Thermal Bridges: How to Detect and Eliminate Them

Thermal bridges are the Achilles heel of insulation. You can stack 25 cm of mineral wool in the loft, but if the junction between the upper floor and the wall is left untreated, that is where heat escapes and condensation sets in. They account for up to 25% of heat loss in a poorly designed house, and the French RE2020 building regulation has tightened the rules: the psi (ψ) value of each thermal bridge is now capped. Here is how to understand them, where to look for them, and above all how to address them from the design stage.

What exactly is a thermal bridge

A thermal bridge (also called a cold bridge) is a zone in the building envelope where the thermal resistance is locally reduced. Heat takes this path of least resistance to escape to the outside, just as water finds a crack in a dam.

In practice, it is a discontinuity in the insulation layer. Either because a conductive material (concrete, metal, timber) passes through the insulation, or because the geometry creates a corner zone where the external exchange surface is larger than the internal one.

Three direct consequences

1. Energy loss: heat escapes and the heating bill rises.
2. Discomfort: the internal surface is cold, giving a sensation of a cold wall or floor nearby.
3. Condensation and mould: warm, humid room air condenses on the cold zone. Over time, black stains appear, plaster deteriorates, and health problems can follow.

The three families of thermal bridges

Not all thermal bridges are treated in the same way. Building standards distinguish three categories based on their geometry.

Linear thermal bridges (psi, in W/m.K)

These are lines where the insulation layer is interrupted. They are expressed in psi (ψ), in watts per linear metre per degree. They are the most numerous and have the greatest impact on the thermal balance.

Examples: intermediate floor/wall junction, ground floor slab/wall, wall/roof, window and door frame surrounds, internal load-bearing wall passing through.

Point thermal bridges (chi, in W/K)

These are points where the insulation is punctured. They are expressed in chi (χ), in watts per degree.

Examples: balustrade fixings, cladding brackets, canopy or point-balcony anchors, service penetrations.

Integrated thermal bridges

Present within the wall thickness itself: timber frame (studs pass through the insulation), metal stud profiles in a steel-frame wall, mortar joints in a blockwork wall insulated from the inside. They are accounted for in the wall’s U-value, not in the junction calculations.

Where they hide: mapping a house

In a typical two-storey house, the same culprits always appear. Here is the list of junctions to treat in priority order, from most to least penalising.

1. Intermediate floor / external wall junction

The worst thermal bridge in a two-storey concrete-built house. The floor slab passes through the internal insulation and exits to the outside. Without treatment, psi can reach 0.9 to 1.1 W/m.K all around the floor perimeter.

2. External wall / ground floor slab junction

When the ground floor slab runs under the wall without a break, it conducts cold from the ground into the interior. Typical psi: 0.5 to 0.9 W/m.K.

3. External wall / roof junction (wall plate)

The top of the wall under the roof wall plate, if poorly treated, is a major thermal bridge. It is also a common air leakage point.

4. Window and door frame surrounds

The perimeter of every opening is a linear thermal bridge. Typical psi: 0.1 to 0.3 W/m.K, but multiplied by the full perimeter of all windows and doors, the total becomes significant.

5. Internal load-bearing walls passing through

An internal structural wall that passes through the facade wall to anchor into the structural concrete creates a T-shaped thermal bridge. Particularly common in poured concrete construction.

6. Cantilever balconies

The king of thermal bridges in concrete construction. A balcony slab continuous with the internal floor slab conducts cold across its entire surface: psi can exceed 1.5 W/m.K at the junction.

7. Flat-roof parapets

The upstand of the parapet wall, if not insulated on all three faces, drains heat from the top floor.

Decision tree: which solution for which case

Practical solutions by zone

Solution 1: External Wall Insulation (EWI)

This is the winning reflex. By wrapping the house in a continuous insulating layer applied to the outside, virtually all linear thermal bridges are eliminated in one go. Only point bridges (fixings) and window/door surrounds then need addressing.

See our full guide External Wall Insulation (EWI): principles and installation.

Solution 2: structural thermal break connectors

When Internal Wall Insulation (IWI) is retained, the continuity of the concrete must be interrupted at junctions. This is the role of thermal break connectors: load-bearing elements incorporating high-performance rigid insulation, cast into the structure at pour time.

