How to Insulate an Unheated Crawl Space

For Vented Crawl Spaces

• 1

  Unroll sheets of 6-mil plastic film across the crawl space floor. The plastic provides a vapor barrier for the space. Overlap each course by a foot and roll out enough plastic to go up each wall around 6 inches. Secure the plastic to the foundation walls with duct tape. Place bricks or stones along the overlaps about every 8 or 10 feet.

• 2

  Fit batting insulation into each floor joist bay. The bays are the open spaces between joists. For vented spaces, use kraft-paper backed insulation, and place the paper up -- against the bottom of the main floor’s subfloor. The paper is another moisture barrier, which is always placed toward the warmer area. Try to eliminate as much air space as possible between the insulation and the subfloor without compressing the insulation.

• 3

  Hammer nails into the joists every 24 inches or so. Twist stiff wire around each nail and its opposite on the other joist. This wire setup will create a hanger for the insulation material.

• 4

  Repeat the process in each joist bay.

For Non-Vented Crawl Spaces

• 5

  Unroll sheets of 6-mil plastic film across the crawl-space floor. The plastic provides a vapor barrier for the space. Overlap each course by a foot and roll out enough plastic to go up each wall around 6 inches. Secure the plastic to the foundation walls with duct tape. Place bricks or stones along the overlaps about every 8 or 10 feet.

• 6

  Cut pieces of insulation using a utility knife. The pieces need to be just long enough to fit between each set of floor joists where they meet the rim joist, the wide-framing lumber that runs on top of the foundation, along the perimeter of the crawl space. Use non-faced batting for unvented crawl spaces.

• 7

  Cut more sections of batting. These must be long enough to reach from the sill plate (which rides directly on top of the foundation wall) to a point around 2 feet past the wall/floor corner.

• 8

  Nail a 1-by-2-inch furring strip along the top of the insulation so that material is sandwiched between the sill plate and the furring strip. Drive the nails through the furring strip, insulation and into the sill plate.

• 9

  Tuck the bottom of the batting tightly against the wall and place pieces of 2-by-4-inch framing lumber on top of the insulation, against the corner to hold the material in place.