U-value of an uninsulated concrete ground floor

A slab on the ground with no insulation, where the floor’s shape sets the U-value.

An uninsulated concrete floor on the ground has a U-value set by the floor’s shape, not its thickness: about 0.98 W/m²K for a typical house floor with 30m of exposed perimeter around 50m² (P/A 0.6), 0.63 for a large, square floor (P/A 0.3) and about 1.2 for a small one such as an extension (P/A 0.9). BR 443 leaves the slab’s own small resistance out, so the ground does almost all the work (BS EN ISO 13370). Most are worse than the 0.70 at which Approved Document L counts a floor as one to upgrade; a large floor can be better than it.

Open this build-up in U-Monkey — pre-filled, free

Every layer editable · live U-value · Part L & Future Homes Standard targets · free PDF report

The build-up (outside → inside)

The pre-filled link opens with U-Monkey’s default 30m perimeter / 50m² area (P/A 0.6), as an existing floor judged against Approved Document L’s 0.70 threshold (2021 edition, Table 4.3) — edit the perimeter and area to your floor.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Large, square floor (P/A 0.3)≈ 0.63Better than the 0.70 threshold
P/A 0.5≈ 0.88
Typical house floor (P/A 0.6)≈ 0.98As pre-filled: 30m perimeter, 50m²
P/A 0.75≈ 1.12
Small floor or extension (P/A 0.9)≈ 1.23
P/A 0.6 with a 50mm screed≈ 0.94The screed counts; the slab does not
P/A 0.6 with 50mm PIR and 22mm chipboard over the slab≈ 0.28See the insulating over a concrete floor page

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

Heat leaves a ground floor mostly through its edges, so the ratio of exposed perimeter to area decides the result: a semi or terrace, with the party walls not counted in the perimeter, does better than a detached house of the same area. The slab’s thickness hardly matters, which is why BR 443 leaves dense concrete out. Insulation over the slab (if there is headroom at the doors) or edge insulation are the ways to improve it; U-Monkey’s edge-insulation option works the second to BS EN ISO 13370 Annex D.

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 uninsulated concrete ground floor?

An uninsulated concrete floor on the ground has a U-value set by the floor’s shape, not its thickness: about 0.98 W/m²K for a typical house floor with 30m of exposed perimeter around 50m² (P/A 0.6), 0.63 for a large, square floor (P/A 0.3) and about 1.2 for a small one such as an extension (P/A 0.9). BR 443 leaves the slab’s own small resistance out, so the ground does almost all the work (BS EN ISO 13370). Most are worse than the 0.70 at which Approved Document L counts a floor as one to upgrade; a large floor can be better than it.

Why does a bigger floor have a lower U-value?

Because the ground under the middle of a large floor is warm, deep and a long way from the outside air, so little heat escapes there; most of the loss is near the edges. BS EN ISO 13370 captures that through the characteristic dimension B′ = A / (½P): the bigger B′, the lower the U-value. That is why an uninsulated floor can be 0.63 W/m²K in a large house and 1.2 in a small extension. Enter your own perimeter and area in U-Monkey.

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.

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.

Open this build-up pre-filled or call 01202 623236