How do you read a window’s energy performance?

On a serious sheet or quote, look mainly at: U-factor (heat loss), SHGC (solar gain), and air-tightness results. ENERGY STAR® often summarizes that a threshold is met.

How to read the numbers

U-factor: the lower the number, the better the window holds heat. SHGC: the lower it is, the less the sun warms the interior (useful on the south in summer; a bit less free gain in winter). Airtightness shows whether the window “whistles” less in strong wind — critical in Quebec.

Concrete examples

Two quotes with the same size can show a different U depending on double vs triple, Low-E, argon or Solarban. A 6 mm + 4 mm double does not always show on a single “U” line: also ask what you’re aiming for (noise vs pure insulation). ENERGY STAR® confirms a threshold is met, not that two windows are identical.

What matters as much as the numbers

Installation. An excellent window poorly installed loses part of its performance. Also compare the frame, opening type and real options — not just a marketing line.