base for any insulation material. If rigid board is used, the vapor barrier can go above or below it; otherwise, the poly sheeting should lie directly on the ground.

Insulating outside the wall is best if cost isn't a major concern.

Any attention to insulation detailing brings up the question of termite control. Merely keeping construction scraps and moisture away from the area is a help, but not a solution. Sound construction practices include the use of a solid bond beam or cap at the top of concrete block, termite shields at the sills, and patios or porches that slope away from the foundations. Sills and wood trim should be visible and at least 6" or 8" above grade on the outside, and, if possible, interior insulation should be fastened in such a way that it can be partially removed for seasonal inspection.

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