ΨMonkey — junction Ψ-values, made easy

What a Ψ-value is — in 60 seconds

U-values describe heat loss through the flat areas of walls, roofs and floors. But where two elements meet — a wall head meeting a flat roof, a floor slab meeting an external wall — the geometry pinches the insulation and extra heat escapes along the length of that joint. The Ψ-value (psi, in W/m·K) is that extra loss, per metre of junction.

In SAP (the energy calculation behind Part L), every junction either uses a default Ψ-value from the standard tables — deliberately pessimistic — or a calculated one. Calculated values are almost always much better: a flat-roof junction defaults to 0.16 W/m·K, while a decent modelled detail often comes out at a third of that. Lower Ψ-values mean a better SAP result, which can be the difference between a design that passes and one that needs more insulation elsewhere.

ΨMonkey computes Ψ the proper way — a 2-D finite-difference heat-flow model of the actual junction, following BS EN ISO 10211 conventions — and also checks f_Rsi, the surface-temperature factor that tells you whether the internal surface is cold enough to risk mould (it must be ≥ 0.75 for dwellings).

The whole job in three steps

That's genuinely it for most jobs — a junction Ψ-value in two or three minutes. The rest of this guide fills in the detail and shows you the powerful bits hiding one level down.

Pick your junction

Click 🧱 Build-up assistant at the top-left of the canvas. In the panel that opens, either choose a junction from the Start from a standard junction dropdown, or click ⊞ Browse the visual gallery and pick by picture — often faster if you know what the detail looks like but not its reference.

The library covers the full SAP 10 set: E1–E25 (lintels, cills, jambs, ground floors, balconies, eaves, gables, flat roofs, corners), the party-wall P-series (P1–P8) and roof windows R1–R4. Each loads with sensible accredited-reference build-ups, one-metre flanking arms, and every surface pre-assigned — a complete, ready-to-solve model.

Make it your build-up

Once a junction is selected, its construction appears as editable layer lists — one list per element (wall, deck, floor…), each labelled with its direction so you always know which end is outside. A live preview redraws as you edit, with the warm side tinted orange and the cold side blue, and a live Ψ figure re-solves after each change — you can watch the number respond to your design decisions before anything touches the canvas.

Start from a wall preset

Above the wall layers there's a build-up preset picker. Cavity masonry (full or partial fill) is the accredited default, and below it sit the constructions that are tedious to type out: timber frame brick-clad, with a service void, or with insulated sheathing; render and rainscreen; twin-stud / closed panel with 300 mm cellulose; and CLT with external woodfibre. Each arrives with its bridging fractions already set — 15% for studs at 600 mm centres once you count openings, headers and sole plates, 9% for a battened service void, 8% for twin-stud webs.

Picking a framed wall also changes the junction, not just the numbers. A masonry opening is closed with a proprietary insulated cavity closer; a timber-frame one is closed with a solid cripple stud, and the window has no closer to lap onto. At the eaves, a framed wall has no cavity to fire-stop and instead carries a solid header plate across the top of the studs, with the rafters running over it. ΨMonkey reads this from your build-up — any layer bridged by a timber means a framed wall — and draws the right detail without being told.

Editing layers

Each row is one layer: a material picker and a thickness in mm. The picker has a search box plus ★ favourites and 🕘 recents, and around 80 built-in UK materials with sourced conductivities. Thicknesses can be typed directly, stepped by 1 mm with the input's own arrows, or stepped by 10 mm with the −10/+10 buttons (or Shift+↑/↓ in the box). Add, remove and drag-reorder layers freely.

Bridged layers — studs, joists and battens

Real layers are often not homogeneous: insulation crossed by timber studs, a joist zone that is mostly air, a service void with battens. Click the ⊞ button on any layer to declare a repeating bridge: choose the bridging material and its percentage (50 mm joists at 400 mm centres = 12.5%). ΨMonkey computes the equivalent conductivity and uses it consistently in the drawing, the solver and the flanking calculation. You can do the same to a material you've already painted on the grid — see Editing on the canvas.

Draw, solve, and read the results

Press 🧱 Draw junction onto grid and the assistant stamps the whole model onto the canvas — construction, air regions, and every boundary condition. Then press ▶ Solve (top right). A few hundred milliseconds later you have your answer.

The two numbers that matter

Trusting the number

The results panel isn't just the headline figure — it shows the full working: total heat flow, the 1-D flanking losses being subtracted (Σ U·l on SAP internal dimensions), the heat balance error, the ISO 10211 extent check and whether the solver converged. Green ticks all the way down means the model is numerically sound. There's also a ✓ Validate button that runs the solver against a suite of 21 published analytical and standard test cases — press it any time you want to see the engine prove itself.

