Two metrics that define window thermal performance
U-value measures the rate at which a window conducts heat across its entire assembly (frame, glass, spacers). A low U-value (under 0.20 W/m2K in high-performance windows) means less heat escapes in winter and less enters in summer. SHGC (Solar Heat Gain Coefficient) is the fraction of solar radiation that enters the building through the window. A south-facing window in a cold climate should have low U-value (to retain heat) and high SHGC (to collect winter sun). The same window in a hot climate should have low U-value and low SHGC (to block summer heat). Neither metric alone determines window performance; both must be considered in context of orientation and climate.
Specifying windows by climate and orientation
Building codes and energy rating systems (NFRC in North America) publish window specifications by geography. New York might call for U-values below 0.22 and SHGC below 0.40 on south facades (prioritizing solar gain control), while Minnesota allows higher SHGC (0.60+) on south to capture free heating. Dynamic window technologies (electrochromic smart glass, switchable films) can adjust SHGC hour by hour, providing optimum performance across seasons, though at higher cost. Low-emissivity (low-E) coatings and argon gas fills between panes are standard in modern windows, reducing U-values to 0.15 or lower. Proper frame sealing and installation are equally critical: a poorly installed premium window performs no better than a standard one.