U-value of a warehouse floor slab

Big sheds need little floor insulation: how a large slab’s size and the ground under it set its U-value, and when a strip round the edge is enough.

A warehouse floor of 100 × 50m — a 200mm ground-bearing concrete slab with no insulation under it — has a U-value of about 0.19 W/m²K on sand or gravel and 0.15 on clay, because a big floor loses heat mostly round its edge: its perimeter-to-area ratio is only 0.06. That is just over the 0.18 Approved Document L Volume 2 sets for a floor on sand or gravel, and a 1m strip of 50mm PIR round the edge under the slab takes it to 0.16 (on clay, 0.13), with no insulation under the rest. BR 443 says it plainly: “Very large floors can have low U-values without all-over insulation.” A smaller shed needs more: 60 × 40m, uninsulated, reads 0.24.

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The build-up (outside → inside)

The pre-filled link opens in the England · non-domestic regime with the floor’s geometry set — exposed perimeter 300m, area 5,000m², wall thickness 300mm, sand or gravel (λ 2.0) — and edge insulation horizontal, 1.0m wide, 50mm at λ 0.022. The concrete slab is not counted in R_f, as BR 443 directs for dense slabs, and finishes are ignored. The U-value is for the floor as a whole; the junctions round it are separate.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100 × 50m (P/A 0.06), no insulation, sand or gravel≈ 0.19Misses 0.18 (0.189)
Same, on clay (λ 1.5)≈ 0.15Meets 0.18 with no insulation (0.146)
Same, sand or gravel, 1m strip of 50mm PIR at the edge≈ 0.16As pre-filled (0.163)
Same, on clay, with the 1m edge strip≈ 0.130.125
Same, sand or gravel, 50mm PIR 1m down the foundation (vertical)≈ 0.150.151: vertical edge insulation does a little more
25mm PIR under the whole slab, sand or gravel≈ 0.130.133
50mm PIR under the whole slab≈ 0.110.111
60 × 40m (P/A 0.083), no insulation, sand or gravel≈ 0.24Fails 0.18
60 × 40m with the 1m edge strip≈ 0.21Still fails: a floor this size needs insulation under the slab
60 × 40m, 25mm PIR all over≈ 0.170.167
120 × 60m (P/A 0.05), no insulation, sand or gravel≈ 0.16Meets 0.18 with no insulation (0.163)
150 × 80m (P/A 0.038), no insulation≈ 0.130.132
200 × 100m (P/A 0.03), no insulation≈ 0.110.108

Figures computed to BS EN ISO 13370 with BR 443 conventions, 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 and the footprint and depth shown: your products, thicknesses, perimeter, area and ground will move the number, and that's what the calculator is for.

What changes the number

A floor on the ground is calculated by BS EN ISO 13370, not layer by layer, because most of its heat goes down into the ground and back out at the edge: the bigger the floor, the further its middle is from the edge and the less it loses. The standard works from the characteristic dimension B′ = A/(½P) — 33m for this 100 × 50m floor, about 3m for a small house — and BR 443 (2019) §9 sums it up: “Very large floors can have low U-values without all-over insulation”, and the floor U-value “decreases as the ratio of perimeter to area decreases”. The perimeter is every exposed wall, including walls to unheated spaces; walls to heated neighbours are left out. The ground matters as much as the size: BR 443 gives clay or silt λ 1.5, sand or gravel 2.0 and rock 3.5 W/m·K and says “the ground type in the UK is most commonly clay”, which takes this floor from 0.19 to 0.15. U-Monkey starts on sand or gravel, the cautious choice, so set yours from the site investigation. Edge insulation is a correction to the floor’s U-value (BS EN ISO 13370 Annex D), horizontal under the slab edge or vertical down the foundation, and only the more effective of the two counts; BR 443’s 2006 edition adds that edge insulation “will usually be needed to avoid thermal bridging at the perimeter” — the floor-to-wall junction’s own loss is a Ψ-value, not part of the floor U-value. Approved Document L Volume 2 sets 0.18 for a floor in its 2021 and 2026 editions. For a new building the test is the whole-building calculation (the BER and BPER against the TER and TPER): the NCM notional building gives a side-lit building’s floor 0.15 and a top-lit one’s 0.22, but where the floor’s own uninsulated value is below 0.15 “this lower value must be used in the Notional building” (NCM Modelling Guide 2021, paragraph 36) — so where a big shed’s uninsulated floor is already under 0.15, the notional floor is no better and the floor’s U-value costs nothing against the target — though the uninsulated slab edge’s thermal bridge still counts in the actual building. BR 443 adds that, as a floor’s U-value falls with its perimeter-to-area ratio, “the U-value can be obtained for the smallest intended building”: worth doing where a shed may be split into smaller units. Insulation under a racked warehouse slab has to carry the racking: specify its compressive strength for the loads.

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 building'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 a warehouse floor slab?

A warehouse floor of 100 × 50m — a 200mm ground-bearing concrete slab with no insulation under it — has a U-value of about 0.19 W/m²K on sand or gravel and 0.15 on clay, because a big floor loses heat mostly round its edge: its perimeter-to-area ratio is only 0.06. That is just over the 0.18 Approved Document L Volume 2 sets for a floor on sand or gravel, and a 1m strip of 50mm PIR round the edge under the slab takes it to 0.16 (on clay, 0.13), with no insulation under the rest. BR 443 says it plainly: “Very large floors can have low U-values without all-over insulation.” A smaller shed needs more: 60 × 40m, uninsulated, reads 0.24.

Does a warehouse floor need insulation?

Often only at the edge, sometimes not at all. On sand or gravel an uninsulated slab meets Approved Document L’s 0.18 once the floor is about 120 × 60m (0.16), and a 100 × 50m floor gets there with a 1m strip of 50mm PIR round the edge (0.16); on clay, a 100 × 50m slab is 0.15 with nothing. A 60 × 40m unit needs insulation under the slab: 25mm of PIR all over gives 0.17. Calculate your own floor in U-Monkey with its real perimeter, area and ground — and the slab edge still needs detailing against thermal bridging.

Is this calculator free?

Yes — building the element, reading the U-value, the compliance targets (Approved Document L Volume 2 for buildings other than dwellings, and the Welsh and Scottish equivalents), 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? For a building other than a dwelling, U-values feed the SBEM calculation that checks the building’s emission and primary energy rates against their targets (BER and BPER against TER and TPER) — and the non-domestic U-value page has the Volume 2 limits. 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.

It never is. Open this one pre-filled, then swap the insulation, change the thicknesses and watch the U-value move — free, in your browser, with a Building Control-ready PDF at the end.

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