U-value of an exposed floor over a passage, undercroft or overhang

A timber floor with outside air beneath it — a room over an archway, a car port or a cantilever — and its soffit.

A timber floor with outside air beneath it — a bedroom over an archway or passage, a room over a car port, a cantilevered or jettied floor — with 200mm of mineral wool between the joists, 50mm of PIR fixed under them and a 12mm cement-particle soffit board, has a U-value of about 0.15 W/m²K: inside the 0.18 limiting value for a new floor, short of the 0.13 notional value, which 70mm of PIR reaches (0.13). Wool between the joists alone gives only 0.22, because the joists bridge it through its full depth. The soffit board itself is worth almost nothing; what it changes is the surface resistance — fixed on battens with a ventilated gap above it, the board and the gap drop out of the calculation and the outside surface takes the inside value instead (0.14).

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The build-up (outside → inside)

Calculated as an exposed floor to BS EN ISO 6946 with outside air below (Rse 0.04); the ventilated-soffit row puts the board on battens over a well-ventilated gap, with a breather membrane under the PIR, so the board and gap are disregarded and Rse is 0.17. The natural row uses STEICOflex 036 between the joists and 60mm of STEICO protect dry M (λ 0.040) as the soffit, with no PIR or polythene. Joists at 10.8% (BR 443 §4.5.3).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Uninsulated: soffit board, empty 200mm joists, chipboard≈ 1.54
200mm mineral wool between the joists only≈ 0.22Misses 0.18; meets the 0.25 for a renovated floor
150mm mineral wool + 50mm PIR under the joists≈ 0.17Meets 0.18 (0.172)
200mm mineral wool + 50mm PIR≈ 0.15As pre-filled
200mm mineral wool + 70mm PIR≈ 0.13Meets the 0.13 notional (0.128)
Soffit on battens with a ventilated gap above it≈ 0.14Soffit and gap disregarded; outside surface 0.17
Natural: 200mm wood-fibre batts + 60mm wood-fibre soffit board≈ 0.16STEICOflex 036 between, STEICO protect dry M under

Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and the air-gap correction included — exactly as the pre-filled calculator works them out, with any pass or fail judged on the unrounded U-value. Indicative for the generic build-up shown: your products, thicknesses and (for floors) footprint will move the number, and that's what the calculator is for.

What changes the number

An exposed floor is calculated like any other element with outside air on one side, with BR 443’s surface resistances for heat flowing downwards — 0.17 inside, 0.04 outside — and no air-gap correction, which BR 443 applies to walls and roofs “but not to floors (because convection is suppressed when the heat flow direction is downwards)”. The joists set the first limit: at 400mm centres they are 10.8% of the floor (BR 443 §4.5.3), so 200mm of wool between them gives only 0.22, and a continuous layer under them does more per millimetre than extra depth between them — the same lesson as the floor over a garage, which this is the outdoor version of. The soffit then decides the outside surface: a board fixed straight to the joists or insulation keeps 0.04, while a board on battens with a ventilated gap above it is disregarded with the gap, and the outside surface resistance becomes the inside value (BS EN ISO 6946 §6.9.4) — slightly better, and the gap lets the soffit drain and dry. Under a concrete floor — flats over an undercroft car park — the insulation goes on the soffit instead, and its fixings matter: Unilin’s table for its SR019/MA soffit board under 200mm of concrete reads 0.25 at 75mm and 0.16 at 120mm, with 12 fixings per board. Approved Document L sets 0.18 for a new floor (Tables 4.1 and 4.2), the notional dwelling 0.13, and 0.25 for a renovated one (Table 4.3). The junction where the exposed floor meets the wall is a thermal bridge of its own — model it in ΨMonkey.

Condensation risk in an exposed floor

With the polythene vapour control layer under the deck, U-Monkey’s free BS EN ISO 13788 check finds a little seasonal condensation (a peak of 6.6 g/m² from November) that dries out fully in summer, and it stays acceptable with every assumed vapour value tested across its range — a pass. Leave the polythene out and the peak rises to about 190 g/m², and the result depends on the chipboard’s vapour resistance: the foil-faced PIR under the joists is far more vapour-tight than anything above it, so the floor needs its vapour control on the warm side, under the deck.

