Rigid Foam Board
Thick rigid foam board is a suitable insulation to put under concrete slabs. Foam insulation has a higher R-value per inch than most other insulation materials, including fiberglass and cellulose. Even 1- or 2-inch-thick foam board will provide good insulation.
The foam board must be installed on a level bed of washed gravel, which is itself covered by a vapor barrier. Use only insulation tape for sealing the seams. After the foam insulation board is installed, cover it with a layer of sand to provide protection when you pour the concrete for your garage.
A type of radiant barrier insulation for concrete slabs is called “Under Concrete Barrier.” It is usually made of a double layer of polyethylene bubbles enclosed between layers of 99 percent pure aluminum on one side and white polyethylene on the other.
This insulation is used in a variety of insulating applications, including driveways, basement floors and concrete garage slabs. Installation is just a matter of rolling the insulation out, fitting it and sealing the seams with foil tape.
Another product to consider is made from polystyrene (a type of rigid foam board insulation) sandwiched between heavy-duty black polyethylene sheets. It is specifically designed to minimize the coldness and dampness usually associated with concrete slabs in a garage. The insulation is very durable and can withstand foot traffic during installation.
The installation steps entail laying the form on top of a level surface. Make sure that the insulation is butted closely together for a seamless installation. For this product, a proprietary double-sided adhesive is used for sealing seams. To completely insulate the slab, it is recommended that you extend the material up the footer form of the slab.