Thermal bridging and psi-values in building envelope design: Definition, calculation and impact

A thermal bridge is a location in the building envelope where the effective thermal resistance drops significantly below that of the surrounding construction, creating a locally elevated rate of heat transfer. Linear thermal bridges occur at junctions between different building elements — where an external wall meets a ground floor slab, where a flat roof meets an external wall, where a floor slab meets a balcony, or where a window or door is set within an external wall. Point thermal bridges occur at discrete locations where a fixing, anchor or service penetration crosses the insulation layer. In buildings with very low U-values, the cumulative heat loss attributable to thermal bridges at junctions, reveals and parapets can represent a substantial fraction of the total fabric heat loss.

What is the psi-value and how is it calculated?

The psi-value, denoted ψ and measured in W/mK, is the linear thermal transmittance — the extra heat loss per metre of a junction beyond what the plane elements already lose, capturing the thermal bridge at the connection. The formula is:

Read moreThermal Bridging: How Cold Bridges Reduce Building Performance and Energy Efficiency

ψ = L_2D – Σ(U_i × l_i)

where L_2D is the two-dimensional thermal coupling coefficient calculated by finite-element modelling, U_i are the thermal transmittances of the plane elements, and l_i their lengths. Calculation follows BS EN ISO 10211 and BS EN ISO 14683. Stansted Environmental Services and Mosart both provide psi-value calculations for residential and commercial projects across the United Kingdom, allowing SAP and SBEM assessors to replace conservative default values from the SAP appendix with calculated values that reflect the real performance of the specified details. The result is typically a better SAP score, a higher EPC rating, and in many cases the ability to demonstrate Part L compliance without additional renewable energy measures.

Classification thresholds and condensation risk

Psi-values are classified by severity according to published thresholds. While the exact boundary values vary by publication, typical ranges distinguish negligible, acceptable, significant and severe thermal bridges. A junction with ψ = 0.05 W/mK over a one-metre length is classed as acceptable. Beyond the linear transmittance, the temperature factor f_Rsi measures how warm the coldest interior surface stays:

f_Rsi = (T_si,min – T_e) / (T_i – T_e)

where T_si,min is the minimum interior surface temperature, T_e the external temperature, and T_i the internal temperature. Per BS EN ISO 13788, an f_Rsi at or above 0.75 indicates no mould risk; below 0.72 condensation and mould are likely. A junction with f_Rsi = 0.80 exceeds the 0.75 threshold, so there is no mould risk. This dual check — quantifying heat loss through ψ and assessing surface condensation risk through f_Rsi — is central to thermal-bridge assessment in high-performance envelopes, including Passivhaus and KfW-Effizienzhaus projects.

Common junction types and mitigation strategies

Pre-calculated psi-value catalogues cover 59 junction types, grouped by element interaction:

Insulation strategy profoundly affects psi-values. Continuous insulation wraps unbroken through the junction, delivering the best thermal performance. Insulation with break is interrupted by a structural element — a slab, beam or column. Partial insulation covers only part of the junction, for example the internal face only. The choice of insulation material and thickness — phenolic foam (λ = 0.020 W/mK), PIR board (λ = 0.022 W/mK), XPS (λ = 0.030 W/mK), mineral wool (λ = 0.035 W/mK), EPS (λ = 0.038 W/mK) — determines both the plane U-value and the residual thermal bridge at junctions. Schöck is among the manufacturers offering proprietary thermal separation products for balconies and parapets.

Impact on whole-building heat loss

The annual heat loss attributable to a single junction is:

Q = ψ × L × HDD × 24 / 1000

where L is the junction length in metres and HDD the heating degree days for the project location (e.g. London 2200, Berlin 3100, Stockholm 3900). Over the entire envelope of a building, the cumulative contribution of thermal bridges can represent a substantial fraction of total fabric heat loss. In projects governed by the Gebäudeenergiegesetz or equivalent UK Part L requirements, accurate psi-values allow designers to quantify the actual linear thermal transmittance of each junction type, avoiding the conservative penalty of default values and enabling compliance without oversizing renewable energy systems.

Standards and calculation tools

The thermal bridge calculator by Reuven Engineering Tools evaluates psi-values per EN ISO 10211 and EN ISO 14683. Users select a junction from the catalogue, set the insulation thickness and conductivity, and the tool returns the psi-value with a severity classification, the temperature factor f_Rsi, and the resulting annual heat loss for the specified junction length. Mosart reports that thermal bridge analysis is used by building physicists, Passive House designers and energy assessors to quantify heat loss at junctions and to check the surface temperature factor against condensation and mould risk. The workflow interpolates the junction psi-value from the catalogue for the chosen insulation thickness and conductivity, classifies the psi-value against thermal-bridging thresholds, computes f_Rsi from the minimum interior surface temperature, and multiplies ψ by junction length and heating degree days to estimate annual heat loss impact.

For further reading on envelope performance in high-efficiency buildings, see the thematic portal on energetic refurbishment and the lexicon entry on DIN 4108, which defines thermal protection requirements.