How to Insulate a Concrete Slab


Concrete slabs in contact with the ground all have consistent heat-loss and moisture management issues. You home’s slab is no different, and these issues can lead not only to cold concrete floors, but a loss in energy efficiency and an increased possibility of moisture-related damage. To lessen the difficulties of heat-loss and moisture, you can insulate a concrete slab. An insulation process you can use consists of laying a plywood subfloor over the concrete slab while placing a layer of caulk around the edges of the slab. By insulating the slab you create a barrier that keeps heat in and moisture out, saving in heating costs and extending the life of your slab.

Things You'll Need

  • Latex-based caulk
  • Latex-based insulating foam sealant
  • Concrete grinder
  • Concrete self-leveling compound
  • pH balanced cleanser
  • Concrete degreaser
  • 6-mil polyethylene sheets
  • Fiberglass tape
  • 1 ½-inch thick compression grade rigid-foam sheets
  • Utility knife
  • ½-inch plywood sheets
  • 7/8-inch screws
  • Place a layer of caulk around the outside of the slab where household siding meets the foundation, or where flashing around doors meet the concrete slab. Caulking these seams where materials meet is a way to prevent cold air from entering the house. Use a latex-based insulating foam sealant for especially large gaps.

  • Check the surface of the slab to determine if it’s level. Drag a leveling bar across the slab’s surface and mark any high or low areas.

  • Level the surface of the slab using a concrete grinder to grind down rises in the slab. Remove any dust created from the grinder and then use a concrete self-leveling compound to fill any dips in the slab. Mix the compound according to the manufacturer’s instructions and place it in any dips in the slab. Spread the compound using a trowel. The compound will level itself with the surrounding surface of the slab as it cures. Allow the compound to cure for the time recommended in the manufacturer’s instructions.

  • Clean the surface of the concrete completely, removing all dirt or debris. Use a pH balanced cleaner on the concrete to remove the dirt. If there is oil present, use a commercial concrete degreaser to remove the oil completely before continuing.

  • Place a layer of 6-mil polyethylene sheeting over the surface of the slab. Unroll the sheets so that the edges overlap by six inches, and the sheets rise up the walls four inches. Tape the seams of the sheeting using fiberglass tape and then adhere the sheets to the edge of the slab, using caulk to prevent the sheet from slipping. The sheeting creates a vapor barrier between the plywood subfloor and the concrete, reducing moisture damage.

  • Cover the surface of the slab with sheets of rigid-foam. Leave a one-quarter inch gap at the edges of the slab when placing the sheets. Cut the sheets to fit using a utility knife and tape the seams where the foam sheets meet using fiberglass tape.

  • Place one-half inch plywood sheets onto the foam layer. Place the sheets so that they lie across the line of the foam sheets, running in a 90-degree angle from the direction the foam sheets were placed. Leave a gap of one-quarter gap between the sheets of plywood and a one-half inch gap between the sheets and the walls.

  • Place a second layer of plywood sheets atop the first, running perpendicular to the first layer of plywood and overlapping the gaps in the first layer. Attach the two layers of sheets together using 7/8-inch screws.

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