Installing insulation prior to the pouring of a concrete slab floor improves the energy efficiency of the floor and makes it more comfortable. While you can apply insulation over the concrete slab, that requires adding wood subflooring and other components that add to the cost and complexity of the project. Adding insulation under the concrete floor requires the same skill set and equipment as forming and pouring the concrete slab.
Excavate the area under the slab to 12 inches below the planned level of the top of the slab. This allows an area for insulation and fill material under the slab.
Place 4 inches of gravel in the excavation. Any under-slab plumbing pipes or wiring goes in this level of gravel. Rake the gravel level across the entire floor area.
Lay a layer of 6mm plastic across the floor. The plastic serves as a vapour barrier preventing moisture from passing up through the gravel to the concrete floor above.
Place 2-inch thick pieces of rigid styrofoam insulation on top of the plastic sheeting. Place the insulation edge to edge, using duct or other construction tape to hold the insulation together. When completed, the entire slab area has a cover of rigid sheets of styrofoam.
Add insulation on the inside edges of the form. This offers an insulation factor protecting the slab from cold infiltration from the outside edges. To accommodate this, build the forms 4 inches larger in each dimension.
Add 2 inches of sand on top of the insulation. Use a rake to level the sand on top of the insulation. Add any reinforcing rod or reinforcing wire to the top of the sand.
Pour the concrete on top of the sand. Use shovels and rakes to level the concrete before using a screed, a straight board, run across the top edge of the forms to level the concrete. Trowel the concrete to a smooth finish.