Thermal Bridge Analysis — BEO Science
BEO Science — Building Science Services

Thermal bridge analysis to ISO 10211 —
psi-values, fRsi, and PHPP-ready outputs

FEA modelling of junction details using Flixo — linear and point thermal bridges calculated to the standard, with surface temperature factors and PHPP-compatible psi-values as deliverables.

ISO 10211 FEA Modelling Ψ Psi-Values fRsi Surface Temp Factor PHPP Compatible NZ & AU
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The Problem

Standard U-value calculations ignore thermal bridges entirely

A U-value tells you how well a flat, uninterrupted section of wall, roof, or floor performs. It says nothing about what happens at the edges — where the wall meets the slab, where the window frame contacts the structure, where a steel element punches through insulation. In a well-insulated building, those junctions can account for a significant share of total heat loss. In a Passive House, unaccounted thermal bridges can be the difference between meeting the heating demand criterion and failing it.

PHPP catalogue values can underestimate heat loss in NZ/AU

In European climates, catalogue defaults tend to be conservative. In NZ and Australian conditions — different climate profiles, different construction typologies — the opposite is often true. Calculated psi-values can be worse than catalogue defaults, meaning a project that looks compliant on paper may not be. That's a certification risk you can only find by calculating.

Cold surfaces cause condensation and mould

A thermal bridge isn't just a heat loss problem. Where the interior surface temperature drops, relative humidity rises. The fRsi surface temperature factor is the metric that determines whether a junction will sustain condensation or mould growth — and it isn't visible in a U-value.

Steel elements are almost always underestimated

Steel has a thermal conductivity roughly 500 times higher than mineral wool. A single unbroken steel element through an insulated assembly — a structural bracket, a balcony connection, a steel stud — can locally eliminate the insulation benefit and create a severe cold spot.

Junctions are where the design ambiguity lives

Flat assemblies are well understood. The junctions — wall-to-slab, parapet, window reveal, roof eaves — are where design intent and buildability diverge, where thermal bridging accumulates, and where analysis is most likely to change something important.

What We Do

FEA thermal bridge analysis — four types of assessment

All modelling is performed in Flixo using finite element analysis (FEA) to ISO 10211. Results are calculated values — not estimates, not rule-of-thumb adjustments.

Linear Thermal Bridges — Ψ Psi-Values

Linear thermal bridges occur at junctions — where two assemblies meet, where a material with high conductivity interrupts the insulation layer. The psi-value (Ψ, W/m·K) quantifies heat loss per metre of junction length.

  • Wall-to-floor and wall-to-slab junctions
  • Wall-to-roof and parapet details
  • Window and door installation — frame to wall
  • Balcony and cantilever connections
  • Intermediate floor and party wall junctions
Point Thermal Bridges — χ Chi-Values

Point thermal bridges are discrete elements — fixings, brackets, fasteners, and structural penetrations. The chi-value (χ, W/K) represents the total heat loss through a single point element, counted by number rather than length.

  • Mechanical fixings through insulation layers
  • Structural brackets and restraint fixings
  • Cladding support systems
  • Anchor bolts and holddown fixings
  • Embedded structural elements
Condensation Risk — fRsi, Glaser & Humidity Distribution

Thermal bridge analysis sits alongside — and feeds into — broader moisture risk assessment. We apply three complementary methods depending on the project need.

The surface temperature factor (fRsi) is derived from the FEA model and determines whether a junction surface is at risk of condensation or mould under design conditions. The Glaser method (ISO 13788) provides a steady-state check of interstitial condensation risk across the assembly — a useful first-pass tool, particularly where a full WUFI hygrothermal simulation isn't warranted. Where more detail is needed, humidity distribution plots extracted from the Flixo FEA model show how relative humidity distributes across a junction under design conditions — a valuable complement to WUFI analysis for complex details.

