Replacing windows in Canada: what U-factor, Energy Rating and ENERGY STAR mean

Replacement windows are expensive, and most of the difference between products is in numbers on a label that buyers rarely read. Knowing what they mean makes quotes comparable.

U-factor and Energy Rating

U-factor measures heat loss through the window, so a lower number is better. The Energy Rating (ER) combines heat loss, air leakage and solar heat gain into one number, so a higher number is better. ER cannot be converted directly into an R-value because it includes solar gain.

One national ENERGY STAR level since 2020

NRCan's version 5.0 specification, effective January 1, 2020, removed the old climate zones and applies one level across Canada: a maximum U-factor of 1.22 W/m2.K, or a minimum ER of 34 for windows and doors. Older guides that list zones 1, 2 and 3 describe the previous rules.

Most Efficient

NRCan's 2025 ENERGY STAR Most Efficient criteria for windows and sliding glass doors list a U-factor of 1.05 W/m2.K or lower, or an ER of 40 or higher.

Compare quotes on the same basis

Ask each supplier for the U-factor and ER of the exact model and glazing being quoted, not of a product line in general. Check that the label or documentation matches the product delivered.

Installation decides performance

A good window installed badly leaks air and water. Flashing, sealing between the frame and the wall, and the rough opening details matter as much as the label; see the article on window flashing and rough-opening protection.

Prevention Checklist

• Ask for the U-factor and ER of the exact model and glazing quoted.
• Check it meets ENERGY STAR (U-factor 1.22 or lower, or ER 34 or higher).
• Ignore old climate-zone charts; they are outdated.
• Compare quotes on the same numbers.
• Ask how the installer will flash and seal the openings.

References

1. Natural Resources Canada. Windows, doors and skylights: ENERGY STAR Canada technical specification, Web page, Version 5.0 levels. Accessed 2026-11-01.
2. Natural Resources Canada. Most Efficient criteria for windows and sliding glass doors 2025, Web page, Most Efficient levels. Accessed 2026-11-01.

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