Solution 3: insulation return into window and door reveals

The window or door frame surround is a key detail. Three good practices can be combined:

1. Position the frame at the outer face of the wall (on the insulation side), not in the middle or at the inner face.
2. Return the insulation into the reveal by a minimum of 2 to 4 cm, to cover the frame outer member.
3. Use an insulating pre-frame (compriband / expansion foam tape or insulated cavity closer) between the frame and the masonry.

This combination brings psi down to 0.05-0.10 W/m.K, compared with 0.3-0.5 without treatment.

Solution 4: cross-layered insulation in timber frame

In timber-frame construction, the vertical studs pass through the insulation and create an integrated thermal bridge. The solution is to add a second, cross-layered insulation layer on the inside (between the counter-batten furring) or on the outside (as wood-fibre board panels). The thermal pathways compensate each other and the wall U-value drops by 15 to 20%.

Solution 5: a continuous vapour control layer (VCL)

In any construction, warm humid air will try to pass through. A continuous, airtight vapour control layer (VCL), fitted on the warm side of the insulation, prevents vapour from condensing within the wall depth or at a thermal bridge. See Airtightness and the blower door test.

Detecting existing thermal bridges

For a renovation project, or to validate a new build, three tools are essential.

Thermal imaging (thermography)

An infrared camera reveals cold zones in winter (viewed from inside) or warm zones (viewed from outside). Thermal bridges show up as dark blue/purple from inside and red/yellow from outside.

• Conditions: minimum 10 °C temperature difference between inside and outside, with no direct sunlight on the facade for 2-3 hours beforehand.
• Cost of a thermographic survey: €200-500.
• Cost of a camera hire: €100-200 for 24-48 hours.

Air permeability test (blower door)

Combined with a smoke pencil or thermal camera, the air leakage test reveals airtightness defects, which often coincide with thermal bridges. Mandatory under RE2020 (French building regulation) and the UK equivalent Part L. See Airtightness and the blower door test.

Visual indicators

Without any equipment, certain signs are unmistakable:

• Black stains in corners, behind furniture: condensation on an angle thermal bridge.
• Dust deposits in vertical or horizontal bands on an internal wall: linear thermal bridge (frame, structural partition).
• Sensation of cold draughts near a window or door frame: airtightness defect or reveal thermal bridge.
• Skirting board or paint blistering at the base of a wall: wall/ground slab thermal bridge.

RE2020 building regulation

RE2020 (French building regulation) does not set a single global Ubat value as RT2012 did. It sets two main indicators: Bbio (bioclimatic need) and Cep (primary energy consumption). However, thermal bridges are calculated and directly affect both indicators.

Capped psi values

Although there is no mandatory psi threshold in RE2020, certain technical documents (Avis Techniques, DTA) impose ceiling values:

To calculate the psi values for a project, the tabulated values from the Th-Bat rules (CSTB document 3501) or dedicated software (Pleiades, ULys) are used. For self-builders, the thermal consultant carrying out the mandatory RE2020 study will provide these values.

Costs and trade-offs

Treating thermal bridges has a cost. Here are the typical figures for a standard RE2020 house of 100 m2, two storeys, concrete construction.

For equivalent performance, EWI is generally more cost-effective than a succession of individual thermal break connectors. This is why it has become the reference for high-performance new builds. Unless there is a planning constraint (local plan requiring a particular facade appearance) or an aesthetic reason (exposed stone wall), EWI resolves 90% of the problem.

Common mistakes to avoid

Further reading

Treating thermal bridges is part of a continuous-insulation approach to the building envelope. See also:

• External Wall Insulation (EWI): principles and installation: the most radical solution.
• Ground floor insulation: techniques and thicknesses for the base.
• Understanding RE2020: what changes for self-builders to frame the requirements.
• Airtightness and the blower door test: the inseparable companion to thermal bridge treatment.
• Tabulated psi values are published in document 3501 of the CSTB (Th-Bat rules).
• Structural thermal break documentation: Schöck (Isokorb) and Halfen (HIT).

Key takeaways

A thermal bridge is a zone where insulation is interrupted: heat rushes through it, condensation sets in. In a two-storey concrete house, thermal bridges can account for up to 25% of heat loss. The simplest and most cost-effective solution remains External Wall Insulation (EWI), which eliminates virtually all linear thermal bridges in a single operation. If you stay with IWI, you will need to order structural thermal break connectors before the slab is poured, then treat every detail rigorously. In all cases, ask your thermal consultant for a detailed psi calculation junction by junction, and schedule a thermographic survey in winter to validate the result.