| Assembly | Area | R-Value | U-Factor | Heat Loss | % of Total |
|---|
Roof and ceiling heat loss fundamentals
Heat rises, so the roof and ceiling are always under pressure from warm interior air trying to escape. In a cold Canadian climate the temperature difference across a flat attic assembly can be 60-80°F (33-44°C) for months at a time. Even a high-R assembly loses significant heat at that scale. A 1,200 ft² attic with R-40 blown insulation at a 65°F ΔT loses about 1,950 BTU/hr — roughly 4% of a typical house heating load.
Vented attic vs. cathedral ceiling vs. flat roof
A vented attic with insulation at the ceiling plane uses outdoor air temperature as the boundary condition — the full indoor-to-outdoor ΔT. A cathedral ceiling (insulation in the rafter cavity) works the same way. A flat roof with a conditioned plenum above uses the plenum temperature, which is warmer than outdoor air — reducing the effective ΔT and heat loss slightly. This calculator uses outdoor design temperature for all assembly types, which is conservative and correct for a peak heating load calculation.
Canadian roof insulation requirements
NBC 2020 requires minimum R-40 effective for ceilings in Climate Zone 6 (most of Ontario and BC). Quebec and Alberta have similar requirements. Many programs (R-2000, ENERGY STAR, Net Zero) require R-50 to R-60 for flat attics. See the energy code calculator for your province's specific requirements. The U-value calculator helps determine effective R-value for complex roof assemblies.
Frequently Asked Questions
NBC 2020 requires minimum R-40 effective (RSI 7.0) for attic/ceiling assemblies in Climate Zone 6 (most of Ontario and BC interior). Climate zones 7 and 8 (northern Canada, prairie winters) require R-50 to R-60. ENERGY STAR for New Homes requires R-60 in most Canadian markets. Many programs like R-2000 and Net Zero Ready require R-60 to R-80. Blown cellulose or fiberglass at R-60 requires about 16-18 inches of depth. The payback on upgrading from R-20 to R-60 in a cold Canadian climate is typically 3-6 years.
A flat roof with insulation on top (inverted or conventional assembly) and a vaulted ceiling with insulation in the rafter cavity lose heat at similar rates for the same effective R-value, because both use outdoor air as the boundary condition. The difference is in how much insulation you can achieve. A flat roof can accept unlimited thickness of rigid insulation. A vaulted ceiling is constrained by rafter depth, typically 2x10 or 2x12 giving maximum R-30 to R-38 in the cavity. Adding rigid insulation above the roof deck or below the rafters is often needed to meet code minimums.