Build speed and thermal performance
SIPs: factory-cut OSB-EPS-OSB panels arrive on site pre-cut for windows and doors. A typical single-storey rear extension at 25m² goes up in 4–7 days from foundation level to weathertight. Walls U=0.15 W/m²K, roof U=0.11 as standard with 150mm EPS core. Airtightness easy to achieve 1–3 m³/h/m². Blockwork: traditional twin-skin cavity wall (102mm brick outer, 100mm cavity with 100mm PIR, 100mm blockwork inner) takes 10–14 days to wall plate height on a single-storey 25m². Walls U=0.18–0.22 standard, can go to U=0.15 with 125mm cavity. Roof traditional rafters with 200mm rockwool + 50mm PIR achieves U=0.13.
Cost breakdown
SIPs (supply only): £140–£220 per m² of panel area for standard 142mm wall, 172mm roof. Install labour £60–£130 per m². Total panel + install £200–£350 per m². Blockwork: brick outer skin £80–£140 per m², blockwork inner skin £40–£70 per m², cavity insulation £15–£35 per m². Total cavity wall £180–£250 per m². Crane and access: SIPs need a crane lift for panels (1–2 days, £600–£1,200) on most London sites. Blockwork doesn't need a crane. Foundation: SIPs lighter than blockwork — can use a smaller foundation in some cases, saving £1,500–£3,500. Internal finishes the same on both — plasterboard, plaster, paint, M&E.
When to choose each
Choose SIPs when: speed is critical (need to move back in fast); high thermal performance target (Passivhaus, EnerPHit); large open spans (SIPs roof spans further without intermediate supports); difficult access (panels craned in vs months of brick delivery); rear-only extensions on enclosed gardens. Choose blockwork when: budget-driven; matching existing fabric in conservation area (London stock brick); double-storey extension where bricklayer can scaffold and continue; complex shape with many internal walls; planning officer prefers traditional construction. London conservation areas almost always require brick outer skin — even SIPs extensions in conservation areas typically have a brick veneer added at £80–£140 per m², eroding the cost advantage.
