Quick answer: y = Σ(L × Ψ) ÷ Aexp, in W/m²K. SAP 10 assumes y = 0.20 when the junctions are unknown; a well-detailed house with calculated Ψ-values comes out around 0.04–0.08. The calculator below starts with a typical detached house at SAP's Table K1 defaults — y ≈ 0.20 — and shows the same house at the notional dwelling's values, y ≈ 0.05.
Y-value calculator
Add a row for each junction type, enter its total length, and set Ψ to your calculated value (it starts at SAP's default for that junction). Everything updates as you type.
| Junction | Length L (m) | Your Ψ (W/m·K) | SAP default | Notional | L × Ψ (W/K) | Remove |
|---|
Per-junction figures: SAP 10.3 specification (12-06-2026): Table K1 (default Ψ, Appendix K) and Table R2 (notional dwelling Ψ, Appendix R). The y-value is a check figure — SAP itself takes the junction lengths and Ψ-values, so the numbers to hand to your assessor are the rows, not the total. Junction Ψ-values from the free psi value calculator can be typed straight into the Ψ column.
What the y-value is
Every junction in a dwelling — where a wall meets a floor, a roof, a window or another wall — loses heat over and above what the U-values of the elements account for. That extra loss is the junction's psi value, Ψ, in watts per metre per kelvin. SAP multiplies each Ψ by the junction's length, adds the results to get the thermal bridging heat loss HTB in W/K, and adds that to the fabric heat loss. The y-value is simply HTB divided by the total exposed envelope area, so that thermal bridging can be read as a figure in the same units as a U-value.
Appendix K of SAP allows one shortcut: where nothing is known about the junctions, the calculation may use y = 0.20 W/m²K across the whole envelope instead of junction-by-junction values. That is an expensive shortcut. On the example house above it is the equivalent of 48.9 W/K of junction loss — about the same as the Table K1 defaults row by row — where the notional dwelling the target is set from assumes 13.6 W/K.
Reading the three totals
All at SAP default is what your dwelling is charged if the assessor has no evidence for any junction. All at notional is what the target dwelling assumes, so it is the line you are effectively competing with. With your Ψ-values is where you are: every row you replace with a calculated value to BS EN ISO 10211 moves that total toward the notional line, and the L × Ψ column shows which rows are worth the effort — usually the ground floor, the lintels and the openings, because they have the most metres and the largest gap between default and calculated.
Where the Ψ-values come from
Three sources are acceptable in SAP: details from an approved or independent database with assessed Ψ-values, values calculated by a suitably experienced person to BS EN ISO 10211 and BR 497, and the Table K1 defaults. The calculated route is the one that gets a design from the default line to somewhere near the notional one. ΨMonkey does the modelling free in the browser for all 38 standard junction templates; the Ψ-value library shows what typical build-ups achieve before you model your own, and the building regulations reference keeps the default and notional figures for the common junctions on one dated page.
Frequently asked questions
What is the y-value in SAP?
The y-value is SAP's summary figure for thermal bridging: the total junction heat loss, Σ(length × Ψ) in W/K, divided by the total area of the exposed envelope in m², so it is in W/m²K like a U-value. A well-detailed dwelling with calculated Ψ-values typically comes out between 0.04 and 0.08; a dwelling with no junction information at all is given y = 0.20 by Appendix K.
How do I calculate a y-value?
List every junction in the dwelling with its length in metres and its Ψ-value in W/m·K, multiply each pair and add them up to get the thermal bridging heat loss H_TB in W/K. Divide H_TB by the total area of exposed elements — all the walls, roofs, floors, windows and doors that lose heat to outside or to unheated spaces — to get y in W/m²K. This page does the arithmetic as you type.
Is a y-value of 0.20 bad?
It is the figure SAP 10 uses when nothing is known about the junctions, and it is deliberately unfavourable — roughly three times what a well-detailed house achieves. A design assessed at y = 0.20 needs noticeably better walls, more insulation or more PV to pass than the same design with calculated Ψ-values, which is why junction calculations are usually the cheapest improvement available.
What is the difference between a y-value and a psi value?
A psi value (Ψ) belongs to one junction and is per metre of junction, in W/m·K. The y-value is the whole dwelling's junction loss spread over its envelope area, in W/m²K. SAP works junction by junction — the assessor enters lengths and Ψ-values, not a y-value — so the y-value is a check figure, not an input, unless the default is being used.
What Ψ-value does SAP use if I don't calculate one?
The default for that junction from Table K1 of the SAP specification: 1.00 W/m·K for a lintel, 0.32 for a ground floor, 0.10 for sills and jambs, 0.12 for eaves at ceiling level, 0.24 for a party wall or a roof-window junction. The notional dwelling the target is set from assumes far lower values — 0.05 for lintels, sills and jambs, 0.16 for a ground floor, 0.06 for eaves — so every default is a penalty against the target.
Which junctions matter most?
The ones with the most metres and the biggest gap between default and calculated value: the ground-floor junction around the whole perimeter, lintels, jambs and sills around every opening, and the eaves. Calculate those first. Use the calculator above to see which rows are contributing most to your total.
Next: calculate the wall-to-floor junction — the row with the most metres on most plots — or read what a good psi value looks like.