Eaves Junction Psi Value Calculator (E9 / E10)

E9 / E10 SAP / Approved Document L reference junction

Schematic cross-section of the eaves junction (E9 / E10) showing the structure, insulation line and the heat-bridge path — Schematic of the eaves junction (E9 / E10) — 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 eaves junction is

The eaves junction is where the top of the external wall meets the roof, with insulation either at ceiling level (E9) or following the rafter line (E10).

Why it matters

Maintaining insulation continuity from the wall up into the loft is difficult at the eaves — the wall-plate, ventilation gap and rafter feet all conspire to interrupt it, creating a heat path at the top of every external wall.

How to model it

Paint the wall, the wall-plate, the ceiling or rafter-level insulation and the ventilation void, making the wall and roof insulation meet. The overlap at the wall-plate is the critical detail.

Getting the loft insulation to fully meet the wall insulation over the wall-plate gives a low Ψ-value; a gap there is a common cause of a poor result.

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

The Ψ-value at the eaves junction 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 eaves junction 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 eaves junction (E9 / E10)?

The eaves junction is where the top of the external wall meets the roof, with insulation either at ceiling level (E9) or following the rafter line (E10). Maintaining insulation continuity from the wall up into the loft is difficult at the eaves — the wall-plate, ventilation gap and rafter feet all conspire to interrupt it, creating a heat path at the top of every external wall.

How do I calculate the eaves junction psi value?

Paint the wall, the wall-plate, the ceiling or rafter-level insulation and the ventilation void, making the wall and roof insulation meet. The overlap at the wall-plate is the critical detail. 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 Getting the loft insulation to fully meet the wall insulation over the wall-plate gives a low Ψ-value; a gap there is a common cause of a poor result.

Other junctions

ΨMonkey models any junction. Jump to another common one:

Eaves Junction Psi Value Calculator