Thermal Bridging: What It Is, Where It Happens & How to Fix It

Summary

Thermal bridging is the silent killer of energy-efficient buildings. You can install 270mm of loft insulation, fill the cavity, and double-glaze every window, but if the junction details are poorly executed, a significant proportion of that invested energy performance is lost through cold bridges that bypass the insulation.

In modern UK building regulations, the effect of thermal bridges is explicitly accounted for in SAP (Standard Assessment Procedure) calculations used for building control compliance and EPC ratings. Poor junction detailing can reduce a building's energy rating by a whole band and dramatically increase heating bills relative to the design intent.

Thermal bridges matter not just for energy performance but for building pathology. A cold internal surface — typically below 12–13°C in a normally heated room — will attract condensation and subsequently mould growth. This is the mechanism behind the persistent mould problem in the corners of rooms above windows, behind radiators at external walls, and at the floor/wall junction in solid-floor properties. Identifying and treating thermal bridges is therefore both an energy and a health issue.

Key Facts

• Psi (Ψ) value — linear thermal transmittance in W/mK; quantifies heat loss per metre length of a junction or linear thermal bridge; lower is better
• Chi (χ) value — point thermal transmittance in W/K; for point bridges such as fixings and wall ties
• Thermal bridging in SAP — total heat loss from thermal bridges (HTB) is calculated as the sum of (Ψ × length) for all junctions; typically adds 10–30% to the total fabric heat loss
• Accredited construction details (ACDs) — pre-calculated Ψ values for standard UK construction details; free from MHCLG; using ACDs allows simplified SAP calculation
• Temperature factor (fRsi) — minimum surface temperature relative to indoor/outdoor temperature difference; fRsi ≥ 0.75 required to avoid condensation risk in BS EN 13788 assessment
• Most common bridge types — wall/floor junction, wall/roof junction, window and door reveals, steel beam penetrations, concrete columns in timber frame
• Mould growth threshold — surface temperature below 12.6°C in a room at 20°C, 70% RH creates mould growth conditions
• Robust Details — pre-approved construction details for separating walls/floors in new build multi-occupancy; include thermal bridge data
• Joist hanger cold bridge — joist hangers penetrating a cavity wall are common; thermally broken versions available (Ancon HoldFast THB)
• Thermally broken fixings — stainless steel/plastic composite fixings (Teplo-BFT ties, Ancon angular brackets) significantly reduce cold bridge effect of metal penetrations
• Party walls — at the party wall/external wall junction, heat can transfer to the neighbouring property; psi values for party wall junctions must be included in SAP

Quick Reference Table

Detailed Guidance

Where Thermal Bridges Occur

1. Ground floor / wall junction The most thermally significant junction in most UK houses. Heat travels from the warm floor and wall directly into the ground and externally through the wall base. In uninsulated ground floor/wall junctions, this can account for 25% of total junction heat loss in a well-insulated house.

Mitigation for retrofits:

• Install vertical perimeter insulation (50–75mm PIR board) on the inner face of the external wall at floor level, extending 500mm above the floor
• Where internal floor insulation is added (floating floor), extend the insulation vertically up behind the skirting by at least 150mm
• For new build: the ACD (Accredited Construction Detail) for the floor/wall junction must be selected and implemented; typically involves 25–50mm PIR wrapped continuously from the floor insulation up behind the wall insulation

2. Window and door reveals Windows and doors are set back in the wall thickness, creating a reveal — a surface that connects the inner warm face of the wall to the window frame. Brick or blockwork reveals have significantly higher thermal conductivity than the insulated wall, creating a cold bridge that shows as condensation and mould at the inner corners of window openings.

Mitigation:

• Slim PIR board (20–25mm) or aerogel blanket (10–15mm) applied to the reveal face, covered with plasterboard or render
• Aerogel is particularly useful in reveals where space is limited (window jambs and sills)
• Ensure the reveal insulation is continuous with the wall insulation — no gaps at the junction
• Deep window sills in historic properties are a persistent thermal bridge issue; the sill board itself is less critical, but the masonry jamb is significant

3. Steel beams (RSJs) penetrating or spanning through walls The worst domestic thermal bridge. A steel RSJ spanning an opening with ends bearing on the inner leaf of a cavity wall has both ends exposed to the cold outer leaf and the ground, conducting heat away. In a 3m wide opening, a typical 203 × 203 UC steel column or RSJ can have a Ψ value of 0.5–1.5 W/mK — an enormous cold bridge.

Mitigation:

• Size the beam to minimise the bearing length penetrating into the cold zone
• Box in the beam end with PIR insulation and plasterboard before closing up the wall
• Use proprietary thermally broken bearing pads (Schöck Isokorb, Ancon structural thermal break)
• Where beams are fully visible inside (exposed steel aesthetic), accept the thermal bridge and ensure the area around the beam is well-ventilated to reduce condensation

4. Lintels above windows and doors Steel cavity lintels span the window opening and bear on the inner and outer leaf. The steel conducts heat between warm inner leaf and cold outer leaf. This is visible in thermal imaging as a horizontal cold band across the wall above every window and door opening.

