What is the formula to determine the U-factor of a building material?

The U-factor of a building material is determined by taking the reciprocal of its R-value. The U-factor measures the rate of heat transfer through a building element, such as walls, windows, or roofs, and is expressed in units of BTUs per hour per square foot per degree Fahrenheit (BTU/h·ft²·°F).

R-value represents the resistance to heat flow, so a higher R-value indicates better insulating properties. The relationship between U-factor and R-value reflects the fact that materials with high R-values (better insulators) will have lower U-factors. Therefore, by using the formula of 1 divided by the R-value, you accurately calculate how much heat will transfer through the material, which is critical for determining energy efficiency in building design.

Understanding this relationship helps energy raters assess materials and ensure they meet energy efficiency standards for residential buildings.