How to Install Insulation Under Concrete Slabs

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Heat loss is reduced, heating costs saved, and below-grade rooms become more pleasant when foundations are insulated. It's not possible to insulate under the slab once it's installed, although exterior insulation can improve energy efficiency after construction. Interior insulation -- usually rigid -- is installed under the concrete slab before the slab is poured, and vertically at the edge of the slab, where it serves as a thermal break to reduce the flow of heat to the outside. Energy specialists recommend a minimum R-15 for perimeter insulation, and where the slab is heated, R-10 insulation should be installed under the entire slab.

Things You'll Need

  • Washed gravel
  • Polyethylene plastic, 10 mil
  • Tape measure
  • Rigid insulation, R-15 (3 inches), high compressive strength, square edge, scored type
  • Utility knife
  • Sand
  • Spread 4 inches of washed gravel on top of the prepared subgrade to the extent of the slab. Place a layer of the 10-mil poly over the gravel, overlapping joints by at least 12 inches.

  • Measure or calculate the thickness of the slab, plus the thickness of rigid insulation. Lay out the line on the insulation with a straight edge. Score the surface of the rigid insulation with the knife held against the straight edge, and snap the board.

  • Install the board on edge against the wall or footing. Continue along the perimeter of the foundation, ensuring joints are tight and the insulation fits snugly around service entrances.

  • Lay rigid insulation horizontally against the vertical insulation, and extend under the slab for 24 inches. Make sure the insulation fully contacts the washed gravel. Follow the vertical insulation around the perimeter, scoring and snapping to fit as necessary.

  • Spread a minimum of 1 inch of sand -- or as deep as recommended -- on the rigid insulation as a protective base for the slab.

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