What is thermal transmittance or U-factor (U) used to measure?

Thermal transmittance, commonly referred to as the U-factor, is a measure of the rate of heat transfer through a building element, such as walls, windows, roofs, and doors, for a unit area per degree of temperature difference between the inside and outside environments. The U-factor is expressed in units of watts per square meter per degree Celsius (W/m²·°C) or BTUs per hour per square foot per degree Fahrenheit (BTU/h·ft²·°F).

This measurement is crucial in understanding how well a building component can insulate against heat loss in the winter or heat gain in the summer. A lower U-factor indicates better insulating properties, meaning that less heat is transmitted through the building material, contributing to energy efficiency and comfort within a structure.

The focus on heat flow through unit areas highlights the U-factor's role in building design and energy calculations, making it an essential concept in energy efficiency evaluation and overall thermal performance assessments for buildings.