Thermal bridging detail at planning stage
Linear thermal transmittance at junctions can move a scheme from passing to failing without changing a single U-value. What to commit to at planning stage, and what to leave open.
Junction detailing decides more of a Part L result than most planning drawings admit. Thermal bridging at planning stage is usually handled by a single line in an energy statement saying that accredited or calculated details will be used, and that line is a commitment the design team may not be able to honour once the structure is fixed. Linear thermal transmittance at the junctions, the psi values, can account for a tenth or more of a dwelling's heat loss, and the difference between a default value and a calculated one is large enough to move a scheme from passing to failing without a single change to the U-values on the elevations.
Why the junctions carry so much weight
Heat leaves a building through the flat areas of wall, roof and floor, and it also leaves along the lines where those elements meet. The second route is measured as psi, a linear transmittance in watts per metre kelvin, applied to the length of each junction type in the building. Add the junction lengths on a modern flat with a balcony on every unit and the linear losses stop being a rounding error.
Approved Document L Volume 1 for dwellings, in the 2021 edition, and the calculation conventions that sit under SAP, treat these losses explicitly. The assessor either enters a psi value for each junction type or falls back on a default. The defaults exist so that a scheme with no detailing information can still be assessed, and they are deliberately cautious for exactly that reason.
There is a second reason to care, and it has nothing to do with energy. A cold junction can drop the internal surface temperature far enough for condensation and then mould to appear. The temperature factor, fRsi, is the check for that, with the conventional minimum of 0.75 for dwellings and 0.50 for most non-residential spaces. A junction can pass on heat loss and still fail on surface temperature.
Default values, accredited details and calculated details
Three levels of evidence are available, and they are not interchangeable.
- Default psi values built into the calculation. No detailing evidence required, penalty applied, useful only as a placeholder.
- Published details with associated psi values, including the accredited construction details and equivalent published sets from system suppliers. These are conditional on the detail being built as drawn.
- Project-specific numerical modelling to the conventions in BR 497, producing psi values and fRsi for each junction as designed.
The gap between the first and the third is wide. A default is set high enough to cover a poorly built junction, because the calculation cannot know what will be built. A modelled value describes the junction actually drawn, and on a well-detailed wall-to-floor junction it can be a fraction of the default. That difference, multiplied across the junction length schedule of a block of flats, is the margin that lets a scheme meet its target without thickening the insulation or adding plant.
Most schemes that get into trouble at building control stage committed to level two or three at planning and then changed the structure. A published detail assumes a particular insulation continuity, a particular cavity closer, a particular slab edge. Substitute a different frame or move a parapet and the published value no longer applies.
Where the architecture makes the bridge
Four families of junction cause most of the difficulty, and all four are architectural decisions rather than technical ones.
Balconies are first. A cantilevered slab that passes through the envelope is a continuous fin of concrete, and the fix is either a structural thermal break at the connection or a self-supporting balcony carried on its own frame. That is a foundation and facade decision, and it cannot be retrofitted after the frame is designed.
Parapets are second. A parapet takes the wall past the roof insulation line, and the wrap of insulation over the top has to be continuous and thick enough, which pushes the parapet detail deeper and changes the elevation. Third are the projecting structural elements that carry canopies, brise soleil and signage zones, each one a penetration. Fourth are the ground floor junctions where a suspended slab meets the wall, particularly where the finished floor level has been set by a flood risk requirement rather than by the construction.
None of these are things an assessor can solve. They are settled on the sections, which is why the Energy Statement is worth writing alongside the sections rather than after them.
What can honestly be committed at planning stage
The useful commitment is narrow and specific. State the junction types the design relies on, say which of the three evidence levels each will use, and state the assumed psi value where one has been taken. If the modelling has been done, say so and reference the convention used. If it has not, say that default values have been assumed and that the result carries headroom as a consequence.
What not to do is promise calculated psi values across the board when the structural form has not been fixed. A condition requiring the development to be built in accordance with the submitted energy statement then locks the scheme to numbers nobody has produced.
Where an overall target has been set by policy rather than by Part L alone, the gap between assumption and detail matters more, because the scheme has less margin. The same discipline applies to a mixed-use building where the residential and non-residential parts are assessed separately, a problem covered in our note on SAP or SBEM on a mixed-use scheme.
Recording assumptions so building control can use them
The handover between the planning-stage energy work and the building control submission is where psi values usually get lost. A short schedule solves it: junction reference, description, evidence level, psi value assumed, fRsi if modelled, and the drawing the detail will appear on. Keep it as a live document through technical design and record every substitution against it.
Councils rarely ask to see this schedule at application stage, and that is exactly why it goes missing. The energy consultant issues a report, the report is appended to the application, and the spreadsheet behind it stays on one person's machine. Ask for it as a deliverable.
Two habits are worth forming. Note the date and version of the calculation software and conventions used, because the numbers are not portable between versions. And record who owns each junction, since a slab edge detail typically sits between the architect, the structural engineer and the facade subcontractor, and an unowned junction is an unmodelled one.
The commitment you cannot detail
The failure mode is consistent. An energy statement claims calculated values, the scheme is consented with a condition tying it to that statement, value engineering changes the frame, and the as-designed junctions no longer reach the assumed psi values. The options at that point are a redesign, a compensating improvement elsewhere in the fabric or services, or a non-material amendment to the condition. All three cost money and the third costs time.
The honest alternative at planning stage is to model the four or five junction types that dominate the length schedule and to use defaults for the rest, then say exactly that in the statement. A scheme that passes with defaults on the minor junctions has margin in hand. A scheme that only passes with calculated values everywhere has none. For a project weighing a heavy structural intervention against reuse, that margin question runs alongside the embodied carbon case for retrofit rather than demolition.
If you are assembling the evidence base for a scheme, browse the report types by category to see how the energy work sits alongside the overheating and sustainability documents a council may ask for.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required, and thermal modelling should be carried out by a competent assessor.
Frequently asked questions
What is a psi value in an energy statement?
Psi is the linear thermal transmittance of a junction, measured in watts per metre kelvin, applied to the length of each junction type in the building. It captures the heat lost along the lines where wall, roof, floor and openings meet, as distinct from the U-values that cover the flat areas of the envelope.
Can I use default thermal bridging values at planning stage?
Yes, and often you should. Defaults exist so a scheme with no detailing information can still be assessed, and they are deliberately cautious. Saying plainly that defaults were assumed leaves the design room to move. Committing to calculated values before the structural form is fixed is what causes trouble later.
What is the fRsi temperature factor and why does it matter?
fRsi is the check on internal surface temperature at a junction, used to avoid surface condensation and mould. The conventional minimum is 0.75 for dwellings and 0.50 for most non-residential spaces. A junction can perform acceptably on heat loss and still fail this check, so both outputs should be reported.
Do balconies really affect the Part L result that much?
On a flatted scheme they can. A cantilevered slab passing through the envelope is a continuous fin of concrete repeated on every unit, so the junction length schedule grows quickly. The remedies are a structural thermal break or a self-supporting balcony frame, both of which are decided early in the structural design.
What happens if the built junctions do not match the energy statement?
If the permission carries a condition tying the development to the submitted energy statement, the options are a redesign, a compensating fabric or services improvement, or an application to amend the condition. All three cost money and the last costs programme time, which is why the assumptions should be stated conservatively at the outset.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.