U-value of a floor over a garage

Quilt between the joists, PIR under them — the floor of a room over a garage.

A timber floor over a garage with 200mm of mineral wool quilt between the joists and 50mm of PIR fixed under them achieves a U-value of about 0.14 W/m²K, inside the 0.18 limiting value for a new floor; 70mm of PIR takes it to 0.13, the notional value. Quilt between the joists on its own gets only to about 0.22, because the joists bridge it through its full depth and nothing covers them. The figures take the garage as outside air — right for an open passage or car port, and the cautious case for an enclosed garage. Counting an integral garage the way SAP does, as an extra resistance (Ru 0.35), takes the pre-filled floor from 0.145 to about 0.138; U-Monkey applies it when you pick the garage under “On the other side”.

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The build-up (outside → inside)

Calculated as an exposed floor to BS EN ISO 6946, with the garage taken as outside air; the Ru rows apply SAP 10.2’s formula (3), U = 1/(1/Uo + Ru), with Ru from Table 3.1 (the same in SAP 10.3). BR 443 applies no air-gap correction to floors, because convection is suppressed when heat flows downwards.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Uninsulated: plasterboard ceiling, empty joists≈ 1.45The starting point
200mm quilt between the joists only≈ 0.22Misses the 0.18 for a new floor; meets the 0.25 for an upgraded existing one
200mm quilt + 50mm PIR under the joists≈ 0.14Inside the 0.18 limiting value; short of the 0.13 notional
200mm quilt + 70mm PIR under the joists≈ 0.13Meets the 0.13 notional value (0.128)
150mm quilt + 50mm PIR under (150mm joists)≈ 0.17Shallower joists still clear 0.18 with the PIR layer
Pre-filled floor over an integral garage, fully integral (Ru 0.35)≈ 0.140.138; 0.140 with Table 3.1’s one-wall-and-floor row (Ru 0.25)
200mm quilt only, over an integral garage (Ru 0.35)≈ 0.21Still misses 0.18: 0.206

Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and the air-gap correction included — 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 shown: your products, thicknesses and (for floors) footprint will move the number, and that's what the calculator is for. Rows that give an Ru include the unheated space beyond the element as an extra resistance, U = 1/(1/Uo + Ru) (SAP 10.2 §3.3 formula (3); BR 443 §5), which the calculator applies under “On the other side”.

What changes the number

The joists are the weak point. Quilt between them is bridged by timber through its full depth, so a continuous layer under the joists does far more per millimetre than extra depth between them. That layer is often foil-faced PIR, which is vapour-tight and sits on the cold side of the quilt, so the vapour control layer belongs on the warm side, under the floor deck: without it, U-Monkey’s free condensation check (at its default settings) puts around 185 g/m² of condensation on the face of the PIR over the winter; with it, the peak drops to about 6 g/m² that dries out each summer. The garage ceiling also has to provide the fire separation Part B asks for between a garage and the house, so use the board and fixings the fire design specifies.

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 floor over a garage?

A timber floor over a garage with 200mm of mineral wool quilt between the joists and 50mm of PIR fixed under them achieves a U-value of about 0.14 W/m²K, inside the 0.18 limiting value for a new floor; 70mm of PIR takes it to 0.13, the notional value. Quilt between the joists on its own gets only to about 0.22, because the joists bridge it through its full depth and nothing covers them. The figures take the garage as outside air — right for an open passage or car port, and the cautious case for an enclosed garage. Counting an integral garage the way SAP does, as an extra resistance (Ru 0.35), takes the pre-filled floor from 0.145 to about 0.138; U-Monkey applies it when you pick the garage under “On the other side”.

Does the garage count as outside air?

Not in a SAP calculation, if the garage is integral. SAP 10.2 (section 3.3.1) adjusts the U-value of the elements between a dwelling and an integral garage with an extra thermal resistance, Ru, for the garage: 0.35 m²K/W where it shares a side wall, an end wall and the floor with the house and sits outside its insulation, 0.25 where it shares one wall and the floor (Table 3.1), applied as U = 1/(1/Uo + Ru). Pick the garage under “On the other side” and U-Monkey makes the same adjustment: the pre-filled floor goes from 0.145 to about 0.138. A floor over an open passage, undercroft or car port is over outside air, so there the exposed-floor figure stands — and it is the cautious figure in any case.

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.

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