Thermal Bridging to ISO 10211 & ISO 14683: Psi-Values, Y-Values and fRsi

You can specify excellent U-values for every wall, roof and floor and still lose a surprising share of your fabric heat through the junctions between them. Those junctions are thermal bridges, and the standards that govern them — ISO 10211 and ISO 14683 — decide whether that loss is calculated properly or estimated with a punitive default. The difference routinely moves a compliance result by several percent.

What a thermal bridge actually is

A thermal bridge is any part of the envelope where the thermal resistance is significantly changed — by a penetration, a change of geometry, or a change of construction. Three things happen there: heat flow increases, the internal surface gets colder, and the one-dimensional U-value maths stops being valid. That last point is the crux: a U-value assumes heat flows straight through a flat element. At a junction it does not, so the extra loss has to be accounted for separately.

Psi (Ψ) and chi (Χ): the two quantities

• Ψ — linear thermal transmittance (W/m·K). The extra heat loss per metre of a linear junction: wall-to-floor, jamb, lintel, sill, eaves, corners, party-wall junctions;
• Χ — point thermal transmittance (W/K). The extra loss from a point penetration, such as a balcony support bracket or a structural fixing through insulation.

Both are additional losses — the heat lost over and above what the adjoining plane elements already account for. That is why a psi-value can legitimately be small, zero, or even slightly negative when a junction is externally wrapped and the plane elements have already "over-counted" the corner area.

ISO 10211 vs ISO 14683: calculate or default

The two standards are alternative routes to the same number, with very different accuracy:

• ISO 10211 — detailed numerical calculation. The junction is modelled as a 2D (or 3D) finite-element heat-flow problem using the real materials, geometry and conductivities. It defines the modelling rules: where to place adiabatic cut planes, how to handle boundary conditions and surface resistances, and the convergence criteria the solution must meet. This is the accurate route, and the one certifiers expect for anything non-standard;
• ISO 14683 — simplified methods and default values. A catalogue of generic junction types with default psi-values. Convenient, but deliberately conservative: the defaults carry a safety margin precisely because they must cover a wide range of real constructions. Use them and you pay for that margin in your energy model.

In practice the modelling is done in tools such as THERM or dedicated psi-value software like AutoPSI; glazing junctions bring in WINDOW for the frame and spacer performance.

The y-value: how bridging enters the energy model

Individual psi-values are aggregated into a single fabric-level term. In the UK and Irish domestic methodologies (SAP and DEAP) this is the y-value, the transmission heat-loss coefficient for thermal bridging:

y = Σ(Ψ × L) / A_exposed  (W/m²K)

Each junction's psi-value is multiplied by its length, summed, and divided by the total exposed envelope area. The assessor then has three routes, and they are not equally priced:

• Default y-value — applied when junctions are not evidenced. Typically around 0.15 W/m²K, and deliberately harsh;
• Accredited / certified construction details — published details with pre-calculated psi-values, commonly landing near 0.08 W/m²K overall when used consistently;
• Project-specific calculated psi-values — junction-by-junction ISO 10211 modelling, which on a well-detailed build frequently reaches the 0.03–0.05 W/m²K range.

Moving from the default to calculated values can be worth several percent on a compliance margin — and on a marginal project it is often cheaper than upgrading glazing or adding insulation thickness. Confirm the exact default and accredited figures against the current version of the methodology applying to your jurisdiction, as they are revised between releases.

fRsi: the number that prevents mould claims

Heat loss is only half of what a thermal-bridge calculation delivers. The other half is the temperature factor, fRsi — a dimensionless measure of how cold the internal surface gets at the junction, independent of the actual weather:

f_Rsi = (T_si − T_e) / (T_i − T_e)

A higher value means a warmer surface. Guidance sets minimum values by building type — commonly 0.75 for dwellings, with higher thresholds for humid occupancies such as pools. Fall below it and you are not merely wasting energy: you are designing in surface condensation and the mould growth that follows. Because ISO 10211 modelling produces the surface temperature field anyway, fRsi comes free with the psi-value — and it is the result that protects you from a defect claim years later.

Where projects go wrong

• Accepting the default because calculations feel like an extra cost — then paying far more to close the resulting compliance gap elsewhere;
• Using accredited details on site but not as built — the psi-value only holds if the insulation continuity in the drawing is actually achieved;
• Ignoring repeating bridges — wall ties, rafters and studs belong in the U-value calculation, not the psi-value; double-counting or omitting them are both common;
• Modelling heat loss but never checking fRsi — passing on energy and failing on condensation;
• Wrong cut planes — truncating the model too close to the junction inflates or deflates the answer and breaks ISO 10211 compliance.

The takeaway

Thermal bridging is the one fabric item where analysis reliably beats specification on cost. ISO 14683 defaults are a safety-margin tax; ISO 10211 modelling removes it, quantifies the real junction losses, and hands you the surface temperatures that keep the building free of condensation. Calculate the junctions that repeat most — and detail them so the calculation stays true on site.

This article is general guidance and reflects information available at the time of writing. Default and accredited thermal-bridging values, fRsi thresholds and calculation conventions are set by the standards and national methodologies in force — always confirm the current requirements for your jurisdiction and project.