For projects requiring full dynamic hygrothermal modelling, see our WUFI condensation analysis service →

  • fRsi calculation at all critical interior surface points
  • Glaser steady-state interstitial condensation check (ISO 13788)
  • Humidity distribution plots from FEA for complex junctions
  • Assessment against condensation and mould risk thresholds
  • Design recommendations where risk criteria are not met
  • Support for E3 Internal Moisture and B2 Durability compliance
PHPP-Compatible Psi-Value Outputs

Calculated psi-values replace conservative PHPP thermal bridge catalogue defaults in the Areas sheet. For well-detailed Passive House projects, this routinely reduces modelled heat demand — and can be the difference between meeting the heating demand criterion and not.

  • Psi-values formatted for direct PHPP Areas sheet entry
  • All junction types covered: IF, BF, RF, W, D, C, P, UF
  • Comparison against catalogue default values
  • Sensitivity analysis — impact on PHPP heating demand
  • PHI certification submission support
Why Calculate

Catalogue defaults versus calculated psi-values

PHPP catalogue values were developed largely from European construction typologies and climate conditions. In NZ and Australia they don't always translate — and the mismatch doesn't always work in your favour. Calculated psi-values can come in higher than catalogue defaults, meaning a project that looks compliant using catalogue values may be closer to the certification limit than it appears — or over it. The table below shows the range of outcomes; the direction depends entirely on the junction type and construction detail.

Junction type PHPP catalogue default Calculated — NZ/AU range
Wall / internal floor (IF) 0.000 W/m·K 0.1 – 1.0 W/m·K (concrete or steel structure)
Wall / base floor slab (BF) 0.040 W/m·K 0.040 – 0.300 W/m·K
Wall / roof (RF) 0.040 W/m·K 0.010 – 0.080 W/m·K
Window installation (W) 0.040 W/m·K −0.010 – 0.050 W/m·K
Corner junction (C) 0.040 W/m·K 0.000 – 0.040 W/m·K

Values are indicative — actual psi-values depend on the specific assembly, junction geometry, and construction typology. In NZ/AU conditions, calculated values can exceed catalogue defaults, particularly at slab junctions and where construction details differ from European norms. The direction isn't predictable without modelling.

Deliverables

What the analysis produces

Each commission is scoped around specific junction details — we don't charge for modelling you don't need. Typical turnaround is five to ten working days depending on complexity and number of details.

Standard deliverables — all commissions
  • Flixo FEA model files with documented material inputs and boundary conditions
  • Isothermal plots, heat flux, and humidity distribution visualisations for each junction
  • Calculated psi-values (Ψ) and chi-values (χ) per junction
  • Surface temperature factor (fRsi) at all critical interior surface points
  • Glaser steady-state condensation check (ISO 13788) where applicable
  • Condensation and mould risk assessment against applicable criteria
  • PHPP-formatted psi-value table for direct Areas sheet entry
  • Written report with findings, risk assessment, and design recommendations
  • Review call to walk through results with your design team
Our Process

From junction detail to calculated psi-value

01
Detail drawing review and junction scoping
We review your junction details and identify which bridges are worth calculating — either because they carry significant heat loss, because fRsi is uncertain, or because the PHPP catalogue value is significantly penalising the project. We confirm the scope before any modelling begins and flag any details where the drawing needs clarification before FEA can proceed.
02
FEA model build in Flixo to ISO 10211
Each junction is modelled in 2D (or 3D where required for point bridges) using material hygrothermal properties from the Flixo database or manufacturer data. Boundary conditions are set per ISO 10211 — internal and external temperatures, surface resistances, and edge conditions. The model is checked against the standard's validation criteria before results are extracted.
03
Psi-value, fRsi, and risk assessment
Calculated psi-values and fRsi factors are extracted from the FEA results. fRsi is assessed against condensation and mould risk thresholds under the relevant design conditions. Where a junction fails the fRsi threshold, we identify the specific design change needed to resolve it — not a generic recommendation to add more insulation.
04
Report delivery and PHPP handover
The report is structured for architects and Passive House designers — findings and risk assessment in plain language, with full technical documentation for the certifier. Psi-values are formatted for direct PHPP entry. A review call is included to walk through results, discuss any design changes, and confirm the PHPP impact before the next submission.
Who We Work With

