What is a psi value?

Updated 19 September 2026 — reviewed against the regs as they stand.

Quick answer: a psi value (Ψ-value) is the linear thermal transmittance of a junction, in W/m·K — the heat that leaks along the line where two elements meet, per metre of junction, per degree of temperature difference, beyond what the elements’ U-values already count. SAP multiplies each Ψ by the junction’s length and adds the results to the fabric heat loss. Leave a junction uncalculated and SAP uses a default from Table K1 (1.00 for a lintel, 0.32 for a ground floor); the notional dwelling you are measured against assumes far lower values (0.05 and 0.16). A calculated Ψ-value, modelled to BS EN ISO 10211, is usually the cheapest way to close that gap.

Psi value meaning: heat loss along a line

Heat leaves a building through areas and along lines. Through the flat middle of a wall the loss is set by its U-value, in watts per square metre per kelvin. But where that wall meets a floor, a roof, a window or another wall, the insulation is interrupted, the geometry changes and heat finds a shorter path out. That extra loss, concentrated along the junction, is the thermal bridge — and its size is the psi value, written Ψ, in watts per metre of junction per kelvin (W/m·K).

The definition matters: Ψ is the loss over and above what the adjoining elements’ U-values already explain. A junction with Ψ = 0.16 W/m·K and a length of 40 m adds 6.4 W/K to the dwelling’s heat loss coefficient — the same as another 35 m² of wall at 0.18. Across a house, junctions typically add 10 to 30 per cent to the fabric heat loss, which is why thermal bridging is treated as a separate line in SAP rather than folded into the U-values.

SAP also uses a summary figure, the y-value (W/m²K): the sum of every Ψ × length, divided by the total area of exposed elements. It is a convenient single number, and a trap — the y-value of a dwelling with no junction information at all is fixed at 0.20, roughly three times what a well-detailed house achieves. The y-value calculator works it out from your junction lengths, at SAP’s defaults, at the notional values and at your own.

What a good psi value looks like

Two figures sit behind every junction in a SAP 10.3 calculation, and both are published in the specification. Table K1 is the default: the value SAP gives a junction when nothing better is available. Table R2 is what the notional dwelling — the target your design is measured against — assumes for the same junction. The gap between them is the penalty for not calculating.

The junctions below are the ones that come up most. The complete set — all 44 E, P and R references with both values and the gap between them — is on the SAP junction reference.

W/m·K, from the SAP 10.3 specification (12 June 2026 version), Appendix K and Appendix R. The default and notional figures for the common junctions, set against ΨMonkey’s calculated values for eight named wall build-ups, are on the building regulations reference; all 38 ΨMonkey templates carry both figures in the Ψ-value library.

Read across a row and the point of calculating is obvious. A steel lintel left at its default costs twenty times what the notional dwelling assumes; a modelled lintel with a thermal break lands somewhere between 0.05 and 0.3 depending on the product. A ground floor at 0.32 against a target of 0.16 is a common single reason a design misses its fabric energy efficiency target. Sills and jambs are small numbers but there are a lot of metres of them.

“Good”, then, is relative to the notional: a calculated value at or below the Table R2 figure is beating the target at that junction. Typical calculated results for a well-detailed masonry or timber-frame wall are 0.02–0.05 for sills and jambs, 0.03–0.10 for eaves, corners and intermediate floors, 0.10–0.20 for a ground floor, and anywhere from 0.05 to 0.30 for a lintel depending on whether it is thermally broken.

How SAP uses psi values

Appendix K of SAP gives four ways to feed junctions into a calculation, in order of preference. Details from a government-approved source with independently assessed Ψ-values. Details from a reputable independent database — the successor to the old Accredited Construction Details, which SAP 10 withdrew. Ψ-values calculated by a suitably experienced person to BS EN ISO 10211 and BRE report BR 497. And, failing all three, the defaults — Table K1 per junction, or y = 0.20 W/m²K for the whole dwelling when the junctions themselves are unknown.

The routes can be mixed within one calculation: a manufacturer’s assessed lintel detail, calculated values for the floor and eaves, defaults for a junction nobody has drawn. Every calculated value needs documentary evidence — the model and its result — which is what the Building Control PDF from a calculation tool is for.

How a psi value is calculated

A Ψ-value comes from a two-dimensional numerical model of the junction, built and run to BS EN ISO 10211 with the UK conventions in BR 497. The junction is drawn as a cross-section — the wall, the floor slab and its insulation, the cavity closer, the lintel — extended at least a metre into each adjoining element so the edges of the model behave like the flat elements they represent. Each material gets its thermal conductivity; the inside and outside surfaces get the standard surface resistances; a temperature difference is applied and the model solves for the heat flow through the whole section.

That total, per metre of junction, is the L2D value. The elements’ own share is already known: U-value × the length of each element in the model. Subtract it and what is left is the junction’s extra loss:

Ψ = L2D − Σ (U × l)

A worked example: a ground-floor junction modelled with 1.5 m of wall at U = 0.18 and 1.5 m of floor at U = 0.13 returns L2D = 0.62 W/m·K. The elements account for 0.18 × 1.5 + 0.13 × 1.5 = 0.465, so Ψ = 0.62 − 0.465 = 0.155 W/m·K — a whisker inside the notional dwelling’s 0.16, and half the 0.32 default.

The same model gives a second number at no extra cost: the temperature factor, f_Rsi: how warm the coldest internal surface at the junction stays, as a fraction of the way from the outside temperature to the inside one (1.0 would be a surface at room temperature, 0 one at outdoor temperature). For dwellings f_Rsi should be at least 0.75 (BRE Information Paper IP 1/06) to keep surface condensation and mould off the junction — a check Building Control can ask for alongside the Ψ-value, and one that a junction can fail even when its Ψ is respectable.

Numerical modelling used to mean desktop software and a consultant’s day rate; it can now be done in a browser. ΨMonkey carries the 38 standard SAP junction templates, solves them to ISO 10211 against your build-up with Ψ and f_Rsi live on screen, is validated against the ISO 10211 reference cases, and produces the Building Control PDF for £25 +VAT a junction.

Psi values for roof windows and rooflights

The roof-window junctions — head, sill and jamb, R1 to R3 — are the ones most often left at default, and among the most expensive to leave. The default is 0.24 W/m·K for each; the notional dwelling assumes 0.08, 0.06 and 0.08. A dormer with three roof windows can carry 15 m of these junctions, so the defaults cost 3.6 W/K against a notional 1.1 W/K — the difference between passing and failing the fabric target on an otherwise good design. Modelled with a properly insulated upstand the calculated values usually sit close to the notional figures.

SAP 10.2, SAP 10.3 and the Home Energy Model

Nothing about psi values changes between the SAP versions: the same Appendix K routes, the same Table K1 defaults, the same calculation method. What changes is the pressure. The Future Homes Standard’s notional dwelling has an air permeability of 4 and a heat pump in place of the gas boiler, so the easy wins elsewhere in the calculation are smaller, and the ten- or twenty-fold gap between a default and a calculated junction is more likely to be the whole margin. The Home Energy Model applies thermal bridges at every half-hourly timestep rather than in a monthly heat-loss sum, but it takes the same Ψ-values as its input.

The bottom line

A psi value is a junction’s heat loss per metre. SAP will happily use a punitive default if you let it, and the target dwelling it compares you with does not. Calculating the value — for the junctions with the most metres and the biggest default-to-notional gap first: ground floor, lintels, roof windows, eaves — is the cheapest fabric improvement on most new-build designs, because it changes the number without changing the building.