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How To Insulate Garage Floors with Plywood & Rigid Foam

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If you are converting your garage to living space and plan to install a new floor, it is usually smart to begin by insulating the concrete floor. There are two ways to handle insulating concrete floors. You can attach wood sleepers to the floor, fill the gaps with rigid-foam insulation and then apply your finish flooring. Or, as explained below, you can cover the slab with rigid-foam insulation, add two layers of plywood and then add the finish flooring.

Either approach requires that the slab be prepared before you begin. It should be in good condition, free of excess moisture and level.

Difficulty: Average
Time Required: 2-3 days

Here's How:

  1. Insulating concrete garage floors begins with covering the entire floor with 6-mil polyethylene, which will serve as a vapor barrier. Overlap the seams about 6-8 inches. Run the polyethylene up the sides of the walls 3-4 inches.

  2. Lay sheets of rigid-foam over the polyethylene. Use a utility knife to cut the foam to size (a hacksaw blade or drywall saw also work well). Leave a 1/4-inch gap around the perimeter. Tape the seams with heavy-duty construction tape.

    Be sure to use compression-grade rigid-foam panels, which are typically installed beneath concrete slabs. Check with your supplier or the manufacturer for advice on the best thickness and R-value to use.

  3. Lay 1/2-inch plywood over the foam, running the sheets perpendicular to the long dimension of the foam. Space the panels 1/4 inch apart, with a 1/2-inch gap around the perimeter.

  4. Add a second layer of 1/2-inch plywood, running this one perpendicular to the first, with the same spacing.

  5. Fasten the two layers of plywood together with 7/8-inch screws or staples. After insulating the concrete floor, install the finish flooring.

What You Need

  • 6-mil polyethylene
  • Rigid-foam insulation (compression grade)
  • Heavy-duty construction tape
  • Utility knife
  • 1/2-inch plywood
  • Circular saw
  • 7/8-inch screws or staples
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