Understanding Window Energy Ratings for Ontario

Window and Door Replacement

Window energy ratings help you compare products and estimate performance before buying — but the numbers need context to be meaningful for Ontario's heating-dominated climate.

ENERGY STAR certification identifies windows that meet minimum energy performance criteria for Canada's climate zones. Ontario falls in Climate Zone 2 in the ENERGY STAR window program, which specifies minimum U-factor and solar heat gain coefficient (SHGC) requirements for certified products.

Energy Efficiency and Style

U-factor measures the rate of heat transfer through the window assembly. Lower is better — a U-factor of 0.20 loses heat at half the rate of a U-factor of 0.40. ENERGY STAR Zone 2 requires a maximum U-factor of 1.22 W/m²K for certification. Premium triple-pane windows achieve U-factors of 0.80 or lower.

Solar heat gain coefficient (SHGC) measures how much solar radiation passes through the glass. In a heating-dominated climate like Ontario's, south-facing windows with higher SHGC (0.40 or above) are desirable — they allow passive solar heat gain during winter. North, east, and west-facing windows benefit from lower SHGC to reduce unwanted heat gain in summer.

Getting a Free Estimate

Visible light transmittance (VLT) measures how much visible light the glass passes. Higher VLT provides brighter interior spaces. Low-e coatings that reduce solar heat gain also typically reduce VLT somewhat — there's a trade-off between solar control and interior brightness that must be balanced based on the window's orientation and function.

Condensation resistance rating predicts how well a window resists interior surface condensation. In Ontario's cold winters, condensation on cold glass surfaces is a comfort and moisture problem. A higher condensation resistance rating indicates better interior glass surface temperature, reducing the likelihood of condensation forming on the glass.