Inverted Corner Psi Value Calculator (E14)

E14 SAP / Approved Document L reference junction

Schematic cross-section of the inverted corner (E14) showing the structure, insulation line and the heat-bridge path — Schematic of the inverted corner (E14) — not to scale. Model your actual build-up in ΨMonkey.

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What the inverted corner is

The inverted corner is a concave corner, such as at a recessed entrance, an inset porch or where the building plan steps inwards.

Why it matters

The opposite of an external corner: the geometry concentrates heat loss, so an inverted corner is typically a positive (heat-losing) bridge.

How to model it

Model the two walls meeting at the concave corner with the insulation wrapped around the internal angle, then solve.

Inverted corners usually carry a positive Ψ-value; calculating yours documents it properly rather than relying on a conservative default.

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

The Ψ-value at the inverted 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 inverted 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 inverted corner (E14)?

The inverted corner is a concave corner, such as at a recessed entrance, an inset porch or where the building plan steps inwards. The opposite of an external corner: the geometry concentrates heat loss, so an inverted corner is typically a positive (heat-losing) bridge.

How do I calculate the inverted corner psi value?

Model the two walls meeting at the concave corner with the insulation wrapped around the internal angle, 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 Inverted corners usually carry a positive Ψ-value; calculating yours documents it properly rather than relying on a conservative default.

Other junctions

ΨMonkey models any junction. Jump to another common one:

Inverted Corner Psi Value Calculator