| Assembly | Area | R-Value | ΔT Fraction | Effective ΔT | Heat Loss |
|---|
Floor heat loss over unconditioned spaces
A floor over an unconditioned space loses heat just like a wall or ceiling, through conduction from warm inside to cold outside. The difference is the boundary temperature. A vented crawlspace isn't as cold as the outdoor design temperature because it's partially sheltered. A cantilevered floor or a floor directly over an open garage is essentially exposed to outdoor conditions and should be treated with the full ΔT. A persistently cold floor over any of these spaces is also a common reason people look at the radiant floor heat calculator as a comfort fix rather than relying on insulation alone.
Crawlspace temperature assumptions
For a vented crawlspace, ASHRAE recommends using 0.75 × (indoor – outdoor) ΔT as the effective temperature difference. This accounts for the crawlspace being somewhat buffered from the worst outdoor conditions, a buffering effect Natural Resources Canada's insulation guide also relies on when recommending crawlspace insulation strategies. For a sealed (conditioned) crawlspace, the floor above it has no heat loss at all, since the crawlspace itself becomes conditioned space. For a semi-conditioned garage, use 0.5 × ΔT. For an open cantilever, use 1.0 × ΔT. For detailed basement perimeter calculations, use the basement heat loss calculator.
Canadian code minimums for floors over unconditioned spaces
NBC 2020 requires effective R-26.5 (RSI 4.67) for floors over unconditioned spaces in Climate Zone 6, per NAIMA Canada's published thermal requirements tables; many contractors round this up to R-28 in practice for a margin of safety. Provincial codes can set their own values on top of this baseline. The U-value calculator helps you determine effective R-value for your specific floor assembly.
Frequently Asked Questions
ASHRAE recommends 0.75 for vented crawlspaces. The crawlspace temperature is warmer than outdoor air because the ground, insulated walls, and limited air exchange buffer it from the coldest conditions. At an outdoor design temperature of -20°C, the crawlspace might be at -10°C, giving an effective delta T of 0.75 times the full indoor-to-outdoor difference. For an open cantilever directly exposed to outdoor air, use 1.0. For a semi-conditioned basement with some heat source, use 0.5.
NBC 2020 requires effective R-26.5 for floors over unheated crawlspaces in Climate Zone 6 (most of Ontario), though many builders round up to R-28 for a margin of safety. Quebec and Alberta have similar requirements. ENERGY STAR requires R-30. For floors over unheated garages, R-26.5 to R-28 is also the standard minimum range. In cold climate zones 7 and 8 (northern Canada), R-36 to R-40 is recommended. Check the energy code calculator for your specific province's requirements. Insulation is typically installed between floor joists with rigid board or spray foam to eliminate thermal bridging at the joist edges.