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Heat loss through building parts

The heat loss through a wall, roof or window is the U-value times the area times the temperature difference. The sum over all surfaces gives the building's transmission loss. Windows often have a much higher U-value than walls.

Φ=U A ΔT\Phi = U\,A\,\Delta Theat loss through a surface
Q=Φ⋅tQ = \Phi\cdot tenergy over time

Symbols

Φ\Phiheat loss (power)W
AAaream²
ΔT\Delta Ttemperature differenceK

Example

A 2 m² window with U=0.8U = 0.8 and a 25 K difference:

Φ=0.8⋅2⋅25=40\Phi = 0.8\cdot 2\cdot 25 = 40 W.

A temperature difference in K and in °C is the same, so either can be used.
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← U-value · Moisture and dew point →

Part of Building and Structures: Building physics: heat and moisture.