SAP requirements, Table K1 defaults, BR 497, Assessment Profiles, who can sign a report, and whether a SAP assessor will accept it.
A ψ-value (psi-value) is a linear thermal transmittance coefficient measured in W/(m·K). It quantifies the additional heat loss at a junction — a wall-floor connection, a lintel, a balcony — beyond what the U-values of the adjacent elements would predict. SAP 10.3 requires every junction in a dwelling to be assigned a ψ-value. If no bespoke calculation is provided, the assessor must use the conservative Table K1 defaults, which are often 4 to 25 times worse than what a modern detail actually achieves. This directly affects the fabric energy efficiency score and can prevent a building from passing.
Table K1 in SAP 10.3 provides default ψ-values for standard junction types. These defaults were set conservatively to represent poor construction practice. A modern insulated lintel detail typically has a ψ-value around 0.041 W/(m·K) — the figure the timber-frame box lintel worked example on this site actually computes; the Table K1 default for the same junction type is 1.000 W/(m·K) — 24 times higher. Across 10–20 junctions in a typical house, using defaults instead of bespoke calculations can add the equivalent of several square metres of uninsulated wall to the fabric energy calculation. The result is a lower fabric energy efficiency score, potentially requiring more expensive measures to compensate.
Failing fabric energy efficiency is not a minor issue — it is a building control blocker. A new dwelling in England cannot receive a completion certificate without a compliant energy assessment. If the fabric energy efficiency target is not met, you have three options, none of them cheap: first, add more insulation to walls, floors, or roof — often impossible once the building is at a late stage of construction without demolishing and rebuilding elements; second, compensate with renewable energy systems such as solar PV, heat pumps, or mechanical ventilation with heat recovery — adding thousands of pounds to the build cost; third, challenge the defaults by commissioning bespoke ψ-value calculations to replace the Table K1 penalties — which is what Psiclops does, and which should have been done at design stage. The critical point is that thermal bridging penalties from Table K1 defaults are often the single largest reason a building fails fabric energy efficiency, and they are entirely avoidable if bespoke calculations are commissioned before construction begins. A £10 ψ-value calculation at design stage is categorically cheaper than a £10,000 retrofit at completion stage.
HEM (Home Energy Model) is the replacement for SAP 10.3 as the UK's standard method for assessing the energy performance of dwellings. It uses a more detailed simulation approach but retains the requirement for bespoke ψ-value calculations at junctions, and the same BR 497 compliance route applies — so Psiclops's underlying calculation method already carries over unchanged. What isn't available yet is a dedicated HEM Assessment Profile (its own citation and Table K1 reference in reports, alongside the SAP 10.3 and SAP 10.2 profiles already offered) — see the Assessment Profile question below for what a Profile actually governs. The calculations themselves are HEM-ready; the named, selectable HEM Profile follows once HEM is formally released.
BR 497 is the BRE publication 'Conventions for Calculating Linear Thermal Transmittance and Temperature Factors'. It defines the methodology for ψ-value and fRsi calculations that SAP assessors are required to accept. Any bespoke ψ-value calculation submitted to a SAP assessor must follow BR 497 conventions and be accompanied by a compliant report. Psiclops generates BR 497 compliant reports automatically for every ψ-value calculation.
Under SAP Route 3, a bespoke ψ-value calculation can be signed by a competent person — someone with appropriate knowledge of thermal bridging calculation methods, typically a structural engineer, energy consultant, building physicist, or similarly qualified practitioner. The competent person declaration is captured at the project level in Psiclops and carried through to every report. Psiclops also offers an architect verification add-on for practices that require independent professional confirmation of the inputs.
Psiclops passes every published, relevant verification test available in this field — live pass/fail checks against EN ISO 10211, EN ISO 13788, ISO 6946, and ISO 13370's own published reference cases, shown on our public Verification page. There is, however, no national or international body able to provide independent certification for Psiclops, or for any other software product used for ψ-value/thermal-bridging calculation. BR 497 itself states there is no 'competent person scheme' for numerical thermal modelling of junctions: it sets out the conventions a calculation must follow, not a certifying authority that approves specific software. Two adjacent schemes are sometimes confused with this, but are genuinely different: BRE's SAP-approved-software list covers whole-dwelling energy assessment tools, not standalone junction calculators, and BRE's Certified Thermal Details and Products Scheme certifies specific generic, pre-designed junction details and physical building products.
A SAP assessor's obligation, under BR 497, is to accept a bespoke ψ-value calculated by a competent person following BR 497's conventions in place of SAP Table K1's conservative default — there is no requirement, and no approved list, naming which software must have produced it (the same reason no certifying body exists, above). Every Psiclops report carries the required competent person declaration — either self-signed by you, if you qualify as a competent person under SAP Route 3, or via the architect verification add-on — and follows BR 497's conventions throughout. An assessor has the same basis to accept it as any other bespoke calculation submitted to them.
An Assessment Profile is the specific standard and its own rules, defaults, and conventions that govern a project's calculations and reports — which junction defaults apply, which thresholds a result is checked against, and which conventions Psiclops Assist's own guidance stays within. It exists because those rules genuinely need to be pinned down for every project: without a named, fixed profile, Table K1 comparisons, the fRsi threshold, party-wall halving, and the rest would have nothing definite to be checked against, and Psiclops Assist's own guidance would have no fixed convention to stay consistent with. Two profiles are available today, selectable per project: BR 497 / SAP 10.3 (UK) and BR 497 / SAP 10.2 (UK). Both are governed by the same underlying BR 497 conventions document and share an identical technical basis — Table K1 junction defaults, fRsi thresholds, corner relaxation, flanking-length rules, party-wall halving, and mesh tolerances are all the same between them — the difference is which SAP edition's own citation and Table K1 reference appears in your report and Declaration, so a project assessed under either edition gets a report that names the standard it was actually assessed against. Every calculation and report is checked programmatically against whichever profile a project selects, not left to interpretation, and Psiclops Assist's suggestions and AI-generated narrative are kept consistent with that same active profile's conventions. Support for further profiles (HEM once it's formally released, Passivhaus, DEAP) is planned but not yet available.
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