How to Insulate a Basement Floor for Heat

Drag a 2-by-4-inch plank across the top of the gravel layer of the foundation floor, laid in the foundation hole for drainage, to level the gravel out. The level gravel will create a sturdy surface for the insulation.

Place a layer of 6 mil polyethylene plastic sheeting over the foundation floor of your basement, covering the layer of gravel. The sheeting will create a vapor barrier between the soil and the basement floor slab, preventing moisture from seeping through the slab into the basement.

Position the edges of the sheets onto the walls, so that the edge rises to a height of about 2 inches along the wall base. Overlap each new row of sheeting by 4 inches over the edge of the plastic already on the floor. Cut the sheets at the end of each row with a utility knife, then tape the overlapping seams with fiberglass tape.

Lay polystyrene insulation boards over the sheets, covering the entire foundation base. Place the boards in rows along the length of the basement, butting the edges together. Cut the board with a utility knife at the end of each row to make it fit snugly. Use the portion cut away to begin the next row of boards, to stagger board placement. Cut the last row of boards lengthwise as well, to make fit them into whatever space is left.