Free Psi Value Calculator (ISO 10211) — ΨMonkey

Runs entirely in your browser. Nothing to install, no account needed.

Heads-up: ΨMonkey is designed for desktop — modelling junctions works best with a mouse and a bigger screen. Feel free to look around, then come back at your desk.

Every paid PDF carries a printed report ID — Building Control can verify it in seconds.

PSIMonkey psi value calculator: a wall-floor junction painted on the modelling grid, with the solved psi-value and f_Rsi temperature factor shown in the results panel — Watch ΨMonkey in action — pick a standard SAP junction, change a thickness and see Ψ re-solve live, then solve for the full temperature field. Watch the 22-second demo →

What is a Ψ-value, and how is it calculated? A Ψ-value (linear thermal transmittance, W/m·K) is the extra heat lost along a junction — an eaves, a lintel, a wall-to-floor — over and above the walls, roofs and floors either side of it. It is found by modelling the junction's real geometry and materials, solving the two-dimensional temperature field to BS EN ISO 10211, then subtracting the one-dimensional heat flow of the flanking elements from the modelled total, on SAP internal dimensions. Calculated values almost always beat the deliberately pessimistic SAP defaults, so thermal bridging stops being a penalty. ΨMonkey solves any of the 38 standard SAP junctions — or a detail you draw yourself — in your browser and shows Ψ and the f_Rsi temperature factor free on screen. You only pay £35 +VAT if you export the Building Control-ready PDF.

Why calculate your own Ψ-values?

SAP 10 and the Home Energy Model penalise default junction Ψ-values heavily. Thermal bridging can be 20–30% of a dwelling's fabric heat loss, so swapping defaults for calculated values is often the cheapest route to a Part L pass — no spec changes, just better numbers for the details you're already building.

How it works

Pick your junction — 38 ready-made SAP details: the full E-series (E1–E25), party-wall P-series (P1–P8) and roof windows (R1–R4). It draws itself, with 1 m flanking arms and surfaces already assigned.
Set your build-up — swap materials and thicknesses in the layer list; Ψ and f_Rsi re-solve as you type, so you can test a spec change in seconds. Add repeating bridges (timber studs through insulation) where you need them.
Solve — a finite-difference solver to BS EN ISO 10211 computes the full 2D temperature and heat-flux field. Not in the library? Draw any detail cell-by-cell on the grid instead.
Export when ready — a Building Control-ready PDF report of the model, materials and results for £35 +VAT (£42 inc VAT). Re-export the same junction free for 24 hours.

New to thermal modelling? The step-by-step ΨMonkey user guide walks a whole junction through from scratch — picking the SAP detail, matching your build-up, reading Ψ and f_Rsi, and what to hand Building Control.

Calibrated, not just computed

Anyone can put a solver in a browser; the question is whether you can trust the number. Every built-in junction is checked against published accredited construction details — modelled on the same build-up, the standard junctions land within about ±0.02 W/m·K of the published Ψ values, across openings, floors, roofs, corners and party walls. Flanking heat loss is subtracted analytically on SAP internal dimensions (BR 497), ground floors use a modelled ground domain to BS EN ISO 13370, and every model reports its f_Rsi against the 0.75 condensation threshold. Edit a build-up and the 2D model, the flanking and the dimension convention all re-derive together.

Validated, and we publish the results

BS EN ISO 10211 publishes a two-dimensional test reference case precisely so that any calculation method can be checked against a known answer. ΨMonkey reproduces all nine of its reference surface temperatures to within 0.04 °C and the reference heat flow to within 0.011 W/m — the standard permits 0.1 of each. Every build runs 21 solver checks and audits all 38 junction templates; all of them pass, and you can run the same suite yourself from the ✓ Validate button inside the app.

See the full validation results →

Who it's for

SAP and HEM assessors who want better-than-default junction values; architects and designers checking details before they're built; builders and developers chasing a marginal Part L pass without spec upgrades. If you'd rather we did it, our SAP team calculates junction Ψ-values as part of full assessments.

No subscription required — pay per junction

You never have to subscribe to anything to use ΨMonkey. Model as many junctions as you like, see every Ψ-value and f_Rsi on screen for free, and pay £35 +VAT (£42 inc VAT) only when you export the Building Control-ready PDF for a junction. If you calculate the occasional detail, paying per junction is far cheaper than a recurring licence — and there's nothing to cancel.

