Where the default hurts most
The five junctions with the widest gap between the SAP default and the notional dwelling — the ones where accepting the default is most expensive:
- E23 Balcony within or between dwellings, balcony support penetrates wall insulation 1.00 vs 0.02 — 0.98 W/m·K worse
- E1 Steel lintel with perforated steel base plate 1.00 vs 0.05 — 0.95 W/m·K worse
- E2 Other lintels (including other steel lintels) 1.00 vs 0.05 — 0.95 W/m·K worse
- P5 Roof (insulation at rafter level) 0.48 vs 0.08 — 0.40 W/m·K worse
- P4 Roof (insulation at ceiling level) 0.48 vs 0.12 — 0.36 W/m·K worse
External wall junctions
| Ref | Junction | K1 default | R2 notional | Gap | Calculate it |
|---|---|---|---|---|---|
| E1 | Steel lintel with perforated steel base plate | 1.00 | 0.05 | 0.95 | Lintel |
| E2 | Other lintels (including other steel lintels) | 1.00 | 0.05 | 0.95 | Lintel |
| E3 | Sill | 0.10 | 0.05 | 0.05 | Window Sill |
| E4 | Jamb | 0.10 | 0.05 | 0.05 | Window Jamb (Reveal) |
| E5 | Ground floor (normal) | 0.32 | 0.16 | 0.16 | Wall-to-Floor Junction |
| E19 | Ground floor (inverted) | 0.10 | 0.07 | 0.03 | Wall-to-Floor Junction |
| E20 | Exposed floor (normal) | 0.32 | 0.32 | 0.00 | Exposed Floor |
| E21 | Exposed floor (inverted) | 0.32 | 0.32 | 0.00 | Exposed Floor |
| E22 | Basement floor | 0.22 | 0.07 | 0.15 | Wall-to-Floor Junction |
| E6 | Intermediate floor within a dwelling | 0.14 | 0.00 | 0.14 | Intermediate Floor |
| E7 | Party floor between dwellings (in blocks of flats) | 0.28 | 0.07 | 0.21 | Intermediate Floor |
| E8 | Balcony within a dwelling, wall insulation continuous | 0.10 | 0.00 | 0.10 | Balcony Junction |
| E9 | Balcony between dwellings, wall insulation continuous | 0.15 | 0.02 | 0.13 | Balcony Junction |
| E23 | Balcony within or between dwellings, balcony support penetrates wall insulation | 1.00 | 0.02 | 0.98 | Balcony Junction |
| E10 | Eaves (insulation at ceiling level) | 0.12 | 0.06 | 0.06 | Eaves Junction |
| E24 | Eaves (insulation at ceiling level — inverted) | 0.15 | 0.24 | −0.09 | Eaves Junction |
| E11 | Eaves (insulation at rafter level) | 0.15 | 0.04 | 0.11 | Eaves Junction |
| E12 | Gable (insulation at ceiling level) | 0.25 | 0.06 | 0.19 | Gable Junction |
| E13 | Gable (insulation at rafter level) | 0.25 | 0.08 | 0.17 | Gable Junction |
| E14 | Flat roof | 0.16 | 0.08 | 0.08 | — |
| E15 | Flat roof with parapet | 0.30 | 0.56 | −0.26 | — |
| E16 | Corner (normal) | 0.18 | 0.09 | 0.09 | External Corner |
| E17 | Corner (inverted — internal area greater than external area) | 0.00 | −0.09 | 0.09 | Inverted Corner |
| E18 | Party wall between dwellings | 0.24 | 0.06 | 0.18 | Party Wall Junction |
| E25 | Staggered party wall between dwellings | 0.24 | 0.06 | 0.18 | Party Wall Junction |
Party wall junctions
| Ref | Junction | K1 default | R2 notional | Gap | Calculate it |
|---|---|---|---|---|---|
| P1 | Ground floor | 0.32 | 0.08 | 0.24 | — |
| P6 | Ground floor (inverted) | 0.32 | 0.07 | 0.25 | — |
| P2 | Intermediate floor within a dwelling | 0.00 | 0.00 | 0.00 | — |
| P3 | Intermediate floor between dwellings (in blocks of flats) | 0.00 | 0.00 | 0.00 | — |
| P7 | Exposed floor (normal) | 0.48 | 0.16 | 0.32 | — |
| P8 | Exposed floor (inverted) | 0.48 | 0.24 | 0.24 | — |
| P4 | Roof (insulation at ceiling level) | 0.48 | 0.12 | 0.36 | — |
| P5 | Roof (insulation at rafter level) | 0.48 | 0.08 | 0.40 | — |
Roof and room-in-roof junctions
| Ref | Junction | K1 default | R2 notional | Gap | Calculate it |
|---|---|---|---|---|---|
| R1 | Head of roof window | 0.24 | 0.08 | 0.16 | — |
| R2 | Sill of roof window | 0.24 | 0.06 | 0.18 | — |
| R3 | Jamb of roof window | 0.24 | 0.08 | 0.16 | — |
| R4 | Ridge (vaulted ceiling) | 0.12 | 0.08 | 0.04 | — |
| R5 | Ridge (inverted) | 0.12 | 0.04 | 0.08 | — |
| R6 | Flat ceiling | 0.12 | 0.06 | 0.06 | — |
