U-value of a garden room (garden office) floor

An insulated timber floor on legs, ground screws or a frame, open to the air underneath: what each thickness of PIR reaches.

A garden room floor of 125mm joists with 100mm of PIR between them, a 9mm OSB soffit underneath and an 18mm chipboard deck has a U-value of about 0.25 W/m²K, taken as an exposed floor with outside air underneath; 75mm of PIR gives about 0.30 and 50mm about 0.39. Filling the joists gains little (125mm: 0.23), because the joists bridge the boards; 25mm more PIR across the joists, under the deck, takes 100mm between them to about 0.19.

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The build-up (outside → inside)

Joists at BR 443’s 10.8% for floors, bridging the PIR and the air space above it. Calculated as an exposed floor to BS EN ISO 6946, with outside air below. In England and Wales, detached (stand-alone) garden rooms of under 50m² that are not dwellings are outside the energy efficiency requirements (Building Regulations 2010, regulation 21(3)(d)), so no U-value is required; the figures are for comparing floors.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
50mm PIR between the joists≈ 0.39
75mm PIR≈ 0.30
100mm PIR≈ 0.25As pre-filled
125mm PIR, filling the joists≈ 0.23
100mm PIR between + 25mm PIR over the joists≈ 0.19

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.

What changes the number

The joists bridge the boards through their full depth — 10.8% of the floor, BR 443’s figure for floor joists — so a layer across them does more than extra depth between them: 25mm of PIR over the joists takes 0.25 to 0.19, while filling the last 25mm between them only reaches 0.23. What is under the floor decides which calculation applies. On legs, ground screws or a frame with the sides open, the air under the room is effectively outside air, so U-Monkey’s exposed-floor element is the right one, as here. Keep the soffit board: it stops wind washing through the insulation from below. U-Monkey’s condensation check finds no condensation in any month in any of these floors.

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 garden room (garden office) floor?

A garden room floor of 125mm joists with 100mm of PIR between them, a 9mm OSB soffit underneath and an 18mm chipboard deck has a U-value of about 0.25 W/m²K, taken as an exposed floor with outside air underneath; 75mm of PIR gives about 0.30 and 50mm about 0.39. Filling the joists gains little (125mm: 0.23), because the joists bridge the boards; 25mm more PIR across the joists, under the deck, takes 100mm between them to about 0.19.

Is a garden room floor an exposed floor or a suspended floor?

It depends on what is under it. With the sides open — on legs, ground screws or a frame, with air moving freely under the room — treat it as an exposed floor, as here: the air below counts as outside air. If a skirting or a low wall encloses the space and it is ventilated only through small openings, it is a suspended floor over a void, which BS EN ISO 13370 calculates from the floor’s size, the void and its ventilation; that comes out a little better — about 0.21 for this floor under a 3 × 4m room at U-Monkey’s default void settings — and U-Monkey has a suspended floor element for it. A garden room on a concrete slab is a ground floor instead.

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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