U-value of a garage conversion floor

Insulating over a garage slab, using the step up to the house floor.

A garage floor usually sits below the house floor, and that step is room for insulation: 100mm of PIR and 22mm of chipboard over the existing slab gives a U-value of about 0.17 W/m²K at P/A 0.6, inside the 0.25 Approved Document L asks when a garage’s floor is upgraded in a conversion, and the 0.18 for a new floor. 75mm gives about 0.21; 50mm gives about 0.28, which misses 0.25. An uninsulated garage slab is about 0.98.

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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) — a single garage with its front and one side exposed is close to that — 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 garage slab≈ 0.98
50mm PIR + 22mm chipboard≈ 0.28Misses 0.25
75mm PIR + 22mm chipboard≈ 0.21
100mm PIR + 22mm chipboard≈ 0.17As pre-filled: meets 0.25 and 0.18
120mm PIR + 22mm chipboard≈ 0.15
100mm PIR, 2.6 × 5.5m garage exposed on three sides (P/A 0.75)≈ 0.18Meets 0.18 (0.176)

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 thickness you can fit is usually the step down from the house, less the chipboard. A single garage of about 2.6 × 5.5m with its front and one side exposed is close to the P/A 0.6 used here, and one exposed on three sides nearer 0.75; for the floor of a conversion Approved Document L treats it as a retained element. Garage slabs often have no damp-proof membrane, so lay one, and keep the new floor above the outside ground with the old door opening built up and damp-proofed.

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 garage conversion floor?

A garage floor usually sits below the house floor, and that step is room for insulation: 100mm of PIR and 22mm of chipboard over the existing slab gives a U-value of about 0.17 W/m²K at P/A 0.6, inside the 0.25 Approved Document L asks when a garage’s floor is upgraded in a conversion, and the 0.18 for a new floor. 75mm gives about 0.21; 50mm gives about 0.28, which misses 0.25. An uninsulated garage slab is about 0.98.

What floor U-value does a garage conversion need?

In England, Approved Document L treats the existing garage floor as a retained element (paragraph 4.12(b)): at about 0.98 W/m²K uninsulated it is worse than the 0.70 threshold, so it should be upgraded to 0.25 (Table 4.3), with a lesser standard allowed where floor levels make that impractical (note 8). A floor replaced completely is a new element at 0.18 (Table 4.2). 100mm of PIR meets both on a typical garage 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.

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