| Window Group | Area | U-Factor | SHGC | Heating Loss | Solar Gain (cooling) |
|---|
Window heat loss and solar gain in HVAC calculations
Windows are the weakest thermal link in most building envelopes. A double-pane low-e window has a U-factor of 0.28-0.32 BTU/hr·ft²·°F — roughly 6-8x worse than a well-insulated wall at R-20 to R-25. In a home with 300 ft² of windows, the window heat loss in Edmonton (-22°F design) can exceed 20,000 BTU/hr — nearly equal to the wall heat loss for the entire house. Every window upgrade has an outsized effect on total heating load.
U-factor vs. SHGC: two different properties
U-factor governs heat loss in winter (conduction through the glass and frame). SHGC (Solar Heat Gain Coefficient) governs solar energy transmitted in summer. These can be optimized independently with modern glazing: a triple-pane low-e window can have U=0.18 (excellent insulation) with SHGC=0.20 (low solar gain for west/south exposures) or SHGC=0.45 (higher solar gain for passive solar south windows). For detailed solar gain analysis by orientation and month, use the solar gain calculator.
Windows in Canadian energy codes
NBC 2020 and provincial energy codes (Ontario SB-12, BC Energy Step Code) set maximum window U-factors for new construction. In climate zone 6 (most of Ontario), the maximum prescriptive window U-factor is 1.6 W/m²·K (0.28 BTU/hr·ft²·°F). Check the compliance calculator for your province and climate zone requirements.
Frequently Asked Questions
NBC 2020 and provincial codes set maximum U-factors for windows in new construction. Climate Zone 6 (most of Ontario, BC lower mainland): maximum U-1.6 W/m2·K (U-0.28 BTU/hr·ft2·°F). Climate Zone 7 (prairies, northern Ontario): maximum U-1.4 W/m2·K (U-0.25). Many programs require better: ENERGY STAR Canada requires U-1.22 (U-0.22) for most Canadian zones. Triple-pane windows typically achieve U-0.8 to U-1.2 W/m2·K (U-0.14 to U-0.21). The investment in triple pane pays back in 8-15 years in most Canadian climates from energy savings alone, with additional comfort benefits from warmer glass surfaces.
In a heating-dominated Canadian climate, south-facing windows benefit from higher SHGC (0.35-0.50) to capture free solar heat in winter. This passive solar gain can reduce heating load by 5-15% in a well-designed home. North, east, and west windows should have lower SHGC (0.20-0.30) to reduce unwanted summer heat gain. The optimal strategy is high-SHGC south glass with proper overhangs sized for the latitude to block summer sun while allowing winter sun. Use the solar gain calculator for detailed monthly analysis by orientation.