Thermal bridging evidence in a planning energy statement

Linear thermal bridging is one of the few parts of a Part L calculation that can move a planning energy statement from compliant to non-compliant without changing a single wall build-up. If you rely on default junction values, the calculation punishes you. If you rely on calculated psi values, the officer is entitled to ask who calculated them, to which convention, and how the detail will survive the site. Getting thermal bridging evidence into an energy statement properly is a half-day exercise at design stage and an expensive retrofit of the submission if you leave it.

The question an officer is really asking is whether the percentage improvement claimed over the Part L baseline is deliverable. Thermal bridging is where that claim is most often thin.

Why bridging appears at planning stage

An energy statement reports modelled performance against the Building Regulations baseline. For dwellings that means the target emission rate and the target fabric energy efficiency generated from the notional building, with the design figures set against them. For non-domestic buildings it means the BRUKL output from an approved dynamic or SBEM calculation.

Linear thermal transmittance at junctions feeds into both the fabric figure and the emissions figure. The aggregate effect is expressed as a y-value in watts per square metre kelvin, derived from each junction's psi value multiplied by its length and divided by the total exposed envelope area. On a compact block of flats the junction lengths are short relative to the envelope. On a terrace of narrow houses with a lot of openings, the junctions dominate, and a scheme that looks fine on U-values can fail on fabric efficiency.

Authorities that set a percentage reduction target, London boroughs applying the London Plan energy hierarchy among them, are effectively auditing the inputs. The hierarchy asks for lean measures to be exhausted before clean and green ones, so a fabric number propped up by renewables invites a challenge at the first stage.

Default values and the penalty they carry

The calculation methodology lets you take default psi values for each junction type without doing any modelling. They are set conservatively, on the basis that an unmodelled junction could be built badly, and the resulting y-value is markedly worse than a reasonably detailed junction would achieve.

Three consequences follow for a planning submission. The fabric energy efficiency figure drops, so the headroom for meeting a local percentage target disappears. The design team then compensates with photovoltaic area or a larger heat pump, which pushes the scheme up the hierarchy in exactly the direction policy discourages. And the statement ends up describing a building nobody intends to build, because the eventual construction will use published or calculated details anyway.

Defaults have one honest use. At outline stage, where the elevations and junction types are still undecided, a default-based calculation is a defensible worst case, provided the statement says so and the reserved matters strategy explains how the figure improves. Carbon reporting at outline stage has its own conventions, and how carbon is reported in an outline application sets out what an officer can reasonably expect before the detail exists.

Calculated junction values and the evidence behind them

There are two credible routes to better numbers. The first is a published set of construction details with psi values already assigned, used exactly as drawn. The second is bespoke numerical modelling of each junction, carried out to the recognised conventions for calculating linear thermal transmittance and temperature factors, with the geometry, material conductivities and boundary conditions recorded.

Either way the statement should carry a short schedule rather than a sentence. Set out, for each junction type, the psi value used, its source, and the drawing reference where that junction appears:

That table is what distinguishes an energy statement that survives consultation from one that attracts a holding objection. Officers rarely check the arithmetic. They check whether the inputs are traceable to a document and a drawing.

Where a junction is modelled, the output should include the temperature factor as well as the psi value, because a junction can perform acceptably on heat loss while sitting close to the surface condensation threshold. Flagging both in the same schedule saves an argument with building control later.

Where the calculation changes the result

The junctions that move the number are the long ones and the repeated ones. Window and door perimeters on a heavily glazed elevation, the ground floor junction on a shallow-plan dwelling, and balcony or walkway connections on a residential block are the usual culprits.

Balconies deserve particular attention. A continuous reinforced concrete slab projecting through the envelope is a severe bridge, and a structural thermal break assembly changes both the psi value and the structural detail. That is a planning-stage decision because it affects slab edge depth and therefore elevation, not a detail-stage one. The same applies to recessed entrances and to parapet details on flat roofs.

Separating wall junctions in terraces and semi-detached pairs behave differently again, since the party cavity can act as a bypass unless it is fully filled and edge sealed. Treating that junction with a default value on a scheme of narrow terraces is often the single reason a fabric target is missed.

Buildability and the site risk

A calculated psi value is a promise about workmanship. Insulation continuity around a window jamb assumes the cavity closer is installed the right way round, with the insulation line unbroken and the seal continuous.

Design out the dependence where you can. Simple junction geometry, a consistent insulation line and fewer penetrations give a result that tolerates ordinary site tolerances. Where the detail is demanding, say so in the statement and describe how it will be checked, for example junction-specific inspection points in the quality plan. An officer who sees that paragraph is more likely to accept calculated values at face value.

The honest limitation is that nothing in the planning process verifies as-built performance. The statement models an intention. Building control checks the calculation and the certificate at completion, and the gap between the two is a known weakness of the system rather than something an applicant can close.

Conditions securing the detail

Expect one of three forms of condition. A straightforward requirement to construct in accordance with the submitted energy statement, which is weak but common. A requirement to submit junction details and the supporting psi value calculations before superstructure works, which is the form that actually bites. Or a requirement to submit as-built calculations and the completion certificate, which addresses verification rather than design.

Where the authority applies a percentage improvement target, a condition tying the development to the modelled reduction is normal, and the wording matters. A condition drafted against a numbered revision of the energy statement locks in the inputs, so check the revision number on the certificate before the decision notice is issued. Baselines cause more confusion in this area than the measures do, and why the notional building confuses officers explains the trap before you negotiate wording.

Presenting the data

Keep the thermal bridging material to two pages inside the Energy Statement and put the modelling outputs in an appendix. The two pages should state the route taken, the schedule of junctions with sources and drawing references, the resulting y-value, and the sensitivity of the headline reduction to that y-value.

Pair it with the fabric section rather than the renewables section, because the two are read together. The U-values an officer looks for and the junction treatment behind them are a single argument, and the fabric figures officers check first is where that argument starts. Where overheating is also in play, the glazing decisions that help the bridging case can work against the Overheating Risk Assessment, so model both before fixing the elevations.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Compliance modelling itself needs an accredited assessor, and this article is general guidance rather than advice on a specific scheme, so check requirements with your local planning authority or a qualified professional. Enter your postcode on PlanScope to see which reports your site is likely to need.