| R7 | Flat ceiling (inverted) | 0.12 | 0.04 | 0.08 | — |
| R8 | Roof to wall (rafter) | 0.12 | 0.06 | 0.06 | — |
| R9 | Roof to wall (flat ceiling) | 0.32 | 0.04 | 0.28 | — |
| R10 | All other roof or room-in-roof junctions | 0.32 | 0.08 | 0.24 | — |
| R11 | Upstands or kerbs of rooflights | 0.24 | 0.08 | 0.16 | — |
All values W/(m·K). Gap is the K1 default minus the R2 notional — how much worse the default is than what the notional dwelling assumes for that junction; bold marks a gap of 0.20 or more. A dash in the last column means we have no dedicated page for that reference yet — ΨMonkey still models it.
What this table is for
Deciding which junctions are worth calculating. A junction with a small gap and a short total length is usually fine on the default. A junction with a wide gap and a long run — lintels and jambs on a heavily glazed elevation, the ground floor perimeter, a balcony that penetrates the insulation line — is where a calculated Ψ-value to BS EN ISO 10211 pays for itself several times over on a single plot.
Our own calculated results for the standard junctions, solved against eight different wall build-ups, are in the Ψ-value library — that is what these defaults are worth beating in practice. The y-value calculator turns a set of junction Ψ-values and lengths into the single figure SAP uses, and the ACD and RCD guide covers the scheme details that were the old shortcut.
Sources
SAP 10.3 specification (12-06-2026): Table K1 (default Ψ, Appendix K) and Table R2 (notional dwelling Ψ, Appendix R). Junction names and an independent check of all 44 Table K1 defaults from BRE SAP 10 technical paper S10TP-05, "Treatment of thermal bridges", Appendix B, checked 19 September 2026 — all 44 agree. SAP also defines a B1–B6 set for junctions with unheated spaces; those are deliberately not listed here, because they are not part of the Table K1 and R2 transcription this page is built from and an unchecked value is worse than none.
Frequently asked questions
What is the difference between the Table K1 default and the Table R2 notional value?
Table K1 is what SAP uses for a junction when you have no calculated value for it — deliberately conservative, because it has to cover the worst plausible version of that detail. Table R2 is what the notional dwelling assumes, which is effectively the figure your design is measured against. Where the default is much larger than the notional, accepting the default costs you real compliance margin on every metre of that junction.
What happens if I know nothing about the junctions at all?
SAP falls back to a single overall figure: y = 0.2 W/m²K applied to the whole thermal envelope. It is the most punitive option available and almost always worth avoiding — even the Table K1 defaults beat it on most dwellings.
Can I just use the Table K1 defaults?
You can, and for a junction with a short total length it is often the sensible choice. It stops being sensible where the default is far above the notional and the junction is long — E1 and E2 lintels at 1.00 W/m·K against a notional 0.05, or E23 balconies at 1.00, are where dwellings fail on thermal bridging alone.
Where do these numbers come from?
SAP 10.3 specification (12-06-2026): Table K1 (default Ψ, Appendix K) and Table R2 (notional dwelling Ψ, Appendix R). The Table K1 defaults were independently re-checked on 19 September 2026 against BRE’s SAP 10 technical paper S10TP-05 "Treatment of thermal bridges", which lists the same junction set; all 44 agree.
Why do some junctions have a negative value?
At an inverted junction — E17 and the other "inverted" references — Ψ is defined against U-values applied over internal dimensions. The internal surface is shorter than the external one, so the one-dimensional calculation over-counts the loss and the correction comes out negative. That is expected, and SAP accepts it.