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Ψ · L · ΔTfRsi ≥ 0.75Free

Thermal Bridge
Calculator

Two checks for a construction junction: the extra heat loss from its linear thermal transmittance (Ψ), and the surface temperature factor fRsi that governs mould and condensation risk.

Who it's for: Building engineers and building physicists computing junction heat loss and checking condensation risk at a detail — you need the Ψ-value from a 2-D calculation or validated catalogue, and the lowest internal surface temperature for the fRsi check, at the construction documents detail stage.

fRsi = (θsi − θe) / (θi − θe)
Linear heat loss Q = Ψ · L · ΔT. The temperature factor fRsi must be ≥ 0.75 for dwellings (ISO 10211/13788, evaluated at 20 °C internal / 0 °C external) to avoid surface mould. Ψ-values come from a calculated junction or a tabulated catalogue.
Junction
Heat loss · fRsi

How to use this calculator

1
Enter the linear transmittance Ψ
The Ψ-value (W/m·K) describes the extra heat loss the junction adds per metre of length, beyond the plane-element U-value. Obtain it from a 2-D thermal model (ISO 10211) or a validated catalogue — it cannot be derived from overall geometry alone.
2
Enter the junction length L
The total length of this junction type around the building, in metres — for example, the combined eave length if you are assessing the eave detail.
3
Set the internal and external temperatures
Standard dwelling assessment uses 20 °C inside and 0 °C outside. Change these to the actual design temperatures if your project differs.
4
Enter the lowest internal surface temperature θsi (optional)
The coldest point on the internal surface of the junction, taken from the 2-D thermal model or an IR scan. Leave blank to get the heat-loss result only, without the fRsi condensation check.
5
Read the heat loss and fRsi result
The calculator returns the extra heat loss Q in watts and, if θsi is supplied, the temperature factor fRsi with a pass/fail against the 0.75 mould threshold.

The formula

Two checks for one junction: the heat-loss equation quantifies how many extra watts the bridge adds to the building fabric; the temperature factor fRsi tells you whether the coldest internal surface stays warm enough to prevent mould growth.

Q = Ψ · L · ΔT  — extra linear heat loss (W)
fRsi = (θsi − θe) / (θi − θe)  — surface temperature factor

Ψ = linear thermal transmittance of the junction (W/m·K) — from an ISO 10211 2-D calculation or validated catalogue
L = junction length (m)
ΔT = θi − θe = design temperature difference (°C)
θsi = lowest internal surface temperature at the junction (°C)
θi = internal design temperature (°C)
θe = external design temperature (°C)

Worked example

Example
An eave junction has Ψ = 0.08 W/m·K and runs L = 12 m around the building. At the standard 20/0 °C assessment condition (ΔT = 20 °C), the extra heat loss is 0.08 × 12 × 20 = 19.2 W. The 2-D model gives a lowest internal surface temperature of θsi = 16.5 °C, so fRsi = (16.5 − 0) / (20 − 0) = 0.825. Because 0.825 ≥ 0.75, the junction passes the ISO 13788 mould-risk threshold.

When this estimate will be off

  • Ψ must come from a 2-D or 3-D finite-element calculation (ISO 10211) or a validated manufacturer catalogue — it cannot be estimated from overall geometry. An incorrect Ψ will produce a proportionally wrong Q and a meaningless fRsi.
  • fRsi ≥ 0.75 is the standard dwelling threshold (humidity class II, 80% RH). Humid spaces such as pools, commercial kitchens, and laundries need higher values — consult ISO 13788 Table 1 for the applicable humidity class.
  • Point thermal bridges (fixings, anchors, brackets penetrating insulation) have a point transmittance χ (W/K), not a linear Ψ, and require a 3-D analysis. This calculator handles linear bridges only.
  • Q is the additional heat loss from the bridge alone, not the total wall transmission loss. The full junction result adds the plane-element contribution U·A·ΔT to the bridge losses.

Frequently asked questions

A common mistake: Ψ must come from a 2-D or 3-D finite-element calculation (ISO 10211) or a validated manufacturer catalogue — it cannot be estimated from overall geometry. An incorrect Ψ will produce a proportionally wrong Q and a meaningless fRsi.

Ψ-values range from roughly 0.01 W/m·K for a well-designed Passivhaus eave to 0.10–0.20 W/m·K for an uninsulated concrete slab edge or parapet. Use values from a validated catalogue (BRE IP 1/06, a BBA certificate, or a manufacturer thermal-break datasheet) or from a bespoke ISO 10211 2-D calculation for the actual junction geometry.

ISO 13788 derives the 0.75 limit from the surface relative humidity at which mould begins to grow — approximately 80% RH — under standard indoor conditions of 20 °C and 50% RH bulk air. Rooms with persistently higher humidity (bathrooms, kitchens, laundries) need fRsi above 0.80; the standard provides a full humidity-class table.

A linear bridge (Ψ, W/m·K) runs along a length — eaves, window reveals, floor–wall junctions. A point bridge (χ, W/K) is localised — a steel anchor or fixing that penetrates the insulation layer. ISO 10211 covers both; this calculator handles linear bridges only. For point bridges, obtain χ from a 3-D finite-element model.

No. Thermal bridges contribute alongside plane-element transmission (U·A·ΔT) in a full fabric calculation. In a typical dwelling, bridge losses can add 10–30% to the plane-element total. A complete SAP, PHPP, or EN ISO 13370 calculation sums all elements and all junctions — this calculator covers one junction at a time.

Sources

  • ISO 10211:2017 — thermal bridges in building construction — heat flows and surface temperatures — detailed calculations
  • ISO 13788:2012 §5.3 — hygrothermal performance of building components — temperature factor fRsi and the minimum acceptable value to avoid surface condensation

This free Thermal Bridge Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.