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    Building physics / Thermal bridges

    Thermal Bridge Calculator

    A thermal bridge is where insulation is interrupted, and the extra heat loss is the linear thermal transmittance times the length. The same calculation gives the interior surface temperature that decides whether the detail condenses.

    Heat loss125 WTemperature difference26.0 KSurface temperature19.9 °C
    Heat loss
    125 W
    Through the detail
    Temperature difference
    26.0 K
    Inside to outside
    Surface temperature
    19.9 °C
    Estimated at the bridge
    Annual energy
    250 kWh
    At 2,000 full-load hours
    Sensitivity to ψ
    ψ is a property of the detail; this multiplies it by the length and the temperature difference.
    Sensitivity to ψ
    ψ (W/m·K)Loss at this length
    0.0231 W
    0.0578 W
    0.08125 W
    0.12187 W
    0.2312 W
    Inspect the model
    Choose a quick result view, a scaled comparison, or the complete diagnostic set.
    Heat loss against outside temperature
    Linear in the temperature difference.
    Heat loss against outside temperature0.00038.7177.41116.12154.83193.54-15-15-10-10-5-50055Thermal Bridge Calculator
    Canonical equations
    The exact relationships implemented by this engine.

    H = ψ · L · ΔT · The linear thermal bridge.

    Tsi ≈ Ti − ψ·Rsi·ΔT·n · The condensation check.

    engineering methodology

    A clear path from inputs to results.

    A thermal bridge is where insulation is interrupted, and the extra heat loss is the linear thermal transmittance times the length. The same calculation gives the interior surface temperature that decides whether the detail condenses.

    Step-by-step methodWorked exampleAssumptions explained

    How to use

    1. Set linear transmittance ψ (w/m·k).
    2. Set detail length (m).
    3. Set inside temperature (°c).
    4. Set outside temperature (°c).
    5. Read the headline metrics, then check the table for the detail.

    The formula

    H=ψ⋅L⋅ΔTH = ψ · L · ΔT

    Reference: H = ψ·L·ΔT; ψ from the detail, ISO 10211 or the manufacturer.

    Worked example

    At the default inputs

    Inputs
    • Linear transmittance ψ (W/m·K): 0.08; Detail length (m): 60; Inside temperature (°C): 21; Outside temperature (°C): -5; Inside surface resistance (m²K/W): 0.13
    Calculation path
    1. 1

      Heat loss: 125 W · Temperature difference: 26.0 K · Surface temperature: 19.9 °C · Annual energy: 250 kWh

    Result

    Heat loss: 125 W · Temperature difference: 26.0 K · Surface temperature: 19.9 °C

    Interpreting the result

    60 m of a ψ 0.08 detail loses 125 W at a 26 K difference.

    Assumptions & limitations

    • ψ is constant along the detail.
    • No three-dimensional corner effects.

    Frequently asked questions

    Where does ψ come from?

    A two-dimensional heat-flow model of the detail, tabulated in ISO 14683 or by the manufacturer.

    Why does it matter for condensation?

    The bridge is colder, so it is where mould appears first.

    Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.