How to Install Reflective Insulation Under Concrete

Smooth out the layer of gravel spread over the concrete installation site by dragging a 2-by-4-inch plank over the surface with a sawing motion.

Roll out the first row of reflective insulation. Make sure the reflective side is pointed upward so that it’s against the concrete. Pull the edges of the insulation 6 inches upwards along the sides of the walls to create an overlap that will contain the sides of the concrete slab once poured. Secure the overlap to the walls with masking tape, and then cut the end of the row of insulation with a utility knife.

Place the next row of insulation onto the gravel surface so that it butts tightly against the first row. Overlap the ends up the walls 6 inches and secure them as well. Continue with the insulation placement until you’ve covered the entire floor. Cut the last row of insulation lengthwise with the utility knife to fit the space against the far wall, but maintain the 6 inch overlap up the wall.

Seal the seams between the adjacent rows of insulation by covering them with a strip of 3-inch-wide white polyethelyne tape.

Cover the foil layer with an inch of sand to create a drainage surface between the poured slab and the insulation.