Thermal bridging psi values and accredited details
A psi value can come from a blanket default, a tabulated default per junction, or bespoke modelling. The route you choose moves the fabric heat loss result more than most fabric upgrades.
Thermal bridging psi values decide a surprising share of the compliance margin on a well-insulated dwelling. Once walls, roof and glazing are good, junction heat loss stops being a rounding error and becomes one of the larger line items in the fabric heat loss calculation. And the figure you get depends less on how the building is detailed than on which of three evidence routes you chose.
The first thing to correct: the old Accredited Construction Details are no longer the answer. Approved Document L in its 2021 form withdrew reliance on them, so a project that still specifies "ACD" on a drawing is quoting a withdrawn document. Check the current Approved Document L before relying on any of this, because the routes and the default figures have moved before and will again.
Why junctions matter to the compliance result
A psi value, written as the Greek letter psi, is a linear thermal transmittance in watts per metre kelvin. It describes the extra heat loss along a junction over and above the plane elements either side. Multiply each junction's psi value by its length, add them up, and you get the thermal bridge heat transfer coefficient for the dwelling.
On a poorly insulated house that total was small relative to fabric losses through the walls. On a dwelling built to current standards, with thick insulation and triple glazing, the plane elements lose little and the junctions lose proportionally much more. Window perimeters are the worst offenders because a modest dwelling has a lot of reveal, sill and lintel per square metre of wall.
The result is an odd incentive. Improving a wall U-value by a small amount costs money and moves the calculation slightly. Getting the window sill and reveal detail right can move it more, and costs a drawing.
Default values and the penalty they carry
The calculation methodology offers a fallback for designers who have not worked out their junctions: a single default value for the whole dwelling, applied to the exposed envelope area. It is deliberately punitive, because it has to cover the worst case.
Using it means the rest of the specification has to absorb the penalty, typically through better glazing, more insulation or a larger renewable contribution. On a marginal scheme it is the difference between passing and failing.
Between the blanket default and full modelling sits a middle route: the default psi values tabulated in the calculation methodology for specific junction types. These are less penal than the blanket figure and more penal than a properly modelled junction, and they require you to identify each junction type and measure its length. That is an afternoon's work on a house type and it is almost always worth doing.
| Evidence route | Effort | Typical result |
|---|---|---|
| Blanket default applied to envelope area | None | Most penal |
| Tabulated default psi per junction type | Junction schedule and lengths | Middling |
| Calculated psi values to the modelling conventions | Thermal modelling per junction | Least penal |
What replaced accredited details
The principle behind the old accredited details survives even though the documents were withdrawn. A designer can still use a library of pre-calculated junction details, provided the psi values in it were produced to the recognised conventions and the detail is built as drawn.
Two conditions attach, and both get skipped.
The values have to come from modelling carried out to the published conventions for linear thermal transmittance, which rest on the numerical modelling standard for thermal bridges. Many manufacturer libraries meet this. Some quote a figure with no stated boundary conditions or dimension system, and a figure without those is not usable.
And the detail has to be built the way it was modelled. A modelled junction assumes a specific insulation continuity, a specific cavity closer, and a specific relationship between the window frame and the insulation line. Move the window 30mm outboard on site and the value no longer applies.
Calculated psi values and who produces them
Bespoke modelling is a specialist task. The modeller builds a two-dimensional or three-dimensional representation of the junction, applies the boundary conditions from the conventions, and reports both the psi value and the temperature factor.
That second output matters as much as the first. The temperature factor describes the internal surface temperature at the coldest point of the junction, and for dwellings the accepted minimum is 0.75, below which surface condensation and mould become likely. A junction can have an acceptable psi value and still fail on temperature factor, usually at a corner or a threshold.
Ask for three things with every value: the psi figure, the temperature factor, and the drawing the model was based on. Without the third, you cannot check that what is on site matches what was modelled.
Junctions most often got wrong on site
The pattern repeats across sites and house types.
- Window and door reveals, where the cavity closer is omitted or the insulation does not meet the frame.
- Window sills, where the cavity tray or the sill detail breaks insulation continuity.
