I keep seeing ER rating and U-factor - what's the difference and which matters more?

Both ratings matter, but they measure different things — and for Ottawa homeowners, ER (Energy Rating) is often the more practical number to focus on when comparing windows.

U-factor measures how well a window prevents heat from escaping. It's expressed as a number — the lower the better. A U-factor of 1.22 W/m²·K or lower is the ENERGY STAR threshold for Canada's most demanding climate zones, which includes Ottawa. Think of U-factor as the window's "thermal resistance" score. It's a straightforward measure of insulation performance.

ER (Energy Rating) is a Canadian-specific number developed by Natural Resources Canada, and it's more comprehensive. It combines three factors into a single score: heat loss through the frame and glass (U-factor), solar heat gain through the glass, and air leakage. The higher the ER number, the better. A window with a high ER is actually doing net positive work — it's capturing more solar energy than it's losing. The minimum ER for ENERGY STAR certification in Ottawa's climate zone is 34 for most window types.

Why ER often matters more in Ottawa: Because our winters are long and cold, a window that captures passive solar heat while minimizing heat loss is genuinely valuable. Two windows can have identical U-factors but very different ER scores depending on their solar heat gain coefficient and air tightness. The ER gives you the full picture in one number.

That said, both ratings work together. If you're comparing windows, look for an ER of 34+ and a U-factor at or below 1.22. Triple-pane windows with Low-E coatings and argon fills typically score well on both. South-facing windows especially benefit from optimized solar heat gain, which the ER captures but U-factor alone doesn't.

If you'd like help comparing specific window options for your home, Ottawa Window Pros offers free consultations — we can walk you through the ratings on the products we carry and recommend the best fit for your exposure and budget.