U-Value Heat Loss Calculator

The U-value (thermal transmittance) measures how readily heat passes through a building assembly — a wall, roof, floor or window — and it drives the conduction portion of a building's heating and cooling load. This calculator computes the conduction heat loss or gain from the assembly's U-value, its area, and the temperature difference across it, using the fundamental relationship Q = U·A·ΔT. It also returns the equivalent R-value (thermal resistance) and the heat flux per square foot, in US units, so you can evaluate insulation, compare assemblies, and feed envelope loads into an ASHRAE or Manual J calculation.

U-value and R-value are reciprocals: U = 1/R. A lower U-value (higher R-value) means better insulation and less heat transfer. The conduction heat loss through an assembly equals its U-value times its area times the temperature difference between inside and outside. This conduction load, summed over all the walls, roof, glazing and floor, is a major component of a building's total heating and cooling load, alongside solar gain, infiltration, ventilation and internal gains. Energy codes such as the IECC and ASHRAE 90.1 set maximum U-values (minimum R-values) by climate zone.