E15 / P1–P8 SAP / Approved Document L reference junction
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What the party wall junction is
The party wall junction covers where a separating wall between two dwellings meets the external wall, floor or roof. A filled and sealed party wall cavity can be assigned zero heat loss; an unsealed one cannot.
Why it matters
An open or unsealed party wall cavity can draw air up and out at the eaves — the "party wall bypass" — turning the separating wall into an unexpected heat-loss route despite sitting between two heated homes.
How to model it
Model the separating wall, its cavity (fully filled and edge-sealed, or open) and its junction with the external wall or roof, then solve to demonstrate the detail.
A fully filled and sealed party wall cavity supports a zero U-value; the junction calculation documents that the edge sealing is in place.
Related build-ups: the U-values either side of this junction
The Ψ-value at the party wall junction depends on the walls, floors and roof around it — so the first step is knowing the U-values of those elements. Typical figures for the build-ups that pair with this junction:
- U-value of a solid brick wall with internal insulation
- U-value of a cavity wall with PIR partial-fill insulation
Model your exact build-up and read its U-value with the free U-Monkey calculator (PDFs included free), then come back here and solve the junction in ΨMonkey.
Free to model — no subscription
ΨMonkey isn't a monthly licence or a per-seat subscription. Build the party wall junction on the grid, solve it, and read the Ψ-value and fRsi on screen for nothing. You only pay £25 +VAT (£30 inc VAT), once, when you need the Building Control-ready PDF for that junction — so calculating a single detail never means signing up to anything. Calculated psi values are often the cheapest route to a SAP 10 pass, so the maths can pay for itself.
Frequently asked questions
What is the party wall junction (E15 / P1–P8)?
The party wall junction covers where a separating wall between two dwellings meets the external wall, floor or roof. A filled and sealed party wall cavity can be assigned zero heat loss; an unsealed one cannot. An open or unsealed party wall cavity can draw air up and out at the eaves — the "party wall bypass" — turning the separating wall into an unexpected heat-loss route despite sitting between two heated homes.
How do I calculate the party wall junction psi value?
Model the separating wall, its cavity (fully filled and edge-sealed, or open) and its junction with the external wall or roof, then solve to demonstrate the detail. ΨMonkey solves it to BS EN ISO 10211 and returns the Ψ-value (W/m·K) and the f_Rsi temperature factor on screen, free.
Is it free, and is there a subscription?
No subscription and no licence — modelling the junction and reading off the Ψ-value is free in your browser. You only pay £25 +VAT (£30 inc VAT) when you export the Building Control-ready PDF for that junction.
Can I use the result in a SAP or HEM assessment?
Yes — a calculated Ψ-value to ISO 10211 can replace the SAP default for this junction, and A fully filled and sealed party wall cavity supports a zero U-value; the junction calculation documents that the edge sealing is in place.
Other junctions
ΨMonkey models any junction. Jump to another common one:
- Wall-to-Floor Junction E5
- Window Jamb (Reveal) E4
- Window Sill E3
- Lintel E1 / E2
- Eaves Junction E9 / E10
- Gable Junction E11 / E12
- External Corner E13
- Inverted Corner E14
- Intermediate Floor E6
- Balcony Junction E8 / E24
- Exposed Floor E20 / E21
Related reading
- Thermal bridging explained — why junctions leak heat and how Ψ-values work
- What is a U-value? — the element-level heat loss this junction links
- SAP 10: what's new — how calculated Ψ-values beat defaults
- SAP calculations — put your Ψ-values to work in the full compliance model
- New Build Compliance Pack — SAP, water, acoustics and air in one