Using Foam Board Insulation to Help Insulate a Floor

Stacks of foam board insulation
  • 8-24 hours
  • Beginner
  • 250-2,000
What You'll Need
Foam board, the thicker the better
Measuring tape
Caulking gun with caulk
Wood glue

Foam board insulation is used on attics and walls, yet it can also be used to insulate your floors. Most people do not think about insulating their floors unless perhaps it's their attic floor. But insulating your floors is a good idea for many reasons including energy efficiency and height adjustments. But it is important to know how to install foam board insulation properly. Here's what you need to know if you wish to do it yourself.

Build the Floors

The first step is to build the floor joist or to strip back the floor until you can see the joist. This is an important step that cannot be skipped. You will not be able to insulate the floor properly without first seeing the joists.

Measure and Prep

Once you can view the joists, start dusting them off and checking the wood for rot and damage. You may also want to measure the space between them so you can cut and fit the foam board between them. If they are damaged, you may need to replace the wood so the foam will stick between them with no problems. You also do not want weak joists, since they would not be a good support for the floor.

Prep the Foam Board

exposed floor joists

If you have yet to get the foam board, go ahead and buy it now. Thicker foamboard will offer more support and insulation for the floors. Buy more than you think you will need to be on the safe side.

Once you have the board, measure and mark it where it will need to be cut. Do not cut it thinner than the width of the joist, try to make it as wide as that. After they are marked, cut with a craft or utility knife.

Place in the Insulation

Next, you will need to place the foam board insulation between each joist as a test. If it fits properly with no space in-between on either side of the board, pick it up and put a thin layer of glue on. If it you need to spread it, use a brush to help cover and smooth out the glue. Place back between the joist and hold it in place for five seconds or for how long the instructions on the glue says.


Once the glue is dry and all the foam board insulation is installed, you can seal around the foam boards with caulking. Try to smooth it out if it is too thick. Allow drying.

Clean Up and Finish Floor

After all the caulking has dried, you can finish the floor or replace everything back on it. Once the floor is finished, clean up and admire your hard work. Your floor should be ready for heavy foot traffic now.