Thermal bridging assumptions officers question

Thermal bridging assumptions in a SAP calculation are the input most often improved without evidence, and the one officers are now most likely to query. The compliance model needs a figure for heat loss at junctions: either linear thermal transmittance values, the psi values, for each junction type, or a single aggregated y-value applied over the whole envelope. The default y-value of 0.15 W/m²K is deliberately penal. Calculated junction values are much better, and the gap between the two is frequently the whole of a scheme's reported carbon improvement.

Which is why an energy statement that claims calculated values has to be able to produce them.

What linear thermal bridging contributes

Heat escapes through the plane elements of a building, measured by U-values, and additionally at the junctions between them, where insulation continuity breaks. A lintel over a window, a floor slab edge, a wall plate at the eaves: each of these loses more heat per metre than the adjacent wall loses per square metre.

The model accounts for this in one of two ways. Each junction type is given a psi value in W/m·K and multiplied by its total length in the building. Or the whole effect is bundled into a y-value in W/m²K and multiplied by the total exposed envelope area.

On a well insulated dwelling, junction losses are no longer a rounding error. As U-values have fallen, the proportion of fabric heat loss occurring at junctions has risen, so the assumption carries more weight in the result than it did under earlier editions of Part L.

Default values against calculated values

Three positions are available, and they produce different answers.

The default y-value exists so that a calculation can be completed where nothing is known about the junctions. It is not a design position and it should not appear in a planning energy statement for a scheme that has been detailed.

Default psi values from the SAP tables sit in the middle. They are tabulated by junction type and require only that the junctions be identified correctly, which makes them the sensible fallback at outline or early design stage. Say in the Energy Statement which of the three positions you have adopted. That single sentence prevents most of the queries.

Accredited details and their limits

For years the shortcut was to adopt published construction details and the psi values that came with them. That approach still has a place, with two caveats that officers and building control both raise.

The published sets were produced against earlier editions of Part L and earlier construction practice. Where a detail has been superseded or where the specification has moved on, the psi value that travelled with it is no longer the value for what you are building.

And the detail only delivers its value if it is built as drawn. Substituting a different lintel, changing the cavity closer or moving the insulation line changes the junction and therefore the psi value, while the energy statement continues to claim the published figure.

So if you rely on published details, say which set, and include the details themselves in the drawing package rather than referring to them generically.

Junctions that dominate the result

Length drives the arithmetic, not severity. A badly performing junction that occurs over two metres matters less than a mildly imperfect one that runs for sixty.

In a typical dwelling the dominant contributors are the window and door surrounds, taken together, because lintels, jambs and sills repeat around every opening. Then the ground floor to external wall junction, which runs the full perimeter. Then the eaves and the verge. Party wall junctions in terraces and semis come next, and they are the ones most often modelled optimistically.

Balconies on apartment schemes deserve separate attention. A continuous concrete slab projecting through the envelope is one of the worst junctions available, and the modelled psi value should reflect whether a structural thermal break has actually been specified. If the break has been priced out, the model needs updating.

Evidence required to claim improved values

What a case officer or a building control body will ask for, and what should be in the file from the start:

• A psi value calculation report for each junction claimed, produced by numerical modelling to the recognised method, with the geometry, material conductivities and boundary conditions stated.
• The junction drawing that was modelled, at a scale that shows the insulation line, cross-referenced to the drawing number in the application.
• Confirmation of who carried out the modelling and their competence or certification.
• A schedule linking each junction type to its claimed psi value and its total length in the building.
• Where published details are relied on, the identity of the set and the detail reference.

The schedule is the document that settles arguments, because it lets anyone check that the lengths are plausible and that no junction has been omitted. An energy statement that reports a y-value with no schedule behind it is asking to be taken at its word.

Effect on the compliance margin

Run the model twice and the sensitivity becomes obvious. Moving from the default y-value to a calculated set of junction values can be worth several percentage points of carbon improvement on a dwelling, which on many schemes is the difference between meeting a local policy target and not meeting it.

That creates a temptation worth naming. The thermal bridging line is invisible on a drawing, costs nothing to improve on paper, and is rarely checked at planning stage. It is also the line most likely to be unpicked at building control, after the design is fixed and the contract let.

The honest approach is to model the junctions you intend to build, keep a margin elsewhere, and resist the suggestion that the bridging assumption can carry a shortfall. Where the margin is thin, the recoverable options are glazing specification, ventilation heat recovery efficiency and renewables, all of which are visible and priceable.

Documenting the basis in the statement

Four sentences in the statement do most of the work.

State the approach adopted, meaning default y-value, tabulated psi values or calculated values. State who produced any calculations. State that the junction details modelled are those in the application drawings, listed by number. And state what happens if the details change, which is that the calculation will be re-run before the relevant condition is discharged.

Where a condition requires construction in accordance with the approved energy statement, that last sentence is the one that keeps you out of difficulty. Planning conditions have to satisfy the tests in the Planning Practice Guidance on the use of conditions, including that they should not duplicate the Building Regulations, so a condition fixing specific psi values is poorly founded. A condition requiring the reported carbon improvement to be achieved is better drafted and leaves room to recover a shortfall elsewhere.

Non-domestic schemes shift the emphasis again, because large flat envelopes with few openings have proportionately less junction length, while loading doors and roof penetrations introduce their own problems. We look at how that plays out in energy statements for warehouse and logistics schemes.

Keep the wider policy narrative in the Sustainability Statement, and where embodied carbon is also in play, the junction detailing interacts with material choices in the Whole Life Carbon Assessment, for which councils expect a suitably qualified author.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance rather than legal advice, so check the position with your local planning authority or a qualified professional before relying on it.