U-factor is a measure of how easily heat flows through a building element, like a window, wall, roof or door. Metric U-factor is measured in watts per square meter per Kelvin (W/ m2·K).
U-factor can be expressed in metric units (W/m2·K) or imperial inch-pound units (Btu/h·ft2·F). To convert imperial U-factor to metric, multiply the imperial number by 5.678. A lower U-factor is better. It means more resistance to heat flow, better insulating value and better energy efficiency.