| 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; the attic is also usually the single cheapest place to add insulation, a point Natural Resources Canada's home insulation guide makes when ranking where retrofit dollars go furthest.
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. For the vented attic case specifically, the attic ventilation calculator checks that soffit and ridge venting are adequate, since poor ventilation can trap moisture and undermine the insulation's performance over time.
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), a figure NAIMA Canada's published thermal requirements table confirms for that zone. 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.