Underfloor heating on a ground floor: insulation and U-value

Wet underfloor heating in a screed on PIR, and how much insulation it needs.

A ground floor with wet underfloor heating — pipes in a 65mm screed on 100mm of PIR over the slab — has a U-value of about 0.17 W/m²K for a typical 30m perimeter / 50m² floor, inside the 0.18 limiting value for a new floor; 125mm gives about 0.15, and 150mm about 0.13, the new-build notional figure. BS EN 1264-4 also sets a minimum for the insulation under the pipes, a thermal resistance of 1.25 m²K/W over the ground (about 28mm of PIR), so for a UK ground floor it is the U-value that sets the thickness.

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The build-up (outside → inside)

BS EN 1264-4 Table 1 (as manufacturers reproduce it): 0.75 m²K/W over a heated room; 1.25 over an unheated or intermittently heated room, the ground, or outside air down to 0°C; 1.50 down to −5°C; 2.00 down to −15°C. The pre-filled link opens at U-Monkey’s default 30m / 50m² (P/A 0.6).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
50mm PIR≈ 0.29Meets BS EN 1264-4, not Part L
75mm PIR≈ 0.22
100mm PIR≈ 0.17As pre-filled: meets 0.18
125mm PIR≈ 0.15
150mm PIR≈ 0.13Meets the 0.13 notional (0.125)
150mm EPS (λ 0.038)≈ 0.20
100mm PIR, small extension floor (P/A 0.9)≈ 0.18Misses 0.18: 0.183

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 insulation under the screed does it all; the screed and the floor finish barely register in the U-value but matter to the heating, because everything above the pipes slows the heat into the room. Run an edge strip of insulation round the screed so it does not lose heat sideways into the walls, and remember that a heated floor runs warmer than the room, so it loses a little more than its U-value at room temperature suggests. On a small extension the perimeter-to-area ratio can tip 100mm over the limit, though Approved Document L lets an extension’s floor be calculated with the perimeter and area of the whole enlarged home (Table 4.2, note 5); enter your own floor.

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

Underfloor heating on a ground floor: insulation and U-value?

A ground floor with wet underfloor heating — pipes in a 65mm screed on 100mm of PIR over the slab — has a U-value of about 0.17 W/m²K for a typical 30m perimeter / 50m² floor, inside the 0.18 limiting value for a new floor; 125mm gives about 0.15, and 150mm about 0.13, the new-build notional figure. BS EN 1264-4 also sets a minimum for the insulation under the pipes, a thermal resistance of 1.25 m²K/W over the ground (about 28mm of PIR), so for a UK ground floor it is the U-value that sets the thickness.

How much insulation do I need under underfloor heating?

Two rules apply and the stricter wins. BS EN 1264-4 asks for a thermal resistance under the pipes of at least 0.75 m²K/W over a heated room, 1.25 over an unheated room or the ground, and up to 2.00 over outside air in a cold climate; 1.25 is about 28mm of PIR. A ground floor in a new home or extension also needs a U-value of 0.18 W/m²K or better under Approved Document L, which for a typical floor takes about 100mm of PIR. So on the ground, the U-value decides.

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