Mitigation:

• Use insulated cavity lintels (IG Lintels Hi-Therm, Teplo-L): these have a thermal break insert that separates the inner and outer steel components
• For retrofit: this is difficult to address without removing the window. Where mould is severe above windows, applying internal insulation above the window and across the lintel reveal is the practical option

5. Wall ties in cavity construction Standard metal wall ties (butterfly pattern, vertical twist) conduct heat between inner and outer leaf. Each tie contributes approximately 0.002 W/K to heat loss — trivial individually, but with 2.5 ties per m² across a building's wall area, the cumulative impact is measurable.

For new build, thermally broken wall ties (Ancon Teplo-BFT, Halfen HTA) dramatically reduce tie conductivity. For retrofit, wall ties cannot be changed.

Thermal Imaging for Diagnosis

Infrared thermography (a thermal camera) is the most effective tool for identifying thermal bridges in existing buildings. Cold areas on internal surfaces appear as darker blue/purple areas on the thermal image.

Effective thermal imaging conditions:

• Temperature difference between inside and outside of at least 10°C (ideally 15°C or more)
• Stable conditions — the building should have been heated normally for at least 24 hours
• No direct sunlight on the external face being imaged (for external surveys)

Thermal imaging is available from specialist surveyors and is increasingly cost-effective (£200–£600 for a domestic survey). It is particularly useful for diagnosing:

• Cold bridges before undertaking targeted remediation
• Quality assurance of new insulation installation
• Pre-purchase surveys of older properties

Accredited Construction Details (ACDs)

MHCLG publishes a set of Accredited Construction Details with pre-calculated Ψ values for standard UK construction types. Using ACDs in a SAP calculation avoids the need for bespoke finite element modelling (ISO 10211) of each junction.

The ACD publication covers:

• Ground floor/wall junctions
• Intermediate floor/wall junctions
• Eaves details
• Gable wall details
• Window and door reveals
• Party wall junctions

Each detail has a schematic and an associated Ψ value. As long as the built construction matches the detail closely, the Ψ value can be used in the SAP calculation. Deviating from the ACD without recalculating the Ψ value is a compliance risk.

Frequently Asked Questions

How do I know if thermal bridging is causing my condensation and mould problem?

Check the location of the mould. Mould in the corners of rooms above windows, at floor/wall junctions, behind furniture pushed against external walls, and at ceiling/wall corners on external walls is classic thermal bridge condensation. If the mould appears on the middle of an external wall surface (rather than at junctions or corners), it may be moisture penetration or ventilation failure rather than a thermal bridge. A thermal camera survey will confirm the diagnosis.

Can I reduce thermal bridging with better ventilation instead of insulation?

Ventilation removes moisture-laden air and reduces the relative humidity, which raises the surface temperature at which condensation occurs. Increased ventilation can manage the symptoms of thermal bridging (preventing condensation on cold surfaces) but does not address the energy loss. In an occupied home, running more ventilation increases heating bills. The correct solution is to address the cold bridge, not to ventilate the problem away.

What is an aerogel blanket and when should I use it?

Aerogel is a silica-based insulation material with an extremely low thermal conductivity (0.012–0.015 W/mK) — approximately twice as effective as PIR per millimetre. It is extremely expensive (£30–£60/m²) but is the only practical solution in constrained spaces. Window reveals, around structural steel, at heritage wall junctions, and in floor perimeter details where only 10–15mm of space is available are the applications where aerogel is worth the cost premium.

Does a thermal bridge need to be included in a SAP calculation?

Yes. All significant linear thermal bridges (wall/floor junction, eaves, window reveals, etc.) must be included in the SAP calculation for new build compliance with Part L. The assessor uses either ACDs (with published Ψ values) or bespoke values from ISO 10211 calculation. Using a blanket thermal bridging factor (y = 0.15 W/m²K applied to total exposed area) is allowed in SAP 10.2 but penalises well-detailed buildings; bespoke values always produce better results when the construction is well-detailed.

Regulations & Standards

• Building Regulations Approved Document L (2022) — thermal bridging must be minimised and calculated in SAP assessments

• BS EN ISO 10211 — thermal bridges in building construction; heat flows and surface temperatures; calculation method

• BS EN 13788 — hygrothermal performance; surface temperatures to avoid critical surface humidity (fRsi method)

• BS EN ISO 14683 — thermal bridges in building construction; linear thermal transmittance; simplified methods and default values

• MHCLG Accredited Construction Details — free published ACD library with Ψ values for standard UK junction types

• MHCLG: Accredited Construction Details — free download of all ACD junction details with Ψ values

• BRE: Thermal Bridging — BRE Report BR 497 on thermal bridges in dwellings

• Kingspan: Thermal Bridging Guide — practical manufacturer guidance

• IStructE: Structural Thermal Breaks — guidance on thermal breaks for structural connections

• solid wall — thermal bridging in solid wall insulation systems

• cavity wall — thermal bridging in cavity construction

• loft insulation — eaves cold bridge detailing

• floor insulation — floor perimeter thermal bridging

• structural steel — steel beam cold bridging through walls