| 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.
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. For a sealed (conditioned) crawlspace, the floor above it has no heat loss at all — 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 R-28 effective for floors over unconditioned spaces in Climate Zone 6. Many provinces match or exceed this: Ontario SB-12 requires R-28 for floors over unheated garages and crawlspaces in most heating degree day zones. The U-value calculator helps you determine effective R-value for your specific floor assembly. Check the energy code calculator for your province's requirements.
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 R-28 effective for floors over unheated crawlspaces in Climate Zone 6 (most of Ontario). Quebec and Alberta have similar requirements. ENERGY STAR requires R-30. For floors over unheated garages, R-28 is also the standard minimum. In cold climate zones 7 and 8 (northern Canada), R-36 to R-40 is recommended. Insulation is typically installed between floor joists with rigid board or spray foam to eliminate thermal bridging at the joist edges.