External Corner Psi Value Calculator (E13)

E13 SAP / Approved Document L reference junction

Schematic cross-section of the external corner (E13) showing the structure, insulation line and the heat-bridge path — Schematic of the external corner (E13) — not to scale. Model your actual build-up in ΨMonkey.

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Runs in your browser. No sign-up, no subscription — pay £25 +VAT only when you export the PDF.

What the external corner is

The external corner is the normal, convex vertical corner where two external walls meet.

Why it matters

At an external corner more heat escapes through the larger outer surface than enters from the smaller inner one, so the geometry alone creates a thermal bridge — though a relatively gentle one.

How to model it

Paint the two wall leaves meeting at the corner with their insulation continuous around it, then solve.

A normal external corner often has a small — sometimes slightly negative — Ψ-value, so calculating it can actually reduce your total junction heat loss versus the default.

Related build-ups: the U-values either side of this junction

The Ψ-value at the external corner depends on the walls, floors and roof around it — so the first step is knowing the U-values of those elements. Typical figures for the build-ups that pair with this junction:

Model your exact build-up and read its U-value with the free U-Monkey calculator (PDFs included free), then come back here and solve the junction in ΨMonkey.

Free to model — no subscription

ΨMonkey isn't a monthly licence or a per-seat subscription. Build the external corner on the grid, solve it, and read the Ψ-value and f_Rsi on screen for nothing. You only pay £25 +VAT (£30 inc VAT), once, when you need the Building Control-ready PDF for that junction — so calculating a single detail never means signing up to anything. Calculated psi values are often the cheapest route to a SAP 10 pass, so the maths can pay for itself.

Frequently asked questions

What is the external corner (E13)?

The external corner is the normal, convex vertical corner where two external walls meet. At an external corner more heat escapes through the larger outer surface than enters from the smaller inner one, so the geometry alone creates a thermal bridge — though a relatively gentle one.

How do I calculate the external corner psi value?

Paint the two wall leaves meeting at the corner with their insulation continuous around it, then solve. ΨMonkey solves it to BS EN ISO 10211 and returns the Ψ-value (W/m·K) and the f_Rsi temperature factor on screen, free.

Is it free, and is there a subscription?

No subscription and no licence — modelling the junction and reading off the Ψ-value is free in your browser. You only pay £25 +VAT (£30 inc VAT) when you export the Building Control-ready PDF for that junction.

Can I use the result in a SAP or HEM assessment?

Yes — a calculated Ψ-value to ISO 10211 can replace the SAP default for this junction, and A normal external corner often has a small — sometimes slightly negative — Ψ-value, so calculating it can actually reduce your total junction heat loss versus the default.

Other junctions

ΨMonkey models any junction. Jump to another common one:

External Corner Psi Value Calculator