Thermal bridges (Ψ, χ)

Thermal bridges are areas where the thermal resistance of the building envelope is weakened: the insulation is no longer continuous, heat escapes more quickly, and the interior surface temperature drops (risk of condensation/mold). A distinction is made between linear bridges, denoted Ψ (psi, in W/m·K) – floor/facade joints, window and door frames, roof parapets – and point bridges, denoted χ (chi, in W/K) – fixings, balcony brackets, anchors. When properly addressed, both Ψ and χ improve the energy efficiency, comfort, and durability of the building envelope.

Thermal bridges: definition and useful measures

A linear thermal bridge (Ψ) occurs along a junction (e.g., floor edge passing through insulation, window/door frame connection, base of facade, parapet). It is expressed in W/m·K and is summed over the entire length concerned.

A point thermal bridge (χ) is linked to a singular point (balcony brackets, cladding brackets, heavy fixings). It is expressed in W/K and is added per room.

In terms of calculations, default (penalizing) values ​​from the Th-Bât/RE rules can be used, or justified values ​​from 2D/3D models (e.g., thermal transfer software) or Technical Approvals (balcony thermal breaks). In STD/SED (Standard Thermal Design/Structural Energy Design), careful consideration of Ψ/χ and surface temperatures allows for the anticipation of Bbio (bioclimatic coefficient), heat loss, and humidity risks. During construction, the continuity of the insulation, the quality of installation (reveals, supports, flashings), and the absence of parasitic bridging (untreated penetrations) determine the actual result.

Treating Ψ and χ: advantages, limitations, points of attention

Interests

• Reduced heating needs (↓ heat loss) and improved Bbio.
• Comfort: warmer walls on the inside, limited cold drafts.
• Sanitation/durability: condensation and mold are avoided in corners/behind furniture.
• Heating/cooling capacities sometimes reduced (sizing).

Boundaries

• More technical construction details (costs, coordination of lots).
• Some difficult points in renovation (continuous balconies, load-bearing partition walls).
• Default values ​​are very penalizing if not justified (studies/ATec).

Points to consider

• EWI is a priority for "wrapping" floor edges and reducing structural Ψ.
• Balcony bridge breakers (insulating brackets) or balconies added in major renovation.
• Joinery panels: insulation return, jamb extensions, sills with flashing and thermal break.
• Fixings/frameworks: choose brackets/anchors with low χ, limit the density of anchors.
• Facade bases & parapets: isolated profiles, vapor barrier/rain barrier continuity, copings.
• Coordination architect / structure / HVAC: avoid “construction bridges” (late penetrations).
• Surface control: check minimum indoor temperature (anti-condensation criterion) in sensitive areas.

Anecdote — “Balconies under control in Annecy”

In Annecy, a 1970s apartment building suffered from cold walls around its balcony slabs and mold in the corners. During the renovation, the team installed external thermal insulation on the facade, added thermal breaks to the new balcony brackets, and refurbished the window and door frames with insulation returns and flashing. The following winter: higher surface temperatures, no more condensation, and significantly reduced heating needs. The building manager summarized: "We didn't get rid of the balconies, we removed their gateway to winter."