Generally, installing insulation under a concrete slab in a garage is not incorporated into the building of a new home. Usually, a homeowner does not think about insulation under the garage floor until the cold of the winter. At that point, it is impractical to insulate under the existing slab without tearing up the existing floor or installing an insulate floor on top of the slab. It is best to plan for and install insulation under the concrete slab from the start.
Rigid foam board
Thick rigid foam board is a suitable insulation to put under concrete slabs. Foam insulation has a higher R-value per cm than most other insulation materials, including fibreglass and cellulose. Even 2.5 to 5 cm (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 vapour 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 polythene bubbles enclosed between layers of 99 percent pure aluminium on one side and white polythene 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 polythene sheets. It is specifically designed to minimise 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.