SIPs vs Timber Frame
The Definitive 2026 Guide
Two construction methods, one question: which delivers better thermal performance, faster build times, and longer lifespan for a London garden room? We answer it with engineering data, not marketing claims.
0.12 W/m²K
Best SIPs U-value
3-5 days
SIPs shell build
60+ years
SIPs lifespan
What Are SIPs?
Structural Insulated Panels (SIPs) are composite building panels consisting of a rigid insulation core — typically expanded polystyrene (EPS) or polyurethane (PUR) — bonded under factory pressure between two structural skins of oriented strand board (OSB). The result is a single panel that acts simultaneously as structure, insulation, and air barrier.
The critical advantage is that the insulation is continuous. In a traditional timber frame, mineral wool or rigid board is cut and fitted between studs on site. Every stud creates a thermal bridge — a line of timber with a conductivity of approximately 0.13 W/mK running through insulation with a conductivity of 0.035-0.040 W/mK. With studs at 400mm centres, these bridges can reduce the effective U-value of a wall by 15-25%.
SIPs eliminate this problem entirely. The foam core runs edge to edge with no structural interruptions. The thermal performance you calculate is the thermal performance you get — factory-locked, not site-dependent.
U-Value Calculation: How SIPs Win
U-value is calculated as U = 1 / R(total), where R(total) is the sum of thermal resistances of each layer. Thermal resistance R = thickness (m) / conductivity (W/mK).
SIPs Wall (120mm PUR core)
Timber Frame (140mm mineral wool)
*Corrected for 15% timber fraction at studs (BS EN ISO 6946). The bridging-corrected U-value is significantly worse than the between-stud figure.
Why This Matters for the London Climate
London sits in a temperate maritime climate with high relative humidity (annual average 75-80%), frequent precipitation (600mm+ annually), and mild but damp winters. These conditions are the worst-case scenario for thermal bridging in timber frame construction.
When warm, moist interior air meets a cold timber stud, the temperature at the inner surface of the wall drops below the dew point. The result is interstitial condensation — moisture forming inside the wall assembly, invisible from both sides. Over time, this degrades mineral wool insulation (which loses up to 50% of its R-value when damp) and creates conditions for mould growth on timber members.
SIPs panels eliminate this risk. The closed-cell foam core does not absorb moisture, the continuous insulation layer prevents cold spots, and the factory-bonded assembly leaves no gaps for air movement within the panel. In the London climate, this is not a marginal advantage — it is the difference between a garden room that performs as designed for 60 years and one that develops hidden damp problems within 5-10 years if the vapour control layer is not perfectly installed.
Factory-Locked vs Site-Dependent
The single most important sentence in this guide: SIPs deliver factory-locked performance that eliminates on-site insulation gaps. A timber frame build is only as good as the installer who fits the insulation batts. A SIPs build performs identically regardless of who assembles it — because the insulation was bonded in a controlled factory environment, not cut and pushed between studs in a London garden.
Head-to-Head Comparison
12 metrics, engineering data, no marketing spin.
| Metric | SIPs | Timber Frame | Edge |
|---|---|---|---|
| Typical U-value (wall) | 0.12-0.18 W/m²K | 0.15-0.20 W/m²K | SIPs |
| Thermal bridging risk | Near zero (continuous foam core) | Moderate (studs every 400-600mm) | SIPs |
| Insulation consistency | Factory-locked, no gaps | Site-dependent, installer skill varies | SIPs |
| On-site build time | 3-5 days (shell) | 7-14 days (shell) | SIPs |
| Airtightness (m³/h/m² @50Pa) | 1.0-3.0 | 3.0-8.0 | SIPs |
| Structural strength | Panel acts as structure + insulation | Frame is structure, insulation is separate | SIPs |
| Wall thickness for same U-value | 120-150mm total | 140-200mm total | SIPs |
| Material cost (per m²) | £85-£120 | £50-£80 | Timber |
| Design flexibility | Fixed panel sizes, CNC-cut to plan | Fully custom on-site | Timber |
| Moisture management | Closed-cell foam resists moisture ingress | Requires separate VCL + careful detailing | SIPs |
| Lifespan | 60+ years (no degradation of foam core) | 40-60 years (if correctly maintained) | SIPs |
| Recyclability | 95% recyclable (OSB + EPS/PUR) | 90%+ recyclable | Draw |
Anatomy of a Wall: Layer by Layer
What sits between you and the London weather — and why every layer matters.
SIPs Wall Assembly
Total thickness: ~195mm | U-value: 0.17 W/m²K
Key: The rigid foam core (highlighted) runs continuously with zero structural interruptions. No thermal bridges.
Timber Frame Wall Assembly
Total thickness: ~205mm | U-value: 0.28 W/m²K (bridging-corrected)
Key: Mineral wool (highlighted) is interrupted every 400-600mm by timber studs. Each stud is a thermal bridge reducing wall performance by 15-25%.
When Timber Frame Still Makes Sense
SIPs are the superior construction method for the vast majority of London garden room builds. But there are two scenarios where a traditional timber frame may be the better choice.
Highly irregular shapes
If the design requires acute angles, curved walls, or non-standard geometries that cannot be efficiently cut from standard SIPs panels, a timber frame offers more on-site flexibility. This applies to perhaps 5% of London garden room projects.
Extremely tight budgets
The material cost per square metre for timber frame is 30-40% lower. If the budget ceiling is absolute and the client accepts the trade-off in thermal performance and build speed, timber frame delivers a functional garden room at a lower capital cost.
Our Position
Every installer in our network builds with SIPs as standard. We made this decision because the data is unambiguous: SIPs deliver superior thermal performance, faster build times, better airtightness, and longer structural life. In a climate as damp as London, the elimination of thermal bridging and interstitial condensation risk is not optional — it is the minimum standard for a garden room designed to last.
If you receive a quote for a timber frame garden room, ask the installer for the bridging-corrected U-value — not the between-stud figure. The difference will tell you everything you need to know.