Runs in your browser. No account, subscription optional — £35 +VAT only when you export a PDF.
One thing to know before the table: ΨMonkey's standard junction templates are solved against eight named walls, free, on the psi values reference page — each figure a click from the model that produced it.
The honest comparison
Let's be fair: AutoPSI is a well-established psi-value tool — cloud-based, drag-and-drop, and calculated to the right standard. For a busy team modelling junctions every week, its subscription and collaboration features earn their keep. The question is simply whether you need a monthly licence, or whether you'd rather pay only for the reports you actually export.
| ΨMonkey | AutoPSI | |
|---|---|---|
| Published validation | Full results published: BS EN ISO 10211 test reference case 2 matched to 0.04 °C and 0.011 W/m, 21/21 solver checks, 38/38 templates audited — and the suite runs on your own machine from the app (see the results) | Not published at the time of writing * |
| Price | Free to model · £35 +VAT (£42 inc VAT) per junction PDF — or Monkey Pro Unlimited at £49/mo, £490/yr ex VAT, which also covers the U-value calculator (pricing) | £59/mo or £649/yr (Professional); £139/mo or £1,395/yr (Teams) — excl. VAT * |
| Commitment | None — pay per report | Monthly or annual subscription |
| Sign-up to calculate | No account needed | Account required |
| Calculation standard | BS EN ISO 10211 (2D finite-difference) | BS EN ISO 10211 (2D finite-difference) |
| fRsi condensation factor | Yes, with every model | Yes |
| Team library & collaboration | No | Yes (Teams tier) |
| Best for | Occasional junctions, architects, one-off developers, smaller assessors | High-volume teams wanting a shared workspace |
* AutoPSI pricing from autopsi.co.uk/pricing, correct at June 2026 and excluding VAT — check their site for the latest.
Same method, different pricing model
The calculation underneath is the same recognised one: 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. ΨMonkey reports the fRsi temperature factor for condensation risk with every model, and the £35 +VAT PDF documents the model, materials and boundary conditions for Building Control. See a sample report or read how thermal bridging works.
When pay-per-junction wins
A monthly licence is dead money in any month you don't use it. An architect checking a detail before it's built, a developer chasing a marginal Part L pass on one plot, or an assessor who needs the occasional better-than-default junction value rarely calculates enough to justify a subscription. For them, paying £35 +VAT only for the junctions they export is usually far cheaper over a year — with nothing to remember to cancel.
And when it doesn't win, we'd rather say so. Past roughly two junction reports a month a subscription is cheaper, which is why Monkey Pro Unlimited exists at £49/month or £490/year ex VAT — still under a psi-only licence, and it covers the U-value calculator too. The difference is that ours is optional: the calculator stays free and the single report stays buyable, whichever way you go. If you'd rather hand the whole job over, our SAP team calculates Ψ-values as part of full assessments.
Frequently asked questions
Is ΨMonkey a free alternative to AutoPSI?
Yes. ΨMonkey models junctions and gives you the Ψ-value and f_Rsi temperature factor free in your browser, with no account and no subscription required. You only pay £35 +VAT (£42 inc VAT), once, when you export a Building Control-ready PDF for a junction — so there is no recurring fee to commit to.
Does ΨMonkey use the same calculation method as AutoPSI?
Both tools calculate junction Ψ-values using a two-dimensional finite-difference solver to BS EN ISO 10211 — the method required for calculated thermal bridging values in SAP 10 and the Home Energy Model. The calculation standard is the same; the difference is how you pay for it.
When is AutoPSI the better choice?
AutoPSI is an established, capable tool with team collaboration, a shared project archive and branded reporting. If you calculate psi values in volume every week, or you are a team that wants a shared library and ongoing support, its subscription tiers are built for exactly that. ΨMonkey is aimed at people who need accurate values occasionally and would rather not pay a monthly licence.
How much does ΨMonkey cost compared with AutoPSI?
ΨMonkey is free to model; you pay £35 +VAT (£42 inc VAT) per junction only when you export the PDF, and a subscription is optional rather than the only way in. If you do want one, Monkey Pro Unlimited is £49/month or £490/year ex VAT and covers the U-value calculator as well. AutoPSI is a subscription — at the time of writing (June 2026) its published pricing is £59/month or £649/year for a single Professional licence, and £139/month or £1,395/year for the Teams tier, both excluding VAT. Always check autopsi.co.uk for current pricing.
Ready to try the free option? Open the main ΨMonkey calculator, see the no-subscription details, or jump to a specific junction.