Psi Value Calculations in London and Nationwide

Thermal bridging calculations for Part L

A psi value (Ψ, in W/m·K) is the heat lost along a junction: wall to floor, wall to roof, around a window, at a lintel or a party wall. Add every junction in the building and you get its y-value, which goes straight into the SAP or SBEM.

Junctions can account for up to 30% of a new home's heat loss. If the SAP has no calculated values, it has to use the defaults, and those are set on purpose to be pessimistic.

Do you need calculated psi values?

Since the 2021 Part L, the old Accredited Construction Details can no longer be used. That leaves four routes for each junction:

• Values calculated by a competent person to BRE's BR 497 conventions, with the temperature factors from BRE IP 1/06.
• Details from a reputable independent database, such as LABC Registered Construction Details.
• The default values in SAP Table K1.
• A blanket default y-value of 0.20 W/m²K.

The last two are the easy option and often the reason a design fails. You need calculated values when:

• the SAP fails, or only passes with more PV, a better heat pump or thicker insulation than you want to pay for;
• your build-up doesn't match a published detail (timber frame, SIPs, steel frame, ICF, external wall insulation, thermally broken balconies);
• it is a multi-unit scheme, where a few calculated junctions repeat across every flat or house;
• you are aiming for a better EPC than the minimum, or an energy statement asks for extra carbon savings.

How our psi value calculations work

1. You send the junction details and build-ups.
2. We model each junction in AutoPSI, two-dimensional thermal modelling software, following BRE's BR 497 conventions and BS EN ISO 10211, and work out its psi value. Most junctions only need a 2D model.
3. We check the internal surface temperature factor (fRsi) against BRE IP 1/06, so you know the detail isn't a condensation and mould risk.
4. If a junction is poor, we tell you which change helps most: moving the insulation line, a thermally broken lintel, a different cavity closer.
5. The values go straight into our SAP or SBEM, so you see the effect on the result straight away.

One team for the junctions and the SAP. No passing results between consultants.

Y-value calculations for SAP

Already have psi values from manufacturers or a published scheme? We measure every junction length off your drawings and work out the y-value for each dwelling type. Manufacturer values are fine where the detail and the U-values match what is being built. If they don't, they need recalculating.

What you get

• A psi value and temperature factor for each junction, with the build-up and assumptions behind it.
• A summary sheet for building control and the SAP or SBEM evidence pack.
• The y-value for each dwelling type, when we do the SAP too.

What we need from you

• Junction details (PDF or DWG): ground floor, intermediate floor, eaves, gable, window head, sill and jamb, party wall, balconies.
• Wall, floor and roof build-ups with products and thicknesses.
• Window and door frame details, and where the frame sits in the wall.
• Plans, sections and elevations, if we are doing the SAP as well.

Price and turnaround

Priced by the number of different junctions, not the number of homes. Send the details for a free quote. Turnaround: 5 working days once we have everything we need. Booking the SAP with us at the same time? We price both together.

Psi values and the Future Homes Standard

The new Part L and SAP 10.3 start on 24 March 2027, and the Home Energy Model follows later. Both still count junction heat loss, so calculated details stay useful. Getting them right now saves a redesign later. See our industry updates.

Our experience

More than 17 years in energy assessment and more than 4,000 SAP calculations. We see every week which junctions sink a design and which fixes cost least.

Questions we get asked

What is a psi value?
It is the heat lost per metre along a junction between two building elements, in W/m·K. The lower the number, the better the detail. The SAP multiplies each psi value by the length of that junction to get the total heat loss through thermal bridges.

Do I need psi value calculations for Part L?
Not always. You can use the SAP default values or a default y-value of 0.20 W/m²K, but they are pessimistic and often cause a fail. Calculated values are usually the cheapest way to pass when a design is close.

How much do psi value calculations cost?
It depends on how many different junctions there are, not on the number of dwellings. Send the details by email or WhatsApp for a free quote.

Can I reuse psi values on another project?
Yes, if the junction detail and the build-up are the same. Change the insulation, the U-value or the frame position and the psi value changes, so it needs a rerun.

Related: SAP calculations and EPCs, SBEM and BRUKL, energy statements, new dwellings, air tightness testing and solar PV for SAP and SBEM.

Email your junction details or WhatsApp us for a free quote.