Party Wall Junction Psi Value Calculator (E18 / E25 / P1–P8)

E18 / E25 / P1–P8 SAP / Approved Document L reference junction

Schematic cross-section of the party wall junction (E18 / E25 / P1–P8) showing the structure, insulation line and the heat-bridge path — Schematic of the party wall junction (E18 / E25 / P1–P8) — not to scale. Model your actual build-up in ΨMonkey.

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What the party wall junction is

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.

What ΨMonkey solves this junction at

ΨMonkey's own results for its standard party wall junction templates, with the external wall set to each of the assistant's eight named build-ups and everything else left at the template default. They are what the tool gives for those exact layers — not accredited values, and not your detail: a different block, a thicker board or a different lintel plate all move the number. Open any figure and the app loads that junction and wall with Ψ live; change what differs and read your own.

Junction	Masonry, full fillU 0.18	Masonry, partial fillU 0.28	Timber frame, brickU 0.27	TF brick + service voidU 0.26	TF + 50 PIR sheathingU 0.16	Timber frame, renderU 0.28	Twin stud, 300 celluloseU 0.11	CLT + EWIU 0.18
E18Party wall / external walli	−0.0010.973	0.0460.951	−0.0070.959	−0.0030.960	0.0170.973	−0.0070.958	−0.0010.983	0.0000.972
E25Staggered party walli	−0.0000.969	0.0490.944	−0.0050.954	−0.0010.955	0.0170.969	−0.0060.953	−0.0010.980	0.0010.968
P1Party wall — ground floor	0.1760.917	0.1760.917	0.1760.917	0.1760.917	0.1760.917	0.1750.917	0.1770.917	0.1760.917
P6Party wall — ground floor (inverted)	0.1190.903	0.1190.903	0.1190.903	0.1190.903	0.1200.903	0.1180.903	0.1200.903	0.1190.903
P2Party wall — intermediate floor (within dwelling)	0.054f_Rsi0.979same for every wall — the external wall isn't part of this junction
P3Party wall — floor between dwellings	0.054f_Rsi0.979same for every wall — the external wall isn't part of this junction
P7Party wall — exposed floori	−0.001f_Rsi0.966same for every wall — the external wall isn't part of this junction
P8Party wall — exposed floor (inverted)	−0.007f_Rsi0.950same for every wall — the external wall isn't part of this junction
P4Party wall — roof (insulation at ceiling)	0.260f_Rsi0.819same for every wall — the external wall isn't part of this junction
P5Party wall — roof (insulation at rafter)	0.017f_Rsi0.968same for every wall — the external wall isn't part of this junction

Ψ in W/(m·K), with f_Rsi beneath. Red is f_Rsi below 0.75, the BRE IP 1/06 condensation criterion for dwellings. Marked negatives on a non-inverted junction mean the template's default detail doesn't suit that wall — a prompt to model your own, not a figure to use. Click any figure to open that junction and wall in ΨMonkey with Ψ live.

The eight walls, and how these figures were produced

Cavity masonry — full fill (accredited default)U 0.176 W/m²K100 mm Facing brick · 100 mm PIR insulation (foil-faced) · 100 mm Aerated concrete block (general) · 20 mm Plasterboard on dabs (12.5 + 15mm)
Cavity masonry — partial fill + 50 mm clear cavityU 0.278 W/m²K100 mm Facing brick · 50 mm Air cavity (ISO 6946, ≥25mm) · 50 mm PIR insulation (foil-faced) · 100 mm Aerated concrete block (general) · 20 mm Plasterboard on dabs (12.5 + 15mm)
Timber frame — brick clad, 140 mm studs (15% bridged)U 0.271 W/m²K100 mm Facing brick · 50 mm Air cavity (ISO 6946, ≥25mm) · 20 mm OSB/3 (18mm) · 140 mm Mineral wool (glass/rock) + 15% Structural timber · 20 mm Plasterboard on dabs (12.5 + 15mm)
Timber frame — brick clad, 140 studs + service voidU 0.258 W/m²K100 mm Facing brick · 50 mm Air cavity (ISO 6946, ≥25mm) · 20 mm OSB/3 (18mm) · 140 mm Mineral wool (glass/rock) + 15% Structural timber · 25 mm Air cavity (ISO 6946, ≥25mm) + 9% Structural timber · 20 mm Plasterboard on dabs (12.5 + 15mm)
Timber frame — 140 studs + 50 mm PIR over the sheathingU 0.163 W/m²K100 mm Facing brick · 50 mm Air cavity (ISO 6946, ≥25mm) · 50 mm PIR insulation (foil-faced) · 20 mm OSB/3 (18mm) · 140 mm Mineral wool (glass/rock) + 15% Structural timber · 25 mm Air cavity (ISO 6946, ≥25mm) + 9% Structural timber · 20 mm Plasterboard on dabs (12.5 + 15mm)
Timber frame — render / rainscreen, 140 mm studsU 0.278 W/m²K20 mm External silicone render · 50 mm Air cavity (ISO 6946, ≥25mm) · 20 mm OSB/3 (18mm) · 140 mm Mineral wool (glass/rock) + 15% Structural timber · 20 mm Plasterboard on dabs (12.5 + 15mm)
Closed panel / twin stud — 300 mm cellulose (8% bridged)U 0.112 W/m²K20 mm External silicone render · 50 mm Air cavity (ISO 6946, ≥25mm) · 60 mm Woodfibre board (Steico) · 300 mm Cellulose fibre (blown) + 8% Structural timber · 20 mm OSB/3 (18mm) · 20 mm Plasterboard on dabs (12.5 + 15mm)
CLT + external insulation — 100 CLT, 160 woodfibreU 0.179 W/m²K20 mm External silicone render · 160 mm Woodfibre board (Steico) · 100 mm Cross-laminated timber (CLT) · 25 mm Air cavity (ISO 6946, ≥25mm) + 9% Structural timber · 20 mm Plasterboard on dabs (12.5 + 15mm)

Each figure is ΨMonkey's own result for its standard template of that junction, with the external wall set to the named build-up and every other element — floors, roofs, party walls, lintels, windows — at the template's shipped default. Boundary conditions to BS EN ISO 10211, Ti 20 °C / Te 0 °C, convergence 10⁻⁶. Solved 2 September 2026 on the app as shipped; regenerated whenever the app changes. How the solver itself is checked is on the validation page.

The full set on one page: the Ψ-value reference table.

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:

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 f_Rsi on screen for nothing. You only pay £35 +VAT (£42 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 (E18 / E25 / 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 £35 +VAT (£42 inc VAT) when you export the Building Control-ready PDF for that junction.

What psi value should I expect for the party wall junction?

It depends on the wall. ΨMonkey's standard party wall junction templates solve between −0.007 and 0.260 W/(m·K) across ΨMonkey's 8 standard walls, with everything else at the template defaults; on the accredited full-fill masonry wall that is E18 −0.001, E25 −0.000, P1 0.176, P6 0.119, P2 0.054, P3 0.054, P7 −0.001, P8 −0.007, P4 0.260, P5 0.017 W/(m·K). Those are the tool's own results for those exact layers, not accredited values — open the junction with your wall preset from the table on this page and change what differs.

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:

Party Wall Junction Psi Value Calculator