U-Factor measures how well a window prevents heat from escaping. It is rated between 0.14 and 1.20. The lower the U-Factor, the better the window insulates. For the DC/Virginia/Maryland area (IECC Zone 4), ENERGY STAR requires U-Factor of 0.30 or less.

Solar Heat Gain Coefficient measures how much solar radiation passes through the window. Rated 0 to 1 -- lower means less solar heat enters. In mixed climates, a balance is key: low enough to reduce cooling costs but high enough for passive solar heating in winter.

R-Value is the inverse of U-Factor (R = 1/U). It measures thermal resistance -- how well the window resists heat flow. Higher R-Values mean better insulation. A double-pane window with Low-E typically achieves R-3 to R-4, while triple-pane can reach R-5 to R-8.

VT measures how much visible light passes through. Rated 0 to 1. Higher VT means more natural daylight. Most Low-E coatings reduce VT somewhat, but modern soft-coat Low-E keeps VT above 0.60 while still blocking infrared heat.

CR rates how well a window resists moisture buildup on a scale of 1 to 100. Higher is better. Warm-edge spacers, multi-pane glazing, and gas fills all improve CR. A rating above 60 is considered good for the DMV climate.

Low-emissivity coatings are microscopically thin metallic layers applied to glass. They reflect infrared heat while letting visible light through. Soft-coat Low-E is the most common in new construction. Triple-silver Low-E offers the best solar control for sun-exposed windows.