- Lintels, where a steel lintel bridges the cavity and the insulated version was value engineered out.
- Eaves, where loft insulation stops short of the wall plate to keep the ventilation path clear.
- Party wall at eaves and at ground, where the cavity is unfilled and unsealed, creating a bypass.
- Balcony and canopy slabs penetrating the insulation line without a thermal break.
- Ground floor to wall, where the perimeter insulation upstand is missed by the screed gang.
The party wall bypass is the one that surprises people, because the party wall separates two heated dwellings and looks like it should not lose heat at all. An unsealed, unfilled cavity in a party wall behaves as a chimney open to the loft and the ground, and the calculation methodology treats it accordingly.
Evidence for building control
Building control will ask for the calculation output and, for the junctions, either the tabulated defaults or the source of the calculated values. Keep a junction schedule that lists each junction type, its length per plot, the psi value used, and where that value came from.
Then keep the site records that show the detail was built. Photographs of cavity closers before plasterboard, of insulation continuity at eaves, and of the party wall cavity seal are worth more at completion than any amount of paperwork, because a discrepancy found then costs little to fix.
Illustrative example: a pair of semi-detached houses where the original calculation used the blanket default and failed by a small margin. Producing a junction schedule with tabulated default psi values, plus a modelled value for the window reveal and sill, brought the fabric heat loss down enough to pass without changing the glazing specification. Figures here are illustrative.
How the choice shows up in a planning energy statement
At planning stage nobody asks to see psi values, and the assumption behind them still drives the headline. An Energy Statement that claims a percentage reduction against the notional building is quoting a calculation, and the junction assumption is part of that calculation. Assume modelled values at planning and build to defaults, and the claimed reduction does not appear.
Where a condition or a policy requires post-construction reporting, and the London Plan energy monitoring requirements are the clearest example, that gap becomes visible. So state the assumption in the energy statement. One sentence naming the junction evidence route is enough, and it protects the applicant if the specification changes later.
The same discipline applies across the technical submission. An overheating assessment that assumes one glazing specification while the energy calculation assumes another is the sort of inconsistency a reviewer finds quickly, and our Overheating Risk Assessment page sets out what that report covers. For larger single-building schemes the coordination problem is worse, as our note on energy statements for warehouse and logistics schemes describes.
And write the junction schedule once per house type rather than once per plot. It is the same work and it gets reused.
Enter your postcode on PlanScope to see which reports a residential scheme at your site is likely to need. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Can I still use Accredited Construction Details?
No. Approved Document L in its 2021 form withdrew reliance on the old Accredited Construction Details, so a drawing that still references them is citing a withdrawn document. Use the tabulated default psi values in the calculation methodology or values modelled to the recognised conventions, and check the current Approved Document L before relying on either.
What is a psi value?
A linear thermal transmittance in watts per metre kelvin, describing the extra heat loss along a junction beyond what the plane elements either side lose. Multiply each junction's psi value by its length across the dwelling and sum them to get the thermal bridge heat transfer coefficient used in the compliance calculation.
Is the blanket default ever acceptable?
It is permitted and deliberately punitive, because it has to cover the worst case. On a marginal scheme it can be the difference between passing and failing, and the rest of the specification has to absorb the penalty through better glazing, more insulation or more renewables. A junction schedule with tabulated defaults usually costs less.
Why does the temperature factor matter?
It describes the internal surface temperature at the coldest point of a junction, and for dwellings the accepted minimum is 0.75. Below that, surface condensation and mould become likely. A junction can have an acceptable psi value and still fail on temperature factor, most often at an external corner or a door threshold.
Which junctions go wrong most often on site?
Window reveals and sills where insulation does not meet the frame, lintels where an insulated version was substituted, eaves where loft insulation stops short of the wall plate, and party walls where the cavity is left unfilled and unsealed. The party wall bypass surprises people because both sides are heated.
Does any of this affect the planning submission?
Indirectly but materially. An energy statement claiming a percentage reduction is quoting a calculation that includes a junction assumption. Assume modelled values at planning, build to defaults, and the claimed reduction does not appear when post-construction reporting is required. Name the evidence route in the energy statement in one sentence.
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.