How to Insulate a Concrete Floor

Insulating a concrete slab reduces heat loss and helps to moderate the temperature of a home, according to the U.S. Department of Energy. Solve moisture problems before insulating a concrete floor to avoid mold growth in or under the insulation. Install the insulation on a concrete floor before beginning work to finish a basement. The insulation will increase the comfort of the basement and save on overall heating costs.

Things You'll Need

• Vacuum
• Soap
• Scrub brush
• Foam board adhesive
• Utility knife
• Caulk gun
• Interior rigid foam insulation
• Vinyl tape
• Drywall saw
• Plywood
• Blanket or pad
• Pressure treated 3/4-inch decking
• Saw
• Concrete nailer (if using concrete nails)
• Concrete nails (if using concrete nails)
• Drill (if using concrete screws)
• Masonry drill bit (if using concrete screws)
• Screwdriver bit for drill (if using concrete screws)
• 3/4-inch tongue and groove subfloor
• 1 1/4-inch stainless steel or exterior screws

Instructions

1. Clean the concrete floor with a vacuum to remove all loose soil and debris. Remove stains and possible mold with hot soapy water and a scrub brush. Allow the floor to dry completely.

2. Cut the tip of the foam board adhesive at an angle with a utility knife. With the wire attachment on a caulk gun or a long nail, puncture the seal inside the tube of foam board adhesive. Insert the tube into a caulk gun.

3. Apply a bead of foam board adhesive to the back of the foam board (the side without a foil or plastic coating) around the perimeter of the insulation approximately 3 inches from the edge. Apply another bead of foam board adhesive in a zig-zag pattern across the center of the foam board insulation. Lay interior rigid foam insulation directly on the concrete floor with the foil side facing up. Do not walk on the foam insulation to avoid punctures or compressing the foam.

4. Tape the seams with vinyl tape. Place a small sheet of plywood on a blanket or pad when taping the seams to distribute your weight on the insulation.

5. Cut the rigid foam insulation with a drywall saw as needed to fit small pieces on the sides and around stationary fixtures, such as plumbing and heating units. Seal around the objects by taping half on the foam insulation and half on the object with vinyl tape.

6. Install pressure-treated strips across the foam board. Space them approximately every two feet, one on each end and two evenly spaced in the middle, on the foam board insulation. Secure with concrete nails or concrete screws. Concrete nails require the use of a concrete nailer. However, concrete screws require a drill and masonry bit to pre-drill the hole and then a screwdriver bit to secure the concrete screw into the pre-drilled hole. Select the masonry bit by the size of the concrete screws. The proper bit size will be printed on the screw package label.

7. Place sheets of 3/4-inch subfloor on top of the pressure treated strips. Secure to the strips with 1 1/4-inch stainless steel or exterior grade screws. Cut as needed to fit around objects such as heaters and plumbing.

Tips & Warnings

• Finish the floor with decorative flooring. Carpet is not recommended for basement areas due to the risk of moisture and potential for mold growth.

References

• U.S. Department of Energy: Slab-on-Grade Foundation Insulation
• Home Construction and Improvement: How to Insulate a Concrete Floor