Building Heat Loss (U-Value)
Building Heat Loss (U-Value)
The U-value combines all the layers of a wall or roof into one heat-transfer coefficient used directly for heat-loss calculations.
The heat loss is Q = U·A·ΔT. where U is the overall heat-transfer coefficient, A the area and ΔT the inside-to-outside temperature difference.
Inputs
U := 0.3 W/(m^2·K)=0.3 kg/K/s^3
A := 25 m^2=25 m^2
dT := 20 K=20 K
Multiply U-value, area and temperature difference.
Q := U·A·dT=150 W
Results
Lower U-values (more insulation) cut the loss directly. Summed over all surfaces and multiplied by degree-hours, this drives the heating energy bill.
Assumptions & limitations
- Steady state.
- U-value includes all layers and films.
- Uniform temperatures.