Thermal bridging at junctions is one of the most significant β and most controllable β sources of heat loss in a new build. Replacing SAP's conservative defaults with professionally calculated PSI values typically improves compliance results significantly, often creating the headroom needed to pass Part L without additional spend on the fabric or renewables.
Energy Compass are accredited members of the Elmhurst PSI-Value Competency Scheme. All calculations are carried out to BR 497 methodology and accepted by building control across England, Wales, and Scotland.
A PSI value (Ο-value) measures the rate of heat loss through a linear junction β the point where two thermal elements meet. In SAP, these junction losses are summed across the whole building envelope and can represent up to 25% of total fabric heat loss.
SAP applies a default Y-value of 0.15 W/mΒ²K to account for thermal bridging when no calculated PSI values are provided. This default is deliberately conservative β it assumes poor detailing at every junction. For most modern new builds, the actual heat loss is considerably lower, but SAP penalises the dwelling as if it were not.
Calculated PSI values are specific to the actual construction details of your building. By modelling the true heat loss at each junction in 2D, we replace the conservative default with an accurate figure β typically reducing the overall thermal bridging contribution significantly, improving all three SAP compliance metrics.
The removal of Accredited Construction Details and the tightening of Part L compliance targets since October 2023 has made thermal bridging defaults a frequent cause of SAP failure. Many new build designs that would have passed under the old regulations now require calculated PSI values simply to achieve compliance β not because the fabric is poor, but because the default penalty is too high.
Improved SAP compliance results from calculated PSI values create headroom against the Part L targets. That headroom can be used to reduce spend elsewhere in the design β specifying less insulation, standard-performance glazing, or avoiding renewables that would otherwise be needed to close a compliance gap. The PSI calculation cost is typically recovered many times over in material savings.
All standard SAP junction types are covered. A typical single new build dwelling requires 10β15 junctions depending on construction type and complexity.
| Ref | Junction Type | Description | Typical New Build? |
|---|---|---|---|
| E1 | Lintel β steel perforated base | Steel lintel with perforated steel base plate over window / door openings | Common |
| E2 | Lintel β other | Insulated steel box lintels, timber inserts, and other lintel types | Very common |
| E3 | Sill | External wall to window sill junction | Very common |
| E4 | Jamb | External wall to window / door frame sides | Very common |
| E5 | Ground floor | External wall to ground floor junction (standard) | Very common |
| E6 | Ground floor (exposed) | External wall to exposed / suspended ground floor | Occasional |
| E7 | Intermediate floor | External wall to intermediate floor within heated space | Common (2+ storey) |
| E9 | Eaves β ceiling level | External wall to roof junction where insulation is at ceiling level | Common |
| E10 | Eaves β rafter level | External wall to roof junction where insulation follows the rafter | Common (warm roof) |
| E11 | Gable β ceiling level | Gable wall to roof junction, insulation at ceiling | Common |
| E12 | Gable β rafter level | Gable wall to roof junction, insulation at rafter | Common |
| E13 | Corner β normal | External corner where two external walls meet (outward-facing) | Very common |
| E14 | Corner β inverted | Internal re-entrant corner (inward-facing) | Occasional |
| E15 | Party wall | Party wall between semi-detached or terraced dwellings at external wall | Semi / terrace |
| P1 | Party wall β ground floor | Party wall to ground floor junction | Semi / terrace |
| P2 | Party wall β intermediate floor | Party wall to intermediate floor junction | Semi / terrace |
| P3 | Party wall β roof | Party wall to roof junction | Semi / terrace |
Bespoke (non-standard) junctions can also be modelled for unusual structural details or proprietary systems not covered by the standard junction library. Contact us to discuss.
PSI modelling is carried out remotely from your architectural drawings and construction specifications. No site visit required.
Architectural drawings showing wall, floor, roof, and window details. The more construction detail, the more accurate the result.
We identify all relevant junctions from the drawings and confirm the scope β number of junctions and any that require bespoke treatment.
Each junction is modelled in 2D to BR 497 methodology, producing a calculated PSI value specific to your construction detail.
A completed PSI report is issued for SAP input and building control submission. Values feed directly into your SAP assessment.
If we are also carrying out your SAP, calculated values are entered directly β no additional coordination required.
PSI modelling is priced per junction β no hidden extras, no surprises.
Per Junction
per calculated junction
Typical new build dwelling: 10β15 junctions
Indicative cost: Β£500 β Β£750
Semi-detached and terraced dwellings include party wall junctions (P1βP3). Detached dwellings typically require fewer junctions.
For larger schemes or multiple dwelling types, contact us for a project-level quote.
Combined service
Need SAP and PSI modelling together?
We provide both services as a seamless combined package β PSI values feed directly into your SAP assessment with no coordination overhead. One point of contact, one report set for building control.
Send us your drawings and we'll confirm the junction scope and provide a fixed quote. No obligation.
What to send us
Architectural drawings showing wall, floor, roof, and window construction details. Section drawings are ideal. The more construction detail provided, the more accurate the calculated values β and the better the SAP result.