Exposed Floor Psi Value Calculator (E20 / E21)

E20 / E21 SAP / Approved Document L reference junction

Schematic cross-section of the exposed floor (E20 / E21) showing the structure, insulation line and the heat-bridge path — Schematic of the exposed floor (E20 / E21) — not to scale. Model your actual build-up in ΨMonkey.

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What the exposed floor is

The exposed floor junction is where a floor with outside air beneath it (such as a floor over an open undercroft, passage or car port) meets the external wall.

Why it matters

With outdoor air on the underside, the wall-to-floor junction is exposed on two faces, so heat can escape around the floor edge unless the wall and floor insulation are carefully joined.

How to model it

Paint the exposed floor build-up, the wall and the insulation wrapping the junction, then solve.

A well-wrapped exposed floor junction calculates well; the SAP default is conservative for this less common detail.

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

The Ψ-value at the exposed floor 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 exposed floor 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 exposed floor (E20 / E21)?

The exposed floor junction is where a floor with outside air beneath it (such as a floor over an open undercroft, passage or car port) meets the external wall. With outdoor air on the underside, the wall-to-floor junction is exposed on two faces, so heat can escape around the floor edge unless the wall and floor insulation are carefully joined.

How do I calculate the exposed floor psi value?

Paint the exposed floor build-up, the wall and the insulation wrapping the junction, 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 well-wrapped exposed floor junction calculates well; the SAP default is conservative for this less common detail.

Other junctions

ΨMonkey models any junction. Jump to another common one:

Exposed Floor Psi Value Calculator