U-value of a concrete floor over a car park

A 200mm concrete slab over a car park, with a 65mm screed and 110mm of phenolic soffit board (λ 0.019) fixed to its underside, has a U-value of about 0.17 W/m²K with the board’s stainless screws counted — meeting the 0.18 Approved Document L sets for the floor of a new home, and for a new floor in an existing one. 100mm gives 0.19 and misses. After the thickness, the screws matter most: twelve stainless ones per board add about 0.01, carbon-steel ones 0.03. Uninsulated, the same slab is about 2.8. Non-combustible stone wool (λ 0.034), which some specifications ask for under flats, needs about 180mm before its fixings are counted.

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The build-up (outside → inside)

110mm phenolic soffit board (Kingspan Kooltherm K110, λ 0.019; foil on the exposed face), screwed at Unilin’s 12 per 2.4 × 1.2m board (4.17 per m²), taken as stainless
200mm reinforced concrete slab
65mm sand/cement screed

Phenolic soffit board at λ 0.019 (Kingspan Kooltherm K110; Unilin SR019/MA is the same) with 5.2mm screws through the full board at Unilin’s 12 per 2.4 × 1.2m board, taken as stainless — U-Monkey’s “Soffit board screws” preset (BS EN ISO 6946 Annex F); Kingspan’s own 2019 datasheet asked for 11 to 15 per board, with thermally broken fixings. Kingspan’s older K110 table used λ 0.018 and so reads about 0.01 better than today’s board. Stone wool: ROCKWOOL Soffit Slab (λ 0.034, A1), fixings left out — enter your system’s. The basement row uses U-Monkey’s unheated-cellar option (BS EN ISO 13370 §7.4) for a 300m² basement 2.8m deep, at BR 443’s default of 3 air changes an hour.

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
Uninsulated: 200mm slab + 65mm screed	≈ 2.83
75mm phenolic soffit board (λ 0.019), stainless fixings	≈ 0.25	Meets 0.25, the value for an upgraded floor
80mm	≈ 0.23
90mm	≈ 0.21
100mm	≈ 0.19	Misses 0.18
110mm	≈ 0.17	As pre-filled: meets 0.18
120mm	≈ 0.16
150mm (Unilin SR019/MA to order, or two layers; the same λ)	≈ 0.13	Meets the notional home’s 0.13
100mm, fixings left out	≈ 0.18	The stainless screws add 0.011
100mm, carbon-steel fixings	≈ 0.21	Three times the correction: 0.031
100mm under a 250mm slab	≈ 0.19	The slab barely matters
Stone wool soffit slab (λ 0.034, A1), 180mm, fixings left out	≈ 0.18	Meets 0.18 before its fixings
Stone wool, 220mm, fixings left out	≈ 0.15
100mm phenolic over an unheated basement instead (3 air changes an hour)	≈ 0.18	0.19 taken as outside air

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

The board, and the screws through it. Every fixing that passes through the insulation is a small thermal bridge, corrected by BS EN ISO 6946 Annex F (BR 443 §4.8.3): at twelve 5.2mm screws per 2.4 × 1.2m board — Unilin’s pattern for its SR019/MA soffit board, taken here as stainless — they add about 0.01, and carbon steel three times that, so ask for stainless or for a system with a certified figure. Unilin’s published U-values for SR019/MA under a 200mm slab (0.25, 0.23, 0.19 and 0.16 for 75, 80, 100 and 120mm) come out here, with the 65mm screed, only with those screws counted. The slab and screed barely matter: 250mm of concrete instead of 200mm improves the U by 0.001. A floor losing heat downwards has an inside surface resistance of 0.17 and an outside one of 0.04 (BR 443 §4.1), and BR 443 adds no air-gap correction to floors (§4.8.1), because convection is suppressed when heat flows downwards. What a U-value can’t show are the beams, columns and wall junctions the boards stop at: they are thermal bridges for the SAP or SBEM calculation, and the bare concrete around them is where condensation and mould appear first.

Condensation risk under a car park

With the insulation below the slab, the concrete stays warm and the inside surface passes U-Monkey’s mould check comfortably (fRsi 0.96); the uninsulated slab is the opposite — its surface fails the mould check (fRsi 0.42), and condensation collects within the floor in winter.

Interstitial condensation needs more care than a screening check gives it here. U-Monkey’s BS EN ISO 13788 check spreads a foil-faced board’s vapour resistance through its thickness, and on that basis finds none in any month for the pre-filled floor. But Kingspan’s K110 has its foil on the car-park face only, with glass tissue against the concrete, so its vapour-tight layer is on the cold side, where vapour moving down from the rooms above can condense behind it — and the check says so when the board is in a build-up. Entered as made, with a vapour control layer under the core for the foil and the core at μ 50, the same check shows condensate building up behind the foil year on year, some 80 to 110 g/m² of it still there at the end of the year, depending on the value taken for the foil. With foil on both faces, as on Unilin’s SR019/MA, it shows about 10 g/m² in spring that dries out over the summer. Ask the maker for a condensation risk analysis for your slab, floor finish and humidity class, and on a new slab let the concrete dry before the boards close it in.

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 (f_Rsi) 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):

Exposed Floor E20 / E21

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

Frequently asked questions

What is the U-value of a concrete floor over a car park?

Does insulation under a car park have to be non-combustible?

Not by regulation 7(2) on its own wording: that applies to the external walls and specified attachments (balconies, solar shading, solar panels) of relevant buildings — those with a storey at least 18m above ground containing dwellings, an institution or rooms for residential purposes — and regulation 2(6)’s definition of an external wall does not mention a soffit. Inside a car park the soffit is a ceiling lining: Approved Document B’s tables of linings (Volume 1 Table 4.1, and Volume 2 Table 6.1 for a car park that isn’t part of a home) ask for C-s3, d2 in rooms, garages included, and B-s3, d2 in circulation spaces. Phenolic soffit boards are classed B-s2, d0 (Kingspan K110, foil side out), enough for either, or C-s2, d0 (Unilin SR019/MA), enough for a room but not a circulation space. Volume 2 also has a section of its own on car parks (Section 11), so ask building control how it treats soffit insulation in yours. Some project specifications ask for A2-s1, d0 or better anyway, which means stone wool — ROCKWOOL’s Soffit Slab is A1 — at nearly twice the thickness, and a 2026 consultation on Approved Document B proposed reviewing the scope of the combustibles ban, so check the current guidance and your specification.

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.