Heat Conduction Through a Wall
Steady Heat Conduction Through a Plane Wall
How much heat leaks through a wall, window or insulation layer? This worksheet applies Fourier’s law of conduction to a single homogeneous layer at steady state.
Fourier’s law for a plane wall is Q = kAΔT/L, where k is the thermal conductivity, A the area, ΔT the temperature difference across the wall and L the thickness.
Inputs
A rigid-foam insulation panel.
k := 0.04 W/(m·K)=0.04 kg·m/K/s^3
A := 12 m^2=12 m^2
dT := 20 K=20 K
L := 0.1 m=100 mm
Conductive heat rate
Q := k·A·dT/L=96 W
Results
The result is the steady heat-flow rate in watts. Lower conductivity, greater thickness or smaller area all cut the loss. For a multi-layer wall, sum the thermal resistances R = L/(kA) in series and divide ΔT by the total.
Assumptions & limitations
- Steady state — no thermal storage/transient.
- One-dimensional conduction, single homogeneous layer.
- Surface (convective/radiative) resistances excluded — add them for a U-value.