What to know
- Lower U-factor means less heat moves through the complete window or door assembly.
- Higher Energy Rating generally indicates better overall energy performance for Canadian heating conditions.
- Compare ratings for the complete product, not a centre-of-glass number.
- Confirm the exact model in Natural Resources Canada’s searchable certified-product list.
Start with the whole product
A window is a system: glass, spacers, sash, frame, weather seals and hardware all affect performance. A centre-of-glass value describes only the glass and can make a product sound better than the complete-unit rating homeowners actually need to compare.
Canadian energy performance is tested under the CSA A440.2 standard. Certified ratings are independently verified, which gives you a common basis for comparing products from different manufacturers.
U-factor: heat loss through the assembly
U-factor measures the rate of heat transfer. The lower the number, the better the assembly resists heat loss. It is the clearest number for comparing insulation performance, especially in colder regions or on exposed elevations.
Do not confuse U-factor with R-value. Contractors sometimes quote an R-value, but Natural Resources Canada notes that R-value is not part of the window energy-performance standards. Ask to see the certified whole-product U-factor instead.
Energy Rating: a Canadian comparison score
Energy Rating, or ER, balances U-factor, solar heat gain and air leakage. A higher ER means better potential annual energy performance under the rating method. ER can be useful when comparing complete products designed for Canadian conditions.
A high ER does not automatically make one window ideal for every wall. Orientation, shading, summer cooling and room comfort still matter. Use the rating as a comparison tool, then discuss glass coatings and solar gain by elevation with the installer.
Solar heat gain and visible light
Solar heat gain coefficient, or SHGC, describes how much solar heat passes through the product. Higher solar gain can help on an unshaded south-facing wall in a heating-dominated climate, while lower solar gain may help control overheating on west-facing glass.
Visible transmittance indicates how much visible light passes through. Darker coatings and additional panes may change daylight as well as thermal performance, so compare samples rather than assuming every high-performance glass package looks the same.
A five-minute quote check
Before signing, ask the company to put the following information in writing for the exact model and size being supplied.
- Manufacturer, model or series, and certified product reference
- Whole-product U-factor and Energy Rating
- Solar heat gain coefficient for the selected glass package
- Whether the quoted ratings apply to the exact operating style and size
- Installation method and who is responsible for air sealing and water management
Sources and further reading
This article provides general information, not building, health, financial or rebate advice. Verify requirements for your home, jurisdiction and program before proceeding.