How Do I Insulate a Deck Floor?


A deck floor is best insulated with paper-backed batts of fiberglass insulation before the deck is installed. If the deck has already been put in place, it is too late to use the rolls of fiberglass, but if there is a ceiling installed on the underside of the floor joists you can use loose fill or blown-in insulation.

Installing Batts of Insulation

  • Floor joists that are placed 16 or 24 inches on center can be filled with paper or foil backed batts of fiberglass insulation. This material comes in large compressed rolls that pop open and expand when you cut the tape seals. Always buy the material in the same thickness as the floor joists (5 1/2, 7 1/2, 9 1/2 or 11 1/2 inches) to get the most insulation value. When you install the fiberglass material you should be wearing a dust mask, eye goggles, gloves, a hat and long-sleeved shirt and pants to keep the irritable fiberglass from contacting your skin. Use a staple gun with long staples to attach the paper backing to the sides of the joists. The paper backing should never be attached to the top of the joists. Just staple the backing on the sides as close to the top as you can and don't scrimp on staples.

Other Methods

  • If a sheetrock or tongue and groove ceiling has been installed on the underside of the deck joists you can install loose fill or foam insulation by drilling holes through the subfloor decking and filling each space with insulation. Make sure the ceiling is installed properly and never drill through the finish floor to do this task. Also check the chemical composition of the spray for undesirable contents such as formaldehyde.

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