U-value of a battened timber floor over a concrete slab

Insulation between timber battens on an existing slab — what the battens cost, and the BR 443 way to count them.

A timber floor laid on battens over an existing concrete slab, with 50mm of PIR between 47mm battens at 600mm centres and 22mm of chipboard over them, has a U-value of about 0.34 W/m²K on a typical floor of 50m² with 30m of exposed perimeter (P/A 0.6). The battens cost about 0.06: the same 50mm laid continuously under a floating deck gives 0.28. The 0.25 Approved Document L asks of a renovated floor takes about 85mm of PIR between battens, or 50mm between the battens with 25mm more laid over them and a plywood deck (0.24). BR 443 counts a battened layer with a combined calculation inside the floor before working out the ground (§9.1), which U-Monkey applies.

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The build-up (outside → inside)

The battens are entered as a bridge through the insulation at 9.4% timber — 47mm battens at 600mm centres with a 47mm cross-piece every 3m, BR 443 §4.5’s width-over-spacing rule — because BR 443 gives no figure for floor battens. U-Monkey’s default floor: 30m of exposed perimeter around 50m² (P/A 0.6), 300mm walls, sand or gravel (λ 2.0); set your own. The pre-filled link opens as an upgraded floor.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Uninsulated: empty 50mm batten void, chipboard≈ 0.70The bare slab: about 0.98
25mm PIR between the battens≈ 0.48
50mm PIR between the battens≈ 0.34As pre-filled
75mm PIR between 75mm battens≈ 0.27
100mm PIR between 100mm battens≈ 0.22Meets the 0.25 for a renovated floor
50mm PIR laid continuous under a floating deck (no battens)≈ 0.28What the battens cost: about 0.06
50mm PIR between battens + 25mm PIR over them, 18mm plywood≈ 0.24Meets 0.25
Battens at 400mm centres (13.3%), 50mm PIR≈ 0.36
100mm mineral wool slab (λ 0.038) between 100mm battens≈ 0.29
100mm wood-fibre batts (STEICOflex 036)≈ 0.28
50mm PIR, a large floor (P/A 0.3)≈ 0.2830m perimeter, 100m²
50mm PIR, a small floor (P/A 0.9)≈ 0.3727m perimeter, 30m²

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

The battens are a bridged layer in a floor that is otherwise a ground calculation, and BR 443 (2019) §9.1 says how to count them: “If a floor does have a bridged layer”, use “the method of BS EN ISO 6946 with Rsi = 0.17 m²K/W (downwards heat flow) and Rse = 0”, subtract Rsi “so as not to count Rsi twice”, then put the result into BS EN ISO 13370 “allowing for the effect of the ground”. That is what U-Monkey does with a bridged layer in a ground floor; taking the layer’s parallel resistance instead would read about 0.01 worse. BR 443 gives no figure for floor battens, so the fraction here follows its general rule — timber width over spacing — for 47mm battens at 600mm centres with a cross-piece every 3m: 9.4%, or 13.3% at 400mm centres. The battens then cost about 0.06 against the same insulation laid continuously, and a thin continuous layer over them (25mm under a plywood deck) recovers about half of it: 50mm between plus 25mm over reads 0.24, against 0.27 for 75mm between the battens and 0.21 for 75mm laid continuously. The floor’s shape does the rest, as on any ground floor: the same 50mm gives 0.28 on a large floor (P/A 0.3) and 0.37 on a small one (P/A 0.9); the ground under it matters too — U-Monkey’s default is sand or gravel (λ 2.0), and BR 443 notes the UK’s ground is most commonly clay (λ 1.5). Approved Document L asks 0.25 of a renovated floor (Table 4.3) and, for a floor kept in a conversion, 0.25 where it is worse than 0.70 (paragraph 4.12(b)), with a lesser standard where meeting it would cause problems with adjoining floor levels (note 8) — a real limit when a battened floor raises the room. Lay the battens, treated, on a damp-proof membrane over the slab; in a traditional building with no damp-proof course, Historic England warns that sealing a solid floor can push ground moisture to the base of the walls.

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 a battened timber floor over a concrete slab?

A timber floor laid on battens over an existing concrete slab, with 50mm of PIR between 47mm battens at 600mm centres and 22mm of chipboard over them, has a U-value of about 0.34 W/m²K on a typical floor of 50m² with 30m of exposed perimeter (P/A 0.6). The battens cost about 0.06: the same 50mm laid continuously under a floating deck gives 0.28. The 0.25 Approved Document L asks of a renovated floor takes about 85mm of PIR between battens, or 50mm between the battens with 25mm more laid over them and a plywood deck (0.24). BR 443 counts a battened layer with a combined calculation inside the floor before working out the ground (§9.1), which U-Monkey applies.

How much do battens reduce the effect of floor insulation?

About 0.06 W/m²K at 50mm: 50mm of PIR between 47mm battens at 600mm centres gives 0.34 on a typical floor, against 0.28 for the same 50mm laid continuously under a floating deck. Closer battens (400mm centres) cost more, 0.36. A thin continuous layer over the battens covers the timber: 50mm between plus 25mm over them, under a plywood deck, reaches 0.24 — inside the 0.25 Approved Document L asks of a renovated floor.

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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