The natural version, with wood-fibre boards as the soffit and no PIR, passes with no condensation in any month and no polythene at all, because every layer below the deck is vapour-open.

Check this build-up for condensation — free

Opens pre-filled. The condensation verdict sits just under the U-value — tap it for the full month-by-month check, the surface-mould (fRsi) check included. If it isn’t a pass, How to fix it tests the usual remedies on your build-up — a vapour control layer on the warm side, a more vapour-open layer on the cold side, more insulation to warm the inside surface — and applies the one you pick in a tap, with Undo.

Need it in writing for Building Control? The full condensation risk analysis report is £25 +VAT (£30 inc VAT), or included on Monkey Pro Unlimited — see an example report (PDF), or read what building control expects the analysis to show.

It is a screening to BS EN ISO 13788 (the Glaser method), honestly labelled as one: it does not model air leakage, rain or moisture stored in materials, so for high-risk or heritage constructions a full hygrothermal simulation (BS EN 15026) is the right next step. Figures above use the app’s defaults — Manchester (Ringway) climate, humidity class 3 (dwellings with high or unknown occupancy), a 1-in-10 design year and, on a roof, the surface taken 2 K below the air for night-sky cooling — so pick your nearest weather station for your own build-up: the result can change from one station to another.

Next: check the junctions

A U-value covers the flat area of an element — but heat also escapes where elements meet, and at modern U-values those junctions can be 20–30% of a dwelling's fabric heat loss. Now you know your U-values, model the junctions with the free ΨMonkey psi value calculator (no subscription; PDF reports £25 +VAT a junction):

Or read thermal bridging explained to see how junction Ψ-values feed your SAP calculation.

Frequently asked questions

What is the U-value of an exposed floor over a passage, undercroft or overhang?

A timber floor with outside air beneath it — a bedroom over an archway or passage, a room over a car port, a cantilevered or jettied floor — with 200mm of mineral wool between the joists, 50mm of PIR fixed under them and a 12mm cement-particle soffit board, has a U-value of about 0.15 W/m²K: inside the 0.18 limiting value for a new floor, short of the 0.13 notional value, which 70mm of PIR reaches (0.13). Wool between the joists alone gives only 0.22, because the joists bridge it through its full depth. The soffit board itself is worth almost nothing; what it changes is the surface resistance — fixed on battens with a ventilated gap above it, the board and the gap drop out of the calculation and the outside surface takes the inside value instead (0.14).

Is a floor over a passage or archway an exposed floor?

Yes: with outside air beneath it, it is an exposed floor, calculated with an outside surface resistance of 0.04 (or 0.17 behind a ventilated soffit) and judged against Approved Document L’s floor values — 0.18 for a new floor, 0.25 for a renovated one. It differs from a floor over an enclosed integral garage, where SAP adds the garage’s resistance (Ru); over an open passage, archway or car port nothing is added, which is why its figures here are close to the floor over a garage taken as outside air.

Is this calculator free?

Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard), the condensation-risk screening check and the thermal-mass figures (decrement factor, time lag, κ) are all free in your browser, and the Building Control-ready PDF report is free too. No sign-up.

Does it follow the official calculation method?

Yes — the combined method of BS EN ISO 6946:2017 with BR 443 conventions, including timber bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and air-gap corrections, BS EN ISO 13370 for ground floors, suspended floors and basements, and ISO 13788 for the condensation screening check.

Other common build-ups

Floors

Walls

Roofs

Different build-up? See the full floor U-value calculator, or start from scratch in the main U-value calculator.

Need the wider compliance picture? U-values feed straight into SAP calculations — or let our New Build Compliance Pack handle the lot. Background reading: what U-values actually are, and why two calculators can give different answers for the same wall.

Your build-up won't be exactly this one.

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