Ask a builder "what's stronger, steel or timber?" and steel wins. So it's tempting to assume a steel-framed garden room must be the higher-spec choice. In reality, for a heated, insulated year-round building, timber frame construction (and specifically the timber-frame-panel evolution of it) dramatically outperforms steel — on thermal efficiency, condensation risk, running cost and long-term durability.
This guide walks through why. If you're comparing quotes and one supplier is offering a steel-frame studio at a lower headline number, this is what you need to know before you sign.
The core problem: steel is a thermal bridge
Every material has a thermal conductivity value — the rate at which heat moves through it. The lower the number, the better the material insulates. Softwood timber sits at around 0.13 W/m·K. Structural steel is roughly 50 W/m·K.
That's not a small gap. Steel conducts heat around 400 times faster than timber. In a domestic building, every steel stud, rafter or column becomes a highway for heat to escape in winter and pour in during summer — even if there's insulation in the wall around it.
What that means in practice
- Higher heating bills — the heat you pay for is constantly escaping through the frame
- Cold spots on interior walls where the studs sit, visible in thermal imaging
- Condensation and mould risk — warm room air hits the cold surface of the steel and drops moisture there
- Slow warm-up in winter and slow cool-down in summer — the frame becomes a heat reservoir
- In extreme cases, corrosion inside the wall as moisture cycles against untreated steel
Manufacturers can mitigate this with thermal-break tapes, cavity insulation and clever detailing — and some do it well. But the honest starting point is: you're compensating for the frame material rather than working with it.
Timber frame — the material that IS the insulation
Timber, by contrast, is a naturally insulating structural material. A softwood stud continues the insulation line rather than breaking it. That's why almost every high-performance house built to Passive House or AECB standards uses timber frame (or its close cousin, cross-laminated timber) — not steel.
At Arden & Oak we take this a step further with insulated timber-frame construction — a factory-bonded sandwich of two OSB timber facings with a rigid PIR foam core. The result is a wall or roof panel where the structural element and the insulation are the same piece, with no cavities, no studs breaking the line, and no thermal bridges.
- Wall U-value around 0.16 W/m²·K — roughly 2× better than UK building regulations require for a new-build home
- Roof U-value around 0.13 W/m²·K — matches Passive House targets
- Airtightness under 2 ACH @ 50 Pa — 3-5× tighter than a comparable stud-frame outbuilding
- No cold bridging anywhere in the panel — the insulation is continuous
Condensation and long-term durability
Cold steel + warm humid air = interstitial condensation, the invisible enemy of any heated building. Because timber is warm to the touch (relative to steel) and has some capacity to buffer moisture, the same air conditions produce far less condensation risk. In a well-detailed timber-frame or stick building, the wall assembly stays dry across the seasons.
Timber is also easier to detail well. Fixing points don't need to be pre-planned or welded — you drive a screw where you need one, add a shelf, hang a light, cut in a socket. A steel frame is far more rigid in what you can change after the build.
But isn't steel stronger?
For a two-storey building carrying heavy loads, yes — steel excels. For a single-storey garden room measuring 4×3 m up to 8×5 m, the loads are trivial and timber is more than strong enough. UK Building Regulations don't require steel at these spans, and structural engineers routinely design timber-framed spans well beyond anything a garden studio needs.
Where steel does earn its place is in specific architectural moments — an exposed corner glazing detail with no visible corner post, or a heavy cantilevered roof. Those elements can be introduced surgically into an otherwise timber-framed building. Building the entire envelope in steel to get one detail is the wrong trade-off.
How the running costs stack up
Over 10 years of use, a poorly insulated steel-frame room can cost £2,000–£4,000 more in heating than an equivalent stick-built room, depending on how many hours per week it's used. On a £15,000–£30,000 build, that difference is meaningful. Combined with better resale value (any surveyor will spot the difference in thermal performance) and better day-to-day comfort, the case for timber frame is straightforward.
Questions to ask any supplier quoting a steel frame
- What is the U-value of the wall assembly, tested and certified?
- What is the thermal-break specification between the steel and the internal lining?
- What airtightness figure has the design been certified to?
- What condensation risk analysis has been done on the wall build-up?
- What warranty covers hidden corrosion inside the wall over 15+ years?
If the supplier can't answer these clearly, that's your answer.