PSI Value and Thermal Bridging Calculations, London and Nationwide

Thermal bridging and PSI value 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.

What is thermal bridging?

A thermal bridge, often called a cold bridge, is any part of the building where heat gets round or through the insulation faster than through the rest of the wall, floor or roof. There are three kinds:

• Repeating thermal bridges. Timber studs, rafters, wall ties and mortar joints. They repeat across the element, so they are already counted in its U-value. See our U-value calculations.
• Linear thermal bridges. Junctions: wall to floor, wall to roof, around windows and doors, at lintels, party walls and balconies. Each one is measured by its PSI value. These are what SAP and SBEM count as thermal bridging.
• Point thermal bridges. Single steel brackets, beams or fixings that pass through the insulation, measured as a chi value (χ, in W/K).

Why thermal bridging matters

• Heat loss. Poor junctions push up the DER and DPER and can fail the SAP on their own.
• Condensation and mould. The inside surface at a poor junction runs cold. Moisture condenses there and mould follows, usually in corners, at window reveals and where the floor meets the wall. We check each junction's temperature factor (fRsi) against BRE IP 1/06, so you know before it is built.
• Building control. Since the 2021 Part L, junction details are part of the design-stage and as-built BREL reports, and photos of the details on site are part of the evidence. What gets built has to match what went into the SAP.

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.

A calculated PSI value that comes out higher than expected is useful too. It shows which junction is weak while the design can still change cheaply.

The junctions that cost most

• Window heads and lintels. A standard steel lintel bridges straight through the insulation. A thermally broken lintel, specified early, is usually a cheap fix.
• Window position. A frame set out towards the brick loses more heat around its edges than one lined up with the insulation.
• Eaves and parapets. Heat escapes where the wall insulation stops short of the roof insulation. Carrying it over the wall head closes the gap.
• Party walls in terraces and flats. The junction is shared, so each home takes half of the heat loss. Get the cavity closer right and every unit on the scheme benefits.
• Balconies and steel through the wall. A concrete slab or steel bracket running through the insulation is one of the worst junctions there is. Thermally broken connectors make a big difference.
• Ground floors and thresholds. Floor-edge insulation and door thresholds are easy to get wrong on the drawings and on site.

Cutting thermal bridging at design stage

Most poor junctions are fixed on the drawing, not with more insulation:

• Keep the insulation line continuous, so wall insulation meets the floor and roof insulation with no gap.
• Use a low-conductivity block or a perimeter insulation strip at the base of the wall.
• Use thermally broken lintels and insulated cavity closers.
• Line the window frame up with the insulation layer.
• Use thermal breaks at balconies and steel connections.

We tell you which change helps most and what it does to the SAP or SBEM result.

Thermal bridging in commercial buildings (SBEM)

SBEM counts thermal bridging too. If no calculated PSI values are entered, it uses default values for each junction type, and those are pessimistic. On steel frame, curtain wall and heavily glazed buildings the defaults can be what holds the BRUKL back. Calculated values for the main junctions often close the gap without changing the spec. We model the junctions and run the SBEM in-house, so you see the effect straight away.

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.
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.

How we build each model

• Every layer of your build-up is drawn, from outside to inside, using the manufacturer's declared thermal conductivity where there is one.
• The walls, floors and roofs either side of the junction are modelled far enough out to capture all the extra heat flow.
• Window and roof-light frames are left out of the PSI value, as BR 497 requires. Where the window is known, a second model with the frame in place gives a more exact condensation check.
• Cold lofts and integral garages are modelled as outside, as BR 497 sets out; the SAP or SBEM then allows for the garage. Underfloor voids get their own temperature, worked out to BS EN ISO 13370.
• Perforated steel lintels are modelled properly, not guessed, because the steel makes such a difference.
• Junctions that sit very close together, like a lintel just under the eaves, are modelled together so the heat loss isn't counted twice.
• Only one-off bridges, such as cavity closers, lintels and steel, go into the junction model. Repeating ones like studs and wall ties are already in the U-value.
• Each junction is calculated for its PSI value and its temperature factor, and gets its own report.
• The mesh is refined until the heat flow and the coldest surface temperature stop changing. PSI values are reported to three decimal places and fRsi to two, as BR 497 asks.

2D or 3D?

Most junctions, such as wall to floor, wall to roof, sills, jambs and lintels, only need a 2D model. BR 497 asks for 3D where ground floors meet at a corner (for the mould check), where repeating steel runs across the junction, and where single fixings pierce the insulation. Corners where several elements meet, balconies and steel beams through the envelope can need 3D. If your scheme has a junction like that, we tell you before we start.

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

• For each junction: a reference, the build-up with each material's thermal conductivity and where it came from, the boundary conditions, the PSI value, and the temperature factor (fRsi) with a pass or fail against the limit.
• An annotated image of each model and a statement that the work follows BR 497.
• 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.

We are members of Elmhurst's Psi-Value Competency Scheme. To join, we passed assessed test cases modelled to BR 497, and the scheme checks further test cases every year.

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.

Does the window frame count in the PSI value?
No. The frame is covered by the window's own U-value. The junction around it is assessed separately, and the frame can be added back in a second model for a more exact condensation check.

What's the difference between a U-value and a PSI value?
A U-value is the heat lost through a flat wall, floor or roof, per square metre. A PSI value is the extra heat lost along a junction, per metre.

Can thermal bridging cause mould?
Yes. A poor junction leaves a cold patch on the inside surface, where condensation and then mould can form. We check every junction we model for this risk.

Do I need 3D modelling?
Rarely. Most junctions are fine in 2D. Complex corners, balconies and steel through the envelope can need 3D, and we tell you if yours does.

What is thermal bridging?
It is extra heat loss where the insulation is broken or bypassed, mostly at junctions between walls, floors, roofs and openings. SAP and SBEM count it through PSI values for each junction.

Do you do thermal bridging calculations for SBEM?
Yes. Calculated PSI values replace the SBEM defaults for each junction type, which often helps a commercial building pass the BRUKL.

Is thermal bridging the same as cold bridging?
Yes, it is the same thing. “Cold bridge” describes the cold spot on the inside; “thermal bridge” describes the heat escaping.

What should a PSI value report include?
A reference for each junction, the software used, the boundary conditions, every layer with its thermal conductivity and where it came from, the modelled lengths and the U-value used for each side, the heat flow, the PSI value and the temperature factor (fRsi), an annotated image of the model and a statement that it follows BR 497. Without these, the SAP assessor or building control can reject the values.

What is fRsi?
The temperature factor. It shows how warm the coldest inside surface of a junction stays compared with the room. The higher it is, the lower the risk of condensation and mould. BRE IP 1/06 sets the minimum: 0.75 for homes, residential buildings and schools, with other values for other uses, such as 0.50 for offices and shops and 0.80 for commercial kitchens and sports halls. We report a pass or fail for each junction. For condensation inside walls, roofs and floors, see our condensation risk analysis.

Can a junction have a good PSI value and still get mould?
Yes. The two checks answer different questions: the PSI value is about heat loss, fRsi is about the coldest spot on the inside surface. We report both.

Can a PSI value be negative?
Yes. At an inside (inverted) corner, BR 497 measures each wall up to the visible inside corner, so the wall areas already count more heat loss than really happens there. With the insulation running round the corner, the junction value comes out below zero. That is normal and helps the SAP slightly.

How are party wall junctions shared between two homes?
BR 497 calculates the junction as a whole, then gives half of it to each dwelling, so each SAP takes half the heat loss.

Related: U-value calculations, condensation risk analysis, 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.