The U-value, also known as the U-factor or coefficient of heat transmission, is a measure of the rate of non-solar heat loss or gain through a material or assembly. U-values gauge how well a material allows heat to pass through. U-value ratings generally fall between 0.20 and 1.20. The lower the U-value, the greater a product's resistance to heat flow and the better its insulating value. The inverse of (one divided by) the U-value is the R-value.


U-value is expressed in units of W/m2 °C or Btu/hr-sq ft °F. In the US, values are normally given for NFRC/ASHRAE winter conditions of 0°F (-18°C) outdoor temperature, 70°F (21°C) indoor temperature, 15 mph wind, and no solar load.


U-values are often quoted for windows and doors. In the case of a window, for example, the U-value may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials.


To reduce U-factors, some manufacturers apply a low-E (low-emittance) coating to glazing surfaces. These low-E coatings reduce heat loss, improving both heating and cooling performance. Windows can also be assembled to improve thermal performance. Some assembly strategies include using two or more layers of panes or films, low-conductance gas fills between the layers, and thermally improved edge spacers, which are placed between the panes.


The sash and frame of a window represent 10% to 30% of a window's total area, depending on the window size and design. The material used to manufacture the frame can thus impact heat loss and related condensation resistance. In colder climates, in non-residential buildings, where aluminum frames are used, thermal breaks should be specified in order to minimize heat transfer and condensation on the frames. In colder climates, with residential buildings, most products use wood, vinyl, or other non-metallic frames.


Some door frames will also conduct heat readily. For solid doors, insulated metal or fiberglass doors are usually the best choice.


Window coverings, such as shades, shutters, and insulating or storm panels, can help reduce heat loss too.