What is the U-value?

• A. The ratio of indoor air temperature to outdoor
• B. The measure of a material's moisture content
• C. The rate of heat transfer through a building element; lower U-value means better insulation.
• D. The total energy used by a building in a year

Answer: (C)

U-value measures how much heat moves through a building element per square meter for each degree of temperature difference between indoors and outdoors. It directly quantifies thermal performance: a lower U-value means less heat loss or gain, so the element insulates better. This is why energy efficiency aims for low U-values on walls, roofs, and windows, since it reflects how effectively a material resists heat flow (the inverse concept is R-value).

It isn’t about the ratio of indoor to outdoor temperatures, nor about moisture content, nor about total yearly energy use. Those factors are not a direct measure of heat transfer through a component, whereas the U-value specifically captures the rate at which heat can transfer through it.