Specialist input for the design and certification team

Architects & Designers
Thermal bridge analysis at developed design stage confirms whether junction details perform as intended — and identifies problems while they're still cheap to redesign. We work with your details, not generic ones.
Passive House Designers & Certifiers
Calculated psi-values replace catalogue defaults in PHPP — reducing modelled heat demand for well-detailed projects and providing defensible documentation for PHI certification submission. We deliver in PHPP-ready format.
Structural & Building Engineers
Structural elements — steel beams, brackets, holddowns, balcony connections — create some of the most significant thermal bridges in a building. We model these accurately, including the interaction between structural geometry and insulation detailing.
Commercial & Green Star Project Teams
Thermal bridge analysis supports Green Star and NABERS submissions where facade performance and thermal envelope quality are assessed. We produce the documented, calculated outputs these submissions require.
FAQ

Common questions

What is a thermal bridge and how is it measured?
A thermal bridge is any area of the building envelope where heat flows more easily than through the surrounding assembly — typically at junctions, at structural elements, or where high-conductivity materials interrupt the insulation layer. Linear bridges (at junctions) are quantified as psi-values (Ψ, W/m·K) — heat loss per metre of junction length. Point bridges (at discrete elements like fixings) are quantified as chi-values (χ, W/K) — total heat loss per element. Both are calculated using finite element analysis (FEA) to ISO 10211.
When do I need calculated psi-values rather than PHPP catalogue defaults?
In NZ and Australia, catalogue values don't always work in your favour. The PHPP catalogue was developed largely from European construction typologies — in NZ/AU conditions, calculated psi-values can come in higher than the catalogue default, which means a project that looks compliant using catalogue values may actually be over the heating demand threshold. If you're within striking distance of a Passive House limit, you need calculated values — not to confirm you're fine, but to confirm you're not worse than you think. We can advise on which junctions are most likely to matter for your specific project.
What is the fRsi factor and why does it matter?
fRsi is the surface temperature factor — it describes the minimum interior surface temperature at a junction as a ratio of the inside-to-outside temperature difference. A low fRsi means a cold interior surface. Where fRsi drops below the threshold for the design conditions (typically 0.75 for NZ conditions), the surface temperature is low enough to sustain condensation and mould growth. fRsi is calculated from the same FEA model as the psi-value — it's an additional output, not a separate analysis.
How many junction details does a typical commission cover?
It depends on the project. A typical residential Passive House might cover four to eight junctions — wall-to-slab, wall-to-roof, window installation, and one or two project-specific details. Commercial projects often require more, particularly where facade systems, structural cantilevers, or complex geometry introduce multiple bridge types. We scope each commission based on what the project actually needs — we're not applying a fixed package.
Can you model steel elements — brackets, beams, and structural fixings?
Yes. Steel thermal bridges are some of the most important to calculate accurately — the conductivity difference between steel and insulation is so large that simplified estimates are unreliable. We model structural brackets, balcony connections, steel studs, and embedded beams. For point elements like fixings, we calculate chi-values and can provide total heat loss estimates based on fixing patterns and counts.
Do you offer thermal bridge modelling as a standalone training course?
Yes — BEO Science offers Thermal Bridge Modelling to ISO 10211 as an online course, covering Flixo FEA methodology, boundary conditions, psi-value calculation, and PHPP integration. See beoscience.com/thermal-bridge-modelling for course details.
Calculate the bridges — don't estimate them
Book a consultation to discuss your junction details and scope. We'll confirm which bridges are worth calculating and what the PHPP impact is likely to be.
Book a consultation
denise@beoscience.com · +64 210 239 4980 · beoscience.com