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.
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
- Draw your junction — paint the construction build-up on a grid: masonry, insulation, timber, steel, whatever the detail needs.
- Solve — a finite-difference solver to BS EN ISO 10211 computes the 2D heat flow through the junction.
- Read the results — Ψ-value (W/m·K) and f_Rsi temperature factor, live on screen, free.
- Export when ready — a Building Control-ready PDF report of the model, materials and results for £25 +VAT (£30 inc VAT). Re-export the same junction free for 24 hours.
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 — pay per junction
ΨMonkey isn't a monthly licence or a per-seat subscription. Model as many junctions as you like, see every Ψ-value and fRsi on screen for free, and pay £25 +VAT (£30 inc VAT) only when you export the Building Control-ready PDF for a junction. If you calculate the occasional detail, paying per junction works out far cheaper than a recurring licence — and there's nothing to cancel. More on pay-per-junction vs a subscription →
Calculate a specific junction
Guides and a calculator set up for each of the common SAP reference junctions:
- Wall-to-Floor Junction E5
- Window Jamb (Reveal) E4
- Window Sill E3
- Lintel E1 / E2
- Eaves Junction E9 / E10
- Gable Junction E11 / E12
- External Corner E13
- Inverted Corner E14
- Intermediate Floor E6
- Party Wall Junction E15 / P1–P8
- Balcony Junction E8 / E24
- Exposed Floor E20 / E21
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 £25 +VAT (£30 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).
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.