Window Jamb (Reveal) Psi Value Calculator (E4)

E4 SAP / Approved Document L reference junction

Schematic cross-section of the window jamb (reveal) (E4) showing the structure, insulation line and the heat-bridge path — Schematic of the window jamb (reveal) (E4) — not to scale. Model your actual build-up in ΨMonkey.

Launch ΨMonkey — model this junction free

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

What the window jamb (reveal) is

Why it matters

Where the cavity is closed at the reveal, the inner and outer leaves can be bridged and the wall insulation interrupted. A poorly insulated cavity closer creates a direct heat path around the frame and raises condensation risk at the reveal.

How to model it

Model the wall, the cavity closer, the frame position relative to the insulation line and the window board. The overlap between the frame and the insulation is the detail that matters.

An insulated cavity closer with the frame set against the insulation line gives a low Ψ-value; an uninsulated closer is a common, avoidable failure.

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

The Ψ-value at the window jamb (reveal) 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 window jamb (reveal) 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 window jamb (reveal) (E4)?

The jamb is the vertical edge of a window or door opening — the reveal where the cavity is closed around the frame. Every opening has two of them, so jamb length mounts up quickly on a glazed elevation. Where the cavity is closed at the reveal, the inner and outer leaves can be bridged and the wall insulation interrupted. A poorly insulated cavity closer creates a direct heat path around the frame and raises condensation risk at the reveal.

How do I calculate the window jamb (reveal) psi value?

Model the wall, the cavity closer, the frame position relative to the insulation line and the window board. The overlap between the frame and the insulation is the detail that matters. 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 An insulated cavity closer with the frame set against the insulation line gives a low Ψ-value; an uninsulated closer is a common, avoidable failure.

Other junctions

ΨMonkey models any junction. Jump to another common one:

Window Jamb (Reveal) Psi Value Calculator