Thermal bridge
A localised area of a building envelope with significantly higher heat flow than its surroundings, caused by geometry or more conductive materials.
A thermal bridge (or cold bridge) is where insulation is interrupted, penetrated or thinner, or where geometry concentrates heat loss. Common examples: concrete balcony slabs cantilevered through the facade, steel lintels, wall-floor and wall-roof junctions, window reveals, timber or steel studs, and fixings through external insulation.
Point thermal bridge: Q = χ x ΔT (χ in W/K)
Total heat loss coefficient: H = Σ U·A + Σ ψ·L + Σ χ
Example: balcony ψ = 0.7 W/m·K, 10 m, ΔT 20 K: 140 W
Typical ψ values range from about 0.01 W/m·K for well-detailed junctions to 0.5–1.0 W/m·K for uninsulated balcony slabs. In conventional buildings, thermal bridges can add 10–30% to fabric heat loss; passive house design targets "thermal-bridge-free" detailing with ψ ≤ 0.01 W/m·K at external dimensions.
Besides heat loss, thermal bridges lower internal surface temperatures, which can cause condensation and mould. The temperature factor f_Rsi (minimum surface temperature relative to indoor-outdoor difference) should typically exceed 0.75 to avoid mould risk (ISO 13788). Solutions include structural thermal breaks for balconies, continuous external insulation and insulated window installation. Bridges are assessed with 2D/3D numerical models per ISO 10211 or with thermography.
Related terms
All terms →- U-value
The thermal transmittance of a building element: heat flow in watts per square metre per kelvin of temperature difference (W/m²K). Lower is better.
- R-value
Thermal resistance of a material layer or assembly, its ability to resist heat flow; the reciprocal of U-value. Higher is better.
- Passive house (Passivhaus)
A rigorous building performance standard, developed in Germany in the 1990s, achieving very low heating demand through insulation, airtightness and heat-recovery ventilation.
- Energy Performance Certificate (EPC)
A standardised rating of a building's calculated energy performance, typically on an A–G scale, required in the EU and UK when buildings are sold or rented.
- Heating degree days (HDD)
A measure of how much and for how long outdoor temperature falls below a base temperature, used to estimate and normalise building heating demand.