How Modern Windows Actually Work

U-factor measures how much heat passes through a window in watts per square metre per degree (W/m²·K). Lower is better. A single-pane window has a U-factor around 5.0, while a modern double-pane with Low-E and argon is 1.4–1.6. For BC's climate (Zone 4–6), aim for 1.40 or lower to qualify for Energy Star and rebate programs.

Solar Heat Gain Coefficient measures how much solar radiation passes through the glass on a scale of 0 to 1. It depends on orientation: south-facing windows benefit from higher SHGC (0.30–0.40) to gain free solar heat in winter, while west-facing windows should have lower SHGC (0.20–0.25) to prevent summer overheating. Most BC homes do well with a balanced SHGC between 0.22 and 0.35.

Both are Cardinal Glass coatings. Low-E 270 is a "passive" double-silver coating with SHGC ~0.34 — it lets in more solar heat, ideal for north and east windows. Low-E 366 is a "solar control" triple-silver coating with SHGC ~0.22 — it blocks more heat gain, ideal for south and west exposures. Many homes benefit from using both types based on window orientation.

Triple-pane adds 30–50% to the glass cost but provides a U-factor of 0.8–1.0 (vs 1.4–1.6 for double). It's most worthwhile for large north-facing windows, bedrooms where you want zero drafts, noisy street-facing windows (adds significant sound reduction), and homes pursuing Passive House or Net-Zero certification. For most BC homes, double-pane with Low-E and argon offers the best value.

Argon is an inert gas that's 34% less conductive than air. When sealed between glass panes, it slows both convection (gas movement) and conduction (direct heat transfer). This improves the insulating value by about 30% compared to air-filled units. It's colourless, odourless, non-toxic, and will slowly dissipate over 20+ years without any safety concern.

Look for four key numbers: U-factor (heat loss, lower is better), SHGC (solar heat gain, context-dependent), VT or Visible Transmittance (natural light, 0.40–0.70 is typical), and ER or Energy Rating (net energy performance, higher is better — 25+ minimum for Energy Star in Zone 2). The ER is unique to Canada and accounts for both heat loss and solar gains.