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

A typical garden room wall — timber cladding on a ventilated batten space, a breather membrane and 9mm OSB on 100mm studs, with 75mm of PIR between the studs, a vapour control layer and plasterboard — has a U-value of about 0.34 W/m²K. Filling the studs with 100mm of PIR only gets to about 0.32, because the studs carry the heat round the boards; 25mm more PIR across the inside of the studs takes it to about 0.22. Makers’ quoted wall figures run from 0.18 to 0.31, so ask for the build-up behind the number.

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The build-up (outside → inside)

20mm timber cladding (not counted: outside a ventilated layer)
25mm ventilated batten space
Breather membrane
9mm OSB sheathing
75mm PIR between 100mm studs (15% timber), 25mm air space behind
Polythene vapour control layer
12.5mm plasterboard

Studs at BR 443’s 15% for timber frame, bridging both the PIR and the air space behind it. The cladding and its batten space are outside a well-ventilated layer and not counted (BS EN ISO 6946 6.9.4). Makers’ published figures for comparison: Green Retreats 0.31, Warwick Buildings 0.29 (50mm) and 0.21 (70mm), Executive Garden Rooms 0.18 (140mm in a 200mm wall).

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
50mm PIR between 100mm studs	≈ 0.43
75mm PIR between the studs	≈ 0.34	As pre-filled
100mm PIR filling the studs	≈ 0.32	Only 0.02 better: the studs bridge it, and the low-e gap goes
100mm PIR between + 25mm PIR over the studs inside	≈ 0.22	Meets the 0.26 a new dwelling’s wall needs
100mm PIR between + 50mm PIR over the studs	≈ 0.17	Meets 0.18

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 studs are the weak point: at 15% of the wall (BR 443’s figure for timber frame) and six times as conductive as PIR, they carry a large share of the heat round the boards, so going from 75mm to 100mm between them gains only 0.02. Part of that is the 25mm gap beside the 75mm boards, which U-Monkey counts as a low-emissivity cavity because it faces the boards’ foil: without that credit — it needs a clean foil facing a sealed gap — the 75mm wall is 0.37 rather than 0.34, and filling the studs gains about 0.05. A continuous layer across the inside of the studs is what moves the figure — 25mm of PIR over 100mm between takes 0.32 to 0.22, and 50mm to 0.17. The cladding and its batten space count for nothing, because the space behind the cladding is ventilated (BS EN ISO 6946). The vapour control layer barely changes the U-value, and U-Monkey’s free condensation check finds no condensation in any month with or without it, because foil-faced PIR resists vapour on its own; but the boards only fill the spaces between the studs, so the membrane, taped at its laps, is what makes the wall airtight.

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) wall?

What U-value should a garden room have?

Usually the Building Regulations don’t set one, in England and Wales at least (Scotland and Northern Ireland have their own rules): a detached garden room or office that isn’t a dwelling and has a floor area under 50m² is outside the energy efficiency requirements (regulation 21(3)(d)), and many are exempt from most of the Regulations (Schedule 2, Class 6), though Part P still applies to electrics fed from the house (regulation 9(3)). That leaves it to you. For a room heated every working day, aim for what a new house in England would meet: about 0.26 W/m²K or better for walls (Approved Document L’s limiting value), and, to keep running costs down, nearer 0.18 — the notional dwelling’s figure, from which a new home’s energy target is set — the second needs a layer of insulation across the studs, not just between them. A garden room with sleeping accommodation is a different matter: it loses the Class 6 exemption, and whether the energy requirements then apply depends on whether it counts as a dwelling, so ask building control before you build.

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

Walls

Roofs

Floors

Different build-up? See the full wall 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.