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How To Insulate A Crawlspace

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Adding insulation to your crawlspace is one of the wisest home improvements that you can undertake. Insulation not only provides a return on the investment but makes your home much more comfortable for your family and reduces drafts and illness.

Heated Crawlspace:

In a heated crawlspace or one with un-insulated ducts or water pipes, the walls should be insulated. R-19 fiber glass insulation or R-25 insulation work best for this, as shown in Figure 1.

First measure and cut small pieces of unfaced insulation and fit them snugly into the band joist. Unfaced insulation does not have kraft paper or other material on one side of the fiberglass insulation batts as shown in Figures 2 & 3.

Unfaced fiberglass insulation batts
Figure 2 - Unfaced fiberglass insulation batts
Faced fiberglass insulation batts
Figure 3 - Faced fiberglass insulation batts

At a minimum, the insulation should be cut long enough to completely cover the crawlspace wall. It is best to carry the insulation out onto the ground by 2 feet or more.

Use long furring strips and standard nails or use cap nails to attach the insulation to the wood plate at the top of the foundation wall. When attaching the insulation try to minimize any compression by only driving the nails part way into the plate. If using the wood furring strips, pre-nail the wood so it is easier to install, construction adhesive can also be used to mount the furring strips to the walls.

crawlspace insulation
Figure 1 - Crawlspace Insulation

After the insulation has been installed, a 4 or 6-mil polyethylene vapor retarder should be spread across the entire floor. It should be placed under the insulation, then - to hold everything in place - rocks or bricks can be set on top of the insulation that extends out on the floor.

Under-Floor Crawlspace:

under crawlspace floor insulation
Figure 4 - Insulating under a crawlspace floor

A lot of energy can leak through the floor, so it's important to insulate it correctly. R-19 or R-25 fiber glass batts can be installed in floor applications as shown in Figure 4.

Always install the insulation with the polyethylene vapor retarder toward the warm side of the structure in heating climates. In a vented crawlspace, the warm side is usually up, closest to the floor.

The insulation should be installed all the way back at the end of each joist run so that it touches the band or rim joist.

You want complete coverage under the house. There will usually be a narrow joist space on the walls that runs parallel to the joist. The insulation should be cut to fit this space.

Water pipe insulation
Figure 5 - Water pipe insulation

There are often both pipes and wires in crawlspace floors and occasionally a junction box. Water pipes should be insulated (Figure 5) and you will need to insulate carefully around electrical wiring and boxes.

Insulation should be placed around cross braces by cutting it and pushing it between the braces.

Once the insulation is in place between the floor joists, insulation hangers or nylon straps can be used to hold the product in place.

A 4 or 6-mil. polyethylene vapor retarder should be laid down to completely cover the ground. Place rocks or bricks around the perimeter to hold it in place.

Additional information on: Insulating a crawlspace.