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Floor Insulation

Underfloor means the floor above an unheated crawl space or basement. Once the underfloors are properly insulated, you will increase the overall comfort of your home because underfloor insulation provides valuable sound control and thermal control.

The procedure for installing insulation in underfloors is similar to installing ceiling insulation. Follow the installation tips below to ensure that you do a professional job. It's a good idea to wear a work helmet when you're installing underfloor insulation. This can help you avoid injuring yourself on exposed framing members such as floor joists, exposed nails and other hazards. Refer to our complete list of Safety Tips for more important ways to protect yourself while installing insulation.

Make sure you are using insulation with the most appropriate R-value. Our R-value Estimator in the Site Toolbox can guide you. Click on the product links below to learn more about the JM insulation products you can use in underfloor applications.

How to Install Underfloor Insulation

  1. Caulk where wiring runs through the floor joists and around the top of the wall.
  2. Gently press the insulation between the joists. If you're using unfaced batts or rolls, allow friction to hold the insulation in place (called "friction fit method.") If you're using faced insulation, install it with the vapor retarder positioned up toward the subfloor. This product would also be installed using the friction fit method. You may want to consider using JM's special ComfortTherm® for Underfloor, specifically designed for floors, with the stapling flanges on the bottom side, and the vapor retarder on the top side. It is stapled to floor joists in the same way as ordinary faced insulation is installed in ceilings and ensures insulation is up against the floor. The vapor retarder side is installed up against the subfloor, and the non-vapor retarder has flanges for stapling to the sides or bottom of the joists. JM's standard ComfortTherm® product would be installed in the same manner as faced insulation, not the special Underfloor product.
  3. Expand the insulation to its full thickness in the joist cavity to insure complete coverage. However, avoid compressing the insulation material, because compression will reduce its R-value.

Related Products

ComfortTherm® Batts and Rolls


ComfortTherm® insulation is a lightweight, thermal and acoustical insulation made of long, resilient glass fibers bonded with an acrylic thermosetting Formaldehyde-free™ binder.

Kraft-Faced Batts and Rolls


JM's Kraft-Faced Batts & Rolls are made of naturally white, Formaldehyde-free™ fiber glass, reducing environmental concerns including the risks of poor indoor air quality and the effects of manufacturing on the environment.

Unfaced Batts and Rolls


JM's Unfaced Insulation is a lightweight thermal and acoustical fiber glass insulation made of long, resilient glass fibers bonded with an acrylic thermosetting resin made without formaldehyde.

Install Floor Insulation
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