How To Insulate Basement Rim Joists

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Guide to insulating your basement rim joists with rigid foam insulation.

Another way to reduce heat loss and cut down on your heating costs this winter is to insulate the rim joists in your basement with rigid foam insulation. Rigid insulation is the best choice as it retains its insulation rating if it gets damp or wet. It is more difficult for mice and other pest to use as nesting material and adds another layer of defense. Gaps around electrical lines and water pipes increase heat loss as well.

In a few hours you can seal and insulate your rim joists, which are major sources of heat loss in many homes. Insulating the rim joists is one of the best things you can do to make your home more energy efficient for homes with basements or crawl spaces.

Tools and Materials

ToolsOnly a few simple tools are required for this do-it-yourself project:

• Tape measure

• Straightedge or 4-foot level

• Hand saw, utility knife, or circular saw

• Caulk Gun

• Stepladder

A table or circular saw are the quickest ways to cut the foam insulation, but that can get messy. A handsaw or long-blade utility knife, such as a box cutter, will also do the job.


Besides the rigid foam insulation board you will also need:

• Acrylic caulk

• Expanding foam sealant

• Loose fiberglass insulation if you have a block wall with open cavities at the top

It will take 4 to 8 hours to complete the job, depending on the size of your space and if you have a finished basement. The materials will cost around $1 per foot of rim joist, so a 30 x 40 basement will cost around $140 to insulate.


$100 - $500

Expect to pay $1 per foot of rim joist.

Insulating the joists

Rigid foam is the best insulation for rim joists and 2-inch thick boards have an R-value of 10, but R-values can range from 3.8 to 4.4 per inch depending on the manufacturer. Use only “extruded polystyrene” and NOT “expanded polystyrene,” which is a less effective air and moisture barrier.

Cut rigid foam insulation into strips with a table saw or a circular saw. If you don’t have a circular saw or table saw, use a straightedge like a 4-foot level or 2x4 to draw out lines on the foam and then cut with a handsaw or knife. Typical 2 x 10 joists are between 9 ¼ and 9 ½ inches wide. Cut the strips about 1/8-inch narrower than your measurement to make it easier to get them into place. Cut the strips to fit between floor joists using a box cutter.

Cross-section of Rim Joist Insulation

If you have engineered joist that look like small wooden I-beams, then you will have to notch each piece to fit inside the joists. I like to use a keyhole saw to cut out the corners of the foam.

Use long sections of foam to cover the rim joists that are parallel to the floor joists.

Tip: Although it may seem tempting, don’t glue your foam board into place. If you have to access the joist with another project in the future, you can cut the caulking and remove the piece of insulation.

 Caulk around openings

Run a bead of acrylic caulk around each section of foam to create an airtight barrier. Fill gaps larger than 1/4 in. with expanding foam sealant.

Use a box cutter with a long segmented blade to cut the insulation to length. Don’t rush and make sloppy cuts, the better the fit and the more energy you’ll save. Most builders stuff fiberglass insulation into the bays and leave only to have it fall out in a few years. If you have an unfinished basement, you can properly insulate the rim joists in two or three hours.

Call your local building inspections department before you begin this project. Some locales may require you to cover the new insulation with drywall to create a fire block or leave some areas uncovered to allow for termite inspections. Unfortunately you won’t be able to insulate your second-floor rim joists in the same fashion unless you happen to tear out a ceiling during remodeling. You may want to contact and insulation professional to determine if you have insulation in place or they can inject foam insulation into the bays. You will have to spackle small ¾ inch holes when the job is complete.

General Tips

Measure wear piping and cables go through the rim joists and then cut the foam board into two pieces with half of the hole in each piece. Seal the seam and around the hole with the expanding foam sealant.

If you have an exterior outlet that protrudes through the rim joist, use a utility knife to plow out the back of the board to fit over the box and wire.

To create an airtight seal around each section of foam use caulk or expanding foam, otherwise moist air from inside the home could condense on the cold rim joist. This can lead to an environment where mold and rot could flourish.

If you have a poured concrete foundation wall you should also run a bead of caulk where the sill plate meets the concrete.

Leave at least a 1-inch space around any piping, flues, or dryer vents to prevent melting. Rigid insulation can typically withstand temperatures up to 200 to 250° F depending on the manufacturer. Fill in any gaps with loose fiberglass insulation.

For more information on foam insulation, go to The U.S. Department of Energy – Energy Savers at  


Posted on Oct 10, 2011
Jerry Walch
Posted on Oct 2, 2010