Free to model. Building Control-ready PDF £35 +VAT a junction, or unlimited on Pro.
The table
Ψ in W/(m·K), fRsi beneath. Columns are the assistant's eight wall build-ups, with each wall's U-value; rows are the templates, grouped as SAP groups them. Click any figure to open that exact junction and wall in ΨMonkey with Ψ live. 27 of the 38 published templates involve the external wall; the rest solve the same whatever wall you pick, and say so.
One reading rule. A negative Ψ is expected at an inverted junction — E17, P8 and their kind — because the internal-dimension convention over-counts the 1-D loss there. On any other junction a negative is marked, and it means the template's default detail doesn't suit that wall: the right response is to open it and model your own, not to use the figure.
Ψ in W/(m·K), with fRsi beneath. Red is fRsi 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.
What these are — and aren't
They are ΨMonkey's own results for its standard templates with those exact layers, produced by the same solver that runs when you press Solve, and checked against the BS EN ISO 10211 test reference case on every build (the published results). They are not accredited construction details, not manufacturer values, and not a claim about any product.
They are also not your junction. A different block density, a thicker insulation board, a lintel with a different plate, a floor with the insulation in another place — each moves the number, sometimes a lot. That is the whole point of the links: the table gets you to the right starting model in one click, and the tool does the rest on your layers.
Calculated or default?
SAP lets you use a default Ψ-value for a junction you have not calculated. Defaults are deliberately pessimistic, and on a well-detailed dwelling the gap between default and calculated thermal bridging is often the cheapest heat loss you will ever recover — no product change, no site change, just arithmetic done properly. How that plays into a SAP 10 pass is in thermal bridging explained and what changed in SAP 10.
Junction by junction
Each junction has its own page with the geometry, why it bridges, how to model it and its rows from this table:
- Wall-to-Floor Junction E5 / E19 / E22
- Window Jamb (Reveal) E4
- Window Sill E3
- Lintel E1 / E2
- Eaves Junction E10 / E11 / E24
- Gable Junction E12 / E13
- External Corner E16
- Inverted Corner E17
- Intermediate Floor E6 / E7
- Party Wall Junction E18 / E25 / P1–P8
- Balcony Junction E8 / E9 / E23
- Exposed Floor E20 / E21
Frequently asked questions
What is a psi value?
A Ψ-value (psi value) is the linear thermal transmittance of a junction — the extra heat lost per metre length where two elements meet, over and above what the U-values of the elements themselves account for. It is measured in W/(m·K) and calculated to BS EN ISO 10211. In SAP every junction in the dwelling carries one, and the sum of Ψ × length is the thermal bridging heat loss.
Where do the numbers on this page come from?
They are ΨMonkey's own results. Each of the 38 standard junction templates was solved with the external wall set to each of the assistant's 8 named build-ups and everything else at the template default — 304 solves, all converged, run on the app exactly as shipped on 2 September 2026. Nothing is typed in or estimated; the table is regenerated whenever the app changes.
Can I put these psi values straight into a SAP calculation?
Only if the detail on your drawings is the detail in the template — and it usually isn't quite. Treat the table as the starting point: click the figure, the app opens with that junction and wall loaded, change the layers that differ from yours, and read your own result. A calculated Ψ-value to ISO 10211 for your actual detail is what replaces the SAP default; a Building Control-ready PDF for a junction is £35 +VAT, or included with Pro Unlimited.
Why does the same junction give a different psi value with a different wall?
Because the bridge is the interaction between the elements. A timber-frame wall moves the insulation line, a full-fill cavity changes how much heat the block inner leaf carries to the floor, and a service void adds a warm layer inside. The wall changes what happens at the junction even though the floor, lintel or roof is identical — which is why a single "default" figure per junction type is so crude.
Why are some psi values negative?
At an inverted junction, because Ψ is defined against U-values applied over internal dimensions: the internal surface is shorter than the external one, the 1-D calculation over-counts the loss, and the correction — the Ψ-value — comes out negative. That is expected, and SAP accepts it. On a junction that is not inverted a negative is marked in the table: it means the template’s default detail does not suit that wall, and the figure is a prompt to model your own rather than one to use.
Why do some rows show one figure for every wall?
Because the external wall is not part of that junction. Party-wall floors, roof windows and the ridge are solved on their own elements, so swapping the external wall preset changes nothing, and the table says so rather than repeating the same number eight times.
Back to the ΨMonkey psi value calculator. The wall build-ups behind the columns are the assistant's own presets; the constants behind the U-value tool are published as open data.