What Insulation Do I Use for a Wooden Crawlspace?

Fiberglass Batts

• Fiberglass batts or blankets are pre-cut lengths of tightly woven material that are stapled to the wooden joists of the crawl space, covering the sub floor of the basement. They must be flush against joist spacing to ensure there are no air gaps, which would allow cool air up into the home. The batts should be covered with a vapor barrier, such as a plastic membrane, particularly if the crawl space is vented. This prevents the absorption of moisture, which can decrease the batts’ effectiveness. This barrier will also repel insects. Fiberglass batts have an R-value -- the unit of thermal resistance, with higher numbers indicating better insulating effects -- between 3.14 and 4.3, depending on the density of the batt. Batts can be used for both vented and unvented crawl spaces.

Rigid Board

• Rigid insulation boards are tightly packed boards of foam, usually made from polystyrene. They are installed in the same way as fiberglass batts, on the subfloor of the crawl space, whether it is vented or not. Some boards are manufactured with their own vapor barrier. They have an average R-value of between 4 and 5 per inch.

Sprayed Foam

• Sprayed foam, also known as foam-in-place insulation, is liquid polyurethane foam that is sprayed into the crawl space using a pressurized air machine. Installation is usually performed by professionals. The foam hardens and expands within seconds of application. According to Sprayfoam.com, it is effective at providing a heat and moisture barrier as it conforms to the contours of the crawl space, minimizing air pockets. It is most effective when applied between the floor joists, particularly in a vented crawl space. It must be covered with a fire-resistant barrier. Sprayed foam has a typical R-value of 6.25 per inch.

Tape and Tubes

• In addition to either fiberglass batts or sprayed foam, insulation tape and foam tubes should be used to insulate pipes, particularly in vented crawl spaces where the pipes are in contact with outside air. According to Energy Savers, pipe insulation is not necessary in unvented crawl spaces. However, the Red Cross suggests that pipe insulation in necessary in vented areas, even in warmer climates. While fiberglass is naturally fire-resistant, sprayed foam must have such a barrier preventing it from abutting hot pipes. Without it, the high temperature can cause the foam to smolder and release chemical fumes. Install the tape and tubes before applying the batts or foam.