If you produce reports regularly, Monkey Pro makes them cheaper: £19/month +VAT puts your practice branding on every report and drops junctions to £15, and Monkey Pro Unlimited at £49/month includes them outright — less than a psi-only subscription elsewhere, and it covers the U-value calculator too. It stays optional, and pay-per-junction is never withdrawn. More on pay-per-junction vs a subscription →

Comparing tools? Here's how ΨMonkey works as a free alternative to AutoPSI — the same BS EN ISO 10211 method, no licence and no seat count, with the Ψ-values and f_Rsi free on screen.

Calculate a specific junction

Guides and a calculator set up for each of the common SAP reference junctions:

Or start from the numbers: the standard templates are solved against eight named walls on the psi values reference page — each figure a click from the model that produced it.

Frequently asked questions

What is a psi value (Ψ-value)?

A Ψ-value, or linear thermal transmittance, measures heat loss through a junction between building elements — wall-to-floor, wall-to-roof, around openings — in W/m·K. In SAP and HEM, junction lengths × Ψ-values feed directly into the dwelling heat-loss calculation.

Is ΨMonkey really free to use?

Yes. Modelling junctions, calculating Ψ-values and f_Rsi temperature factors is completely free in your browser. You only pay £35 +VAT (£42 inc VAT) when you want a PDF report for Building Control or your SAP assessor.

What calculation method does it use?

A two-dimensional finite-difference solver to BS EN ISO 10211, the standard required for calculated junction Ψ-values in SAP 10 and the Home Energy Model (HEM).

How do I know the Ψ-values are right?

Every built-in junction is checked against published accredited construction details. Modelled on the same build-up, the standard junctions land within about ±0.02 W/m·K of the published figures across openings, floors, roofs, corners and party walls. The solver itself reproduces test reference case 2 of BS EN ISO 10211 — the standard’s own benchmark — to within 0.04 °C on all nine reference temperatures and 0.011 W/m on heat flow, against a permitted 0.1. All 21 checks in its validation suite pass and all 38 templates are audited on every build; the full results are published at /tools/psi-calculator/validation/. Flanking heat loss is subtracted on SAP internal dimensions per BR 497.

Has the solver been independently checked?

It is checked against the benchmark the standard itself publishes. BS EN ISO 10211 sets out a two-dimensional test reference case with nine reference surface temperatures and a reference heat flow; ΨMonkey matches all nine to within 0.04 °C and the heat flow to within 0.011 W/m, where the standard allows 0.1. It also reproduces exact analytical 1D solutions to six figures. The suite runs on every build — 21/21 pass — and you can run it yourself from the ✓ Validate button in the app. Results are published in full at /tools/psi-calculator/validation/.

Which junctions are included?

All 38 of the common SAP reference junctions: the full E-series (E1–E25) covering openings, floors, balconies, eaves, gables, flat roofs and corners; the party-wall P-series (P1–P8); and roof windows plus the ridge (R1–R4). Anything else you can draw yourself on the grid.

Can I use the results in SAP or HEM assessments?

Yes — calculated Ψ-values to ISO 10211 can be used in place of default or accredited values, and almost always improve the result. The PDF report documents the model, materials and boundary conditions for Building Control.

What is the f_Rsi temperature factor?

f_Rsi indicates condensation and mould risk at the coldest internal surface of a junction. Building Regulations require f_Rsi ≥ 0.75 for dwellings. ΨMonkey calculates it automatically with every model.

Can I re-export a report after tweaking my model?

Yes — you can re-export the same junction free for 24 hours after payment. Substantial changes (a different junction) need a new report.

Can I get a refund if something goes wrong?

Yes — our refund practice is deliberately generous. An undelivered report refunds automatically, normally within the hour. If a report is faulty — it doesn't match your inputs or misapplies the stated methodology — we'll repair, replace or refund it under the Consumer Rights Act 2015, and minor corrections can be re-exported free for 24 hours after purchase.

Will Building Control accept the report?

It models the junction to BS EN ISO 10211 and shows the inputs, the calculated Ψ-value and f_Rsi, so a building control body or SAP assessor can check every line. Every paid report also carries a printed report ID that can be verified in seconds at energycount.co.uk/verify — and if anything raises a question, we're a real consultancy on 01202 623236, not just a website.

ΨMonkey — the free psi value calculator