Heat Conduction Calculator
Enter layer conductivities and thicknesses (up to three) plus area and ΔT to get the U-value, R-value and heat flow through the assembly.
Last updated: 2026-09-28
How the calculation works
- Each layer adds resistance: thickness ÷ conductivity. Series resistances simply add.
- U-value is the reciprocal of total R — the number energy codes quote.
- Heat flow multiplies U by area and temperature difference; watts convert to BTU/h for US cross-reference.
Formula
R = Σ (t/k) U = 1/R Q = U · A · ΔT
| Symbol | Meaning | Unit |
|---|---|---|
R | Total thermal resistance | m²·K/W |
U | U-value (thermal transmittance) | W/m²·K |
Q | Heat flow | W |
Worked example
Interpreting the result
The U-value here is conduction-only: real assemblies add inside/outside air films (~R 0.18 m²·K/W combined) and lose performance to thermal bridging — studs every 400 mm can degrade a wall's effective R by 15–25%. Doubling insulation thickness halves conduction through that layer but the law of diminishing returns bites: the first 100 mm saves far more than the next 100. Compare walls on R, not thickness — 50 mm of good insulation beats 100 mm of concrete every time.
Assumptions
- Steady-state one-dimensional conduction; no air films or bridging included.
- Room-temperature conductivities; k rises slightly at extreme temperatures.
Limitations
- Not a whole-building energy model — windows, infiltration and thermal mass are separate.
- Cavity walls with ventilated air gaps need the gap treated explicitly, not as a solid layer.
Frequently asked questions
What is the difference between U-value and R-value?
They are reciprocals: U = 1/R. R measures resistance to heat flow (higher is better, US convention); U measures how much heat passes (lower is better, European convention).
How do I calculate heat loss through a wall?
Q = U × A × ΔT. A 20 m² wall at U 0.36 with a 25 °C inside-outside difference loses 180 W — multiply by heating hours for the energy figure.