Where the flanking came from

Ψ is a subtraction — the 2-D heat flow minus the 1-D losses the flat elements would have had anyway — so the number being subtracted matters as much as the model. The results panel names which basis was used, and the report carries it as a quality check:

You'll see that amber note in a few other situations too: a project file saved by an older version that didn't record its build-ups, a layer too thin to draw on that junction's grid, or a fixed-resistance material (an air cavity) painted more than one cell thick. Each one is a case where the drawing and the arithmetic could quietly disagree, so the tool says it out loud rather than printing a confident number.

Editing on the canvas

Once a junction is drawn, the Editing panel on the left keeps a slider for every layer. Drag a slider and the model redraws and re-solves as you watch — the fastest way to answer "what if the insulation were 20 mm thicker?" The − and + buttons at each end nudge by exactly 1 mm for precise tweaks, and ✎ Edit build-up takes you back to the assistant with everything preserved.

Drag a boundary

The sliders move whole layers of the build-up. When you want to move one interface — thicken the insulation at the cavity's expense, drop the sheathing back a few millimetres — pick the ↔ Move boundary tool and hover where two materials meet. The cursor turns into a resize arrow, the interface highlights, and the status bar reads out both thicknesses. Drag perpendicular to it and one material grows exactly as much as the other shrinks.

Two things make this safe. It only moves the stretch where those two materials meet, so dragging a wall/insulation interface doesn't disturb the same pair somewhere else in the drawing. And the travel is clamped to the layer being pushed: drag hard at a 50 mm cavity and it stops at 50 mm rather than quietly starting on the blockwork behind it. Because both sides move together, you can't open a gap — and a gap is not cosmetic here, since an unpainted cell is outside the model.

Add bridging to what you've drawn

Realised afterwards that a layer has studs in it? Any material that's actually in the model carries a ⊞ in the palette. Click it, pick the bridging material and fraction, and every cell of that material is replaced with the equivalent-λ version — the same derived material the assistant uses, so the solver, the U-value and the flanking all stay in step. Bridged materials show a ⊟ to take it back off again.

Handy canvas controls: mouse-wheel over the canvas zooms at the pointer (up to 800%), middle-button drag pans, + − 0 zoom from the keyboard, and the Fit button reframes the whole model. Zooming matters more than you'd think — some junctions (ground floors especially) model several metres of soil, so the interesting detail is a small part of the drawing.

Drawing your own detail

Two routes when your junction isn't in the library:

1. Build elements from layers, join them on the canvas

The lower half of the Build-up assistant builds a single element — a wall, floor or roof — from a layer list, exactly like the junction editor. Choose vertical or horizontal, set the length, and stamp it; ΨMonkey picks a cell size that reproduces your thicknesses and adds the surface boundary conditions. Stamp two elements so they meet, tidy the intersection with the paint tools, and you have a custom junction.

2. Paint it cell by cell

The canvas is ultimately a grid you can paint: pick a material, choose a tool (paint, line, fill, rectangular marquee) and draw the section directly — then tune it with the boundary drag and the ⊞ bridging control. Three rules keep a hand-drawn model honest:

Getting the number into SAP

The results panel tells you, in one line, exactly what to enter: the Ψ-value for that junction type in the SAP software's junction schedule (the y-value / thermal-bridging page). Give your assessor the value and the junction reference; they enter it as a calculated value in place of the default.

When the calculated value needs evidence behind it — Building Control ask, or the assessor wants it on file — Export PDF produces a report with the drawing, the temperature field, the build-up schedule with every conductivity and its source, the full ISO bookkeeping, f_Rsi, the validation status and a verification reference. It's £35 +VAT per report; the calculator itself, and everything above, is free without sign-up.

Good habits & common pitfalls

If a result looks wrong

Read the amber notes first — under the Ψ figure and in the results panel. They cover the things that most often make a number wrong rather than merely surprising: the flanking basis, a layer too thin for the grid, a fixed-resistance material painted too thick.

Then zoom out and look at the whole drawing. Nine times out of ten a surprising Ψ is geometry: a construction run stopping short of the canvas edge, boundary-condition cells painted onto material (they're flagged with a diagonal cross), or inside and outside air meeting around the end of an element. The assistant-drawn junctions get all of this right automatically — hand-edits are where to look. And be suspicious of a result that looks too good: a Ψ well below the published value for that junction type usually means something has been subtracted that shouldn't have been, not that you've designed something remarkable.

Quick answers