The flow rate of heat through a building product is known as the U-value. The U-value (or U-factor) is a measure of the flow of heat (thermal transmittance) through a material, given a difference in temperature on either side. In the inch-pound (I-P) system, the U – factor is the number of British thermal units (Btu) of energy passing through a square foot of the material in an hour for every degree Fahrenheit difference in temperature across the material (Btu/ft2-h-°F). In the metric system, it is usually given in watts per square meter per degree Celsius (W/m2*°C).

Since the U-value is a measurement of heat flow, the lower the li­vable, the more slowly does the material transfer heat in and out of the home. The U-value typically is used in expressing overall thermal con­ductance, since it is a measurement of the rate of heat flow through the complete heat barrier, from room air to outside air. The lower the U-value, the better is the insulating value. U-value is the customary unit used by the fenestration industry to quantify conducted heat gain or loss. With other building materials, such as insulation, roofing, and flooring materials, the R-value is frequently used for conducted heat gain or loss.