Floors located over an unheated crawl space or unheated garage need to be insulated to prevent loss of heat from the house. The higher cost of energy in today’s economy makes adding flooring insulation in these areas cost effective. These spaces will often have no finished ceiling on the floor underside; in these cases, some form of support will have to be added for the insulation materials.
Type of Insulation
For fiberglass batt insulation or rigid foam, a good support solution is wire lathing (chicken wire) attached to the bottom of the floor joists with tacking. If loose-fill insulation, such as cellulose fiber or vermiculite, then some type of solid support is used. It must be permeable so that water vapor does not become trapped if it should penetrate the vapor barrier. For example, sheets of 11mm thick fiberboard sheathing can be attached to the bottom of the floor joists; a thin film vapor barrier will go between the joists and the fiberboard sheathing.
Take particular care to tightly fit around any bridging or blocking between floor joists with rigid or batt insulation. Avoid compression and bunching in batts in these areas.
Insulation RSI Value
With carpeting and a T & G plywood subfloor, loose fill insulation should provide you with a floor with an RSI-Value of around 5. Due to the lack of a solid support, batt or rogid insulation will provide a floor with a slightly smaller RSI-value of around 4.75.
Although insulating floors over unheated spaces will help reduce heat loss, the floor may still feel cold if it is over a larger area like a garage. In such cases, the best solution will be to provide a heated space under the subfloor. This can be accomplished by installing heating registers along floor joists, then attaching studs to the floor joists and mounting a plywood subfloor onto the studs.
Photo by Giles Douglas, Creative Commons Attribution License