U-value of insulation laid over an existing concrete floor

PIR and chipboard on top of an old slab, where the door thresholds set the thickness.

Laying 50mm of PIR and 22mm of chipboard over an existing uninsulated slab takes a typical house floor (P/A 0.6) from about 0.98 to 0.28 W/m²K; 25mm gives about 0.41 and 75mm about 0.21. Approved Document L’s figure for a renovated floor is 0.25, which takes 60mm of PIR here (0.2496). Its Table 4.3 allows a lesser standard where the extra height would cause significant problems with adjoining floor levels, which is usually what limits this job.

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The build-up (outside → inside)

The pre-filled link opens at U-Monkey’s default 30m / 50m² (P/A 0.6), as a retained floor being upgraded, judged against Approved Document L’s 0.25 (2021 edition, Table 4.3).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Uninsulated slab≈ 0.98See the uninsulated floor page
25mm PIR + 22mm chipboard≈ 0.41
40mm PIR + 22mm chipboard≈ 0.32
50mm PIR + 22mm chipboard≈ 0.28As pre-filled
60mm PIR + 22mm chipboard≈ 0.25Meets 0.25 (0.2496)
75mm PIR + 22mm chipboard≈ 0.21
25mm / 50mm PIR, large floor (P/A 0.3)≈ 0.33 / 0.24

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

Height is the constraint: every millimetre comes off the room and has to be found at doors, stairs, kitchen units and skirtings, which is why thin, high-performance boards are used here. The floor’s perimeter-to-area ratio moves the result as much as the board does, so enter your own. Check the old slab has a damp-proof membrane; if it has none, lay one under the insulation. With a floating chipboard floor the joints want gluing and an expansion gap at 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 insulation laid over an existing concrete floor?

Laying 50mm of PIR and 22mm of chipboard over an existing uninsulated slab takes a typical house floor (P/A 0.6) from about 0.98 to 0.28 W/m²K; 25mm gives about 0.41 and 75mm about 0.21. Approved Document L’s figure for a renovated floor is 0.25, which takes 60mm of PIR here (0.2496). Its Table 4.3 allows a lesser standard where the extra height would cause significant problems with adjoining floor levels, which is usually what limits this job.

How thin can floor insulation over a slab be?

As thin as the floor levels force, but each step costs: 25mm of PIR takes a typical uninsulated floor from about 0.98 to 0.41 W/m²K, 50mm to 0.28 and 60mm to 0.25, the figure Approved Document L asks of a renovated floor. Where that height would cause significant problems with adjoining floor levels, its Table 4.3 (note 8) allows a lesser standard. Products with a lower λ, such as phenolic or vacuum panels, get closer in less height.

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