The formula
P = k · A · ΔT / e
| Symbol | Quantity | Unit |
|---|---|---|
| P | Heat per second | W |
| k | Thermal conductivity | W/(m·K) |
| A | Area | m² |
| ΔT | Temperature difference | K |
| e | Thickness | m |
What it means
Conduction moves heat through a material without the material moving. The rate depends on what it is made of, how large the area is, how big the temperature difference is, and how thick it is.
How to work it out by hand
- Multiply the conductivity by the area
- Multiply by the temperature difference
- Divide by the thickness
What is worth knowing
The thickness sits in the denominator, so doubling the insulation halves the heat loss — but only halves it, which is why insulation has diminishing returns and why the first centimetres matter far more than the last. Conductivities span four orders of magnitude: copper is about 400 W/(m·K), glass around 1, and still air 0.026. That last figure is the point of most insulation: fibreglass, wool and foam work by trapping air, and the fibres are there to stop it circulating rather than to insulate themselves. Conduction is also only one of three routes — convection and radiation carry heat in parallel, and a real wall loses heat by all three.