U Value measures the overall rate of heat transfer through a building assembly, expressed in Btu/(h·ft²·°F), with lower values indicating better insulation performance. U Value is the inverse of R Value and accounts for all materials, air films, and resistance in a complete building section. A U value of 0.10 means 0.10 Btu of heat passes through one square foot per hour for each degree Fahrenheit of temperature difference.
Calculation and Standards
U Value is calculated by adding the R values of all layers in an assembly, then dividing 1 by the total. A wall with R-13 insulation, sheathing (R-2), and air films (R-1.5) totals approximately R-16.5, yielding a U value of 0.061. Building codes require maximum U values ranging from 0.027 in cold climates to 0.055 in moderate climates.
Application in Building Design
U values guide decisions for wall, roof, and foundation assemblies. Manufacturers provide U values for windows, doors, and skylights to help builders meet energy code requirements. Lower U values reduce heating and cooling loads, lowering energy costs and improving occupant comfort year-round.