Floors over Unconditioned Spaces
Floors over unheated crawl spaces or over garages must be insulated. It is common to fill the space between the floor sheathing and a gypsum board ceiling under the floor with insulation.
Where there is no finished ceiling on the underside of the floor, the insulation must be supported (to prevent the insulation from falling through). For friction-fit batts or rigid insulation (Figure 23), attach wire lath, “chicken wire”, wood furring or a vapour permeable sheet material to the bottom of the joists.
For loose fill-type insulation, provide solid support with a vapour-permeable material (to avoid creating a vapour barrier on the cold side of the insulation).
Fit insulation around cross-bridging or blocking between joists and insulate small spaces such as those between blocked double joists or joist spaces below a wall. The NBC requires that a vapour barrier be installed on the warm side of the insulation, which is usually the upper side in a floor over an unconditioned space. A plywood or OSB subfloor with tight-fitting or sealed joints can serve as the vapour barrier.
To provide the required insulating value, additional insulation may have to be installed below the insulated framing space. Install wood framing perpendicular to and underneath the floor framing. Fill the framing cavity with insulation, which will cover the underside of the floor framing and thus prevent thermal bridging.
Source : Canada Mortgage and Housing Corporation (CMHC)