Skip to content
ENExplore Calculators

Thermal Resistance Calculator

Combine layers of insulation into a total thermal resistance and U-value.

About Thermal Resistance Calculator

Thermal resistances in series add, exactly like electrical resistors. The U-value is the reciprocal of the total, which is why adding insulation gives diminishing returns as R grows. A single thermal bridge can dominate the effective U-value of an otherwise well-insulated assembly, because it provides a parallel low-resistance path.

Formula

Thermal Resistance

R_total = Σ (thickness ÷ conductivity) + surface resistances; U = 1 ÷ R_total

R
Thermal resistance of a layer
U
Overall transmittance, the reciprocal of total resistance

Worked example

With the default values below, Thermal Resistance Calculator returns Total Thermal Resistance of 2.8843. Change any input above and every figure here is recalculated in your browser.

Inputs used

Layer 1 thicknessminimum 0 m
0.1 m
Layer 1 conductivityminimum 0 W/m K
0.04 W/m K
Layer 2 thicknessminimum 0 m
0.15 m
Layer 2 conductivityminimum 0 W/m K
0.7 W/m K
Combined surface resistancesminimum 0 m2 K/W
0.17 m2 K/W

Results

Total Thermal Resistance
2.8843 m2 K/W
U-Value
0.3467 W/m2 K
Layer 1 Resistance
2.5000 m2 K/W
Layer 2 Resistance
0.2143 m2 K/W
Heat Loss per Square Metre at 20 K Difference
6.934 W/m2

Step by step

  1. 1Values used
  2. 2Thermal Resistance
  3. 3Total Thermal Resistance= 2.8843 m2 K/W
  4. 4U-Value= 0.3467 W/m2 K
  5. 5Layer 1 Resistance= 2.5000 m2 K/W
  6. 6Layer 2 Resistance= 0.2143 m2 K/W
  7. 7Heat Loss per Square Metre at 20 K Difference= 6.934 W/m2

Assumptions and limits

  • The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Thermal Resistance calculated?

R_total = Σ (thickness ÷ conductivity) + surface resistances; U = 1 ÷ R_total Thermal resistances in series add, exactly like electrical resistors. The U-value is the reciprocal of the total, which is why adding insulation gives diminishing returns as R grows.

Why does Thermal Resistance matter?

A single thermal bridge can dominate the effective U-value of an otherwise well-insulated assembly, because it provides a parallel low-resistance path.

What values do I need to enter?

This calculator takes 5 inputs: Layer 1 thickness, Layer 1 conductivity, Layer 2 thickness, Layer 2 conductivity, Combined surface resistances. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Related Calculators

Thermal Resistance Calculator | Calcrivo