What is thermal bridging?

Thermal bridging (also called cold bridging) occurs where heat passes more easily through a localised area of a building's fabric than through the surrounding insulated construction. Common examples are steel lintels that span the cavity (conducting heat directly from inside to outside), concrete floor slabs that project through the insulation layer, window reveals, and metal wall ties. At a thermal bridge, the inner surface temperature drops, increasing the risk of surface condensation and mould growth.

A psi value (linear thermal transmittance, symbol psi, unit W/mK) quantifies the additional heat loss per metre length of a junction due to thermal bridging, beyond what the U-values of the intersecting elements would predict. Common junctions with significant psi values include the wall-floor junction, wall-roof junction, and window reveals. Psi values for standard junction details are published in the BRE Conventions (BR 497) and are required inputs for SAP calculations under Approved Document L.

How do you prevent thermal bridging?

Thermal bridging is minimised by maintaining a continuous, unbroken insulation layer around the building envelope. Practical measures include: using insulated cavity closers at window and door reveals; specifying thermally broken lintels (a steel lintel incorporating an insulation break); extending floor insulation over the floor edge; using wider wall ties with reduced conductivity; and ensuring the insulation layer wraps around structural projections. On Passivhaus projects, every junction detail is specifically designed to eliminate thermal bridges.

Why does thermal bridging cause damp and mould?

At a thermal bridge, the inner surface of the wall or ceiling is colder than the surrounding construction. When moist indoor air comes into contact with a surface that is at or below the dew point temperature, water vapour condenses on the surface as liquid water. This condensation promotes mould growth and can eventually cause damage to plaster, paint, and structural timber. This is why eliminating thermal bridges is as important as achieving good U-values.

Thermal & Energy noun

Thermal Bridging

/ ˈθɜːm(ə)l ˈbrɪdʒɪŋ /

Also known as: cold bridge, cold bridging, thermal bridge

Definition

Thermal bridging (cold bridging) is a localised area of a building envelope where heat flows significantly more readily than through the surrounding insulated construction. It occurs wherever a material of higher thermal conductivity - such as steel, concrete, or masonry - interrupts or bypasses the insulation layer, creating a path of least resistance for heat to escape from the warm interior to the cold exterior.

In practice

The most common thermal bridges in UK construction are: steel lintels spanning the full cavity width; concrete floor slabs that project through the wall to form a balcony or floor edge; window reveals where the two masonry leaves come together; and metal wall ties. Each of these creates a localised cold spot on the inner surface of the wall.

The inner surface temperature at a thermal bridge is lower than the surrounding wall. When this temperature falls below the dew point of the room air, condensation forms on the wall surface, often causing mould growth and damage to finishes. Even if condensation is not visible, extra heat is lost at every bridge, increasing energy bills. Thermally broken lintels, insulated cavity closers, and careful junction detailing are the standard solutions.

Building Regulations

Relevant PartPart L - Conservation of fuel and power

MetricPsi (Ψ) values in W/mK - linear thermal transmittance

ReferenceBRE IP 1/06 and BR 497 junction library

Approved DocumentAD L (2021) - required in SAP calculation

Approved Document L requires that thermal bridging is accounted for in SAP energy calculations for new dwellings. Where Accredited Construction Details (ACDs) are used, published psi values may be applied. Where non-standard junctions are used, psi values must be calculated from first principles using numerical modelling software. Approved Document C also requires that the inner surface temperature at repeating thermal bridges is assessed to avoid surface condensation risk.

Full Building Regulations guidance

Related Calculators

Insulation Calculator

Thermal Bridging

Just so you know...