Wall-to-Floor Junction Psi Value Calculator (E5)

E5 SAP / Approved Document L reference junction

Schematic cross-section of the wall-to-floor junction (E5) showing the structure, insulation line and the heat-bridge path — Schematic of the wall-to-floor junction (E5) — not to scale. Model your actual build-up in ΨMonkey.

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What the wall-to-floor junction is

Why it matters

Heat escapes sideways through the slab edge and foundation, bypassing both the wall insulation and the floor insulation. Because it follows the full perimeter, even a modest Ψ-value adds up across a dwelling — it is usually one of the biggest single contributors to junction heat loss.

How to model it

SAP default Ψ-values for E5 are deliberately pessimistic; a calculated value for a well-detailed junction is frequently far lower, which is why this is often the most cost-effective junction to calculate.

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

The Ψ-value at the wall-to-floor 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 wall-to-floor 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 wall-to-floor junction (E5)?

The wall-to-floor junction is where the external wall meets the ground floor slab or beam-and-block deck at the base of the building. It runs around the whole heated perimeter, so its length is large on every plot. Heat escapes sideways through the slab edge and foundation, bypassing both the wall insulation and the floor insulation. Because it follows the full perimeter, even a modest Ψ-value adds up across a dwelling — it is usually one of the biggest single contributors to junction heat loss.

How do I calculate the wall-to-floor junction psi value?

Paint the wall build-up, the floor slab or beam-and-block deck, the perimeter and under-slab insulation, the DPC and the foundation, then solve. Edge insulation and getting the insulation lines to meet are what move the number. 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 SAP default Ψ-values for E5 are deliberately pessimistic; a calculated value for a well-detailed junction is frequently far lower, which is why this is often the most cost-effective junction to calculate.

Other junctions

ΨMonkey models any junction. Jump to another common one:

Wall-to-Floor Junction Psi Value Calculator