Window U-Value Explained: Uw, U-Factor, ER and SHGC for Canadian Homes

A U-value, called U-factor on North American labels, is the rate of heat flow through a product, in watts per square metre Kelvin (W/m²K). A window at 1.0 loses one watt per square metre for every degree between inside and outside.

Imperial values (Btu/h·ft²·°F) convert by a factor of 5.678263: divide metric by it, or multiply imperial by it. So 1.22 W/m²K is about 0.21 imperial, and the KF 410’s best value of 0.62 is about 0.11. It matters: NRCan says windows, doors and skylights can account for up to 25% of a home’s energy loss.

Both use W/m²K, but under different standards. A European Uw is calculated to EN ISO 10077 for one reference size. Canadian and US ratings follow CSA A440.2 or NFRC 100/200, at standard sizes that vary by operator type. NFRC assumes −18 °C outside and 21 °C inside; ISO uses 0 °C and 20 °C.

A 2014 paper by RDH Building Science found that NFRC, ISO and Passive House U-values “cannot be compared”, with differences of up to 25% for the same frame and glass; for triple-glazed, low-conductivity frames the European figure is typically lower. Use Uw to compare European windows with each other, not as a code number: SB-12 and ENERGY STAR Canada both require ratings to CSA A440.2 or NFRC.

ENERGY STAR Canada v5.0, in force since 1 January 2020, certifies windows at U 1.22 W/m²K or less, or ER 34 or more, with air leakage of 1.5 L/s/m² or less. NRCan says certified windows are about 20% more energy efficient than the average window. Rebates such as Ontario’s Home Renovation Savings Program need the exact model to be certified, so check NRCan’s list first.

SB-12 places every GTA location in Zone 1, under 5,000 heating degree-days: Toronto City Hall is 3,520, Oakville 3,760 and Vaughan (Woodbridge) 4,100. Most Zone 1 packages with a high-efficiency furnace need windows at U 1.6 or ER 25. Above 17% glazing-to-wall area the limit tightens a step, and above 22% the performance path applies, which is common for glassy custom homes in Toronto.

Condensation forms when room air meets a surface below its dew point, usually the glass edge or the frame. For GTA climates the Ontario Building Code requires windows at U 2.0 or better, or a Temperature Index of at least 68 from the CSA A440.2 test. Low Uf and Psi keep those edges warm: with Internorm’s standard double Low-E coating, at −10 °C outside and 23 °C inside, the room-side glass stays around 21 °C. More in our article on condensation-resistant European windows.

InterHaus is the official Internorm dealer in Canada, with a showroom in Oakville and local warehousing in Ontario. Our product specialists have an architectural background and support architects, builders and energy modellers with data sheets for each glass build-up, Passive House Institute certified window systems and installation supervision. Compare every model on our tilt and turn windows page, or start with the compact KF 310.