What Is a Thermal Bridge? — Cold Bridges Explained

Definition

A thermal bridge (also called a cold bridge) is a localised area in the building envelope where heat escapes more readily than through the surrounding fabric. This occurs where there is a break or reduction in the insulation layer, typically at junctions between building elements such as wall-to-floor, wall-to-roof, and around openings. The additional heat loss is quantified by the psi-value (Ψ), measured in W/(m·K), which represents the extra heat flow per metre length of the junction.

When Is It Used?

Thermal bridging must be considered in all new construction and major renovations to comply with Part L of the Building Regulations. SAP calculations require either default thermal bridging values, Accredited Construction Details, or bespoke psi-value calculations for each junction. Thermal bridge analysis is also critical for Passivhaus design, where thermal bridge-free construction (Ψ ≤ 0.01 W/(m·K)) is targeted.

Key Facts

• Thermal bridging can account for 20-30% of total fabric heat loss in a well-insulated building
• SAP default y-value (0.15 W/(m²·K)) is three times worse than ACDs (0.05-0.08)
• Steel lintels without thermal breaks can have psi-values of 0.3-0.5 W/(m·K)
• Thermally broken lintels (e.g., Catnic, IG) reduce psi-values to 0.05-0.15 W/(m·K)
• The temperature factor (f-factor) must exceed 0.75 to prevent mould growth
• Insulated cavity closers are essential at window and door reveals
• Thermal imaging (thermography) can reveal thermal bridges in existing buildings
• Passivhaus targets thermal bridge-free junctions: Ψ ≤ 0.01 W/(m·K)

Related Calculators

Use the U-Value Calculator for planar fabric performance, or the Heat Loss Calculator to see the total impact of thermal bridging. See also the SAP glossary entry and the Part L U-Values reference table.