The “U-value,” also known as the thermal transmittance or heat transfer coefficient, is a measure of the rate of heat transfer through a building component, such as a wall, roof, or window. It quantifies the amount of heat that passes through one square meter of the component for every degree Celsius of temperature difference between the indoor and outdoor environments.
U-value is expressed in units of watts per square meter per degree Celsius (W/m²·°C) or sometimes in British thermal units per hour per square foot per degree Fahrenheit (BTU/h·ft²·°F). A lower U-value indicates better thermal insulation properties and reduced heat transfer.
The U-value of a building component depends on several factors, including the material properties, thickness, and the presence of insulation. Generally, materials with higher thermal resistance, such as insulation materials, have lower U-values.
U-values are used in building energy calculations and thermal analysis to assess the energy efficiency and thermal performance of buildings. By reducing the U-value of a building envelope, less heat is lost during the winter or gained during the summer, resulting in improved energy efficiency and thermal comfort.
It’s worth noting that U-values are often used in conjunction with other factors, such as solar heat gain coefficient (SHGC) for windows or air leakage rates, to provide a comprehensive understanding of a building’s energy performance. National and international building codes often specify maximum U-values for different building components to ensure energy efficiency standards are met.