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What Is a Thermal Bridge? — Cold Bridges Explained
A plain-English guide to thermal bridges in UK construction: psi-values, common junctions, SAP impact, condensation risk, and mitigation strategies.
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.
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How We Calculate This
Frequently Asked Questions
They are the same thing. "Thermal bridge" is the technical term, while "cold bridge" is the common trade term. Both refer to an area of the building envelope where heat transfers more readily than through the surrounding fabric, creating a localised cold spot. The technical measure is the linear thermal transmittance or psi-value (Ψ), measured in W/(m·K).
The most significant thermal bridges occur at: wall-to-floor junctions (especially at ground floor level), window and door reveals, lintels (particularly steel lintels without thermal breaks), wall-to-roof junctions (eaves and verge), corners of external walls, balcony connections, and around steel beams penetrating the insulation layer. Each junction type has a psi-value that quantifies the additional heat loss.
SAP uses the y-value (thermal bridging factor) to account for heat loss through junctions. If Accredited Construction Details (ACDs) or calculated psi-values are used, the y-value can be as low as 0.05 W/(m²·K). If default values are used (because junction details have not been assessed), the y-value is 0.15 W/(m²·K) — three times higher. This can add 15-25% to the total fabric heat loss and significantly worsen the SAP rating.
Accredited Construction Details (ACDs) are a set of standardised junction details published by the government that have been thermally modelled to minimise thermal bridging. Using ACDs allows a y-value of 0.08 W/(m²·K) in SAP calculations. Better psi-values can be achieved with bespoke thermal modelling of each junction using software such as Therm or Psi-Therm, which may be worthwhile for Part L compliance.
Yes. Thermal bridges create cold spots on internal surfaces where the surface temperature drops below the dew point, causing condensation. This leads to mould growth, which is a health hazard and damages building fabric. The f-factor (temperature factor) at a junction must be above 0.75 to avoid mould risk in normal living conditions (as per BS EN ISO 13788). Proper insulation continuity and thermal break details are essential.
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Last updated: March 2026
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