Use crawl space insulation with a whole-system insulating value of R-26 or greater

Crawl space insulation may be located in the floor system or at the perimeter wall. If there are any pipes or ductwork, the insulation should be installed on the perimeter walls. Install insulation on the inside of the perimeter wall, and extend it below grade to the footing.

If insulating is done as part of a renovation, save money with scrim-reinforced double-faced fiberglass roll insulation. Run the material from the subfloor down the perimeter wall and out over about two feet of the floor. It is important to keep the insulation continuous to get  full R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. from the foundation up to the bottom of the subfloor, particularly at the band-joist area.

Eliminate air leakage from the inside or outside of the building to the space between the insulation and the perimeter wall, and don't vent a crawl space that has perimeter insulation. It's also important to have a continuous vapor barrier between the earth and the crawl space to reduce moisture and its associated potential for mold growth and/or wood failure. A vapor barrier will also reduce the amount of soil gas that gets into the building.