Heat conduction through a plane wall

P = k · A · ΔT / L

Good insulation has low conductivity; area and temperature difference increase loss.

• Heat flow P — Steady heat flow without surface resistances.
• Thermal conductivity k — Material property; mineral wool about 0.035–0.045 W/(m·K).
• Area A — Wall area normal to heat flow.
• Temperature difference ΔT — Inside minus outside temperature magnitude.
• Wall thickness L — Thickness in heat-flow direction.

0.04 W/(m·K) · 10 m² · 20 K / 0.8 m = 10 W.

Plane homogeneous wall and steady one-dimensional heat flow.