Structural Insulated Panels (SIPs) are one of the most advanced and efficient construction systems available for UK house extensions. Combining structure, insulation and airtightness in a single prefabricated element, SIPs deliver fast construction, excellent thermal performance and high quality. This guide explains what SIPs are, how they compare to traditional construction methods, and when they represent the best choice for your extension in 2025.

What Are SIPs?

Structural Insulated Panels (SIPs) are sandwich panels consisting of two rigid facing boards (typically Oriented Strand Board — OSB) bonded to a rigid foam insulation core (typically EPS — Expanded Polystyrene — or PIR — Polyisocyanurate foam). The facing boards provide structural rigidity and form the basis for fixing finishes; the foam core provides insulation and transfers loads between the faces.

SIPs panels are manufactured to precise dimensions in a factory and delivered to site ready to assemble. Standard panel thicknesses range from 100mm to 200mm+ depending on the insulation specification required. Panels can be used for walls, roofs and floors, and can be cut to accommodate windows, doors and penetrations at the factory.

How SIPs Compare to Masonry and Timber Frame

Speed of Construction

SIPs construction is exceptionally fast. A typical single-storey extension shell can be erected and made weathertight in 3–5 days, compared to 2–3 weeks for timber platform frame and 4–6 weeks for masonry cavity wall construction. This speed reduces labour costs, minimises weather exposure and allows the fit-out phase to start sooner.

Thermal Performance

A 165mm SIPs wall panel with EPS core achieves a U-value of approximately 0.18 W/m²K — meeting current Part L requirements in a slimmer wall profile than most insulated masonry or timber frame alternatives. Increasing the insulation core thickness to 200mm achieves U-values of 0.13–0.15 W/m²K — well above minimum requirements.

The key thermal advantage of SIPs over platform frame timber is the continuous insulation core without the interrupted thermal bridges created by studs. Even with stud cavities fully filled with mineral wool, the timber studs in platform frame conduct heat significantly better than the insulation — thermal bridging that SIPs virtually eliminates.

Airtightness

SIPs panels bond the OSB facing boards directly to the insulation core, creating a very airtight construction when panels are properly jointed and sealed. Airtightness levels of 1–3 m³/m²/h at 50Pa are routinely achieved with SIPs construction — significantly better than average masonry or timber frame construction (typically 5–10 m³/m²/h). Good airtightness dramatically reduces heat loss by convection and is a key component of Passivhaus and low-energy construction.

Structural Performance

SIPs panels are structural diaphragms — they resist both vertical loads (compressive) and horizontal loads (racking/shear from wind). The rigid panel acts as a stressed skin, distributing loads efficiently across the full panel area. SIPs are used in multi-storey residential construction (up to 3–4 storeys) and are suitable for all domestic extension types.

Material Cost

SIPs panels are more expensive per m² than the equivalent masonry or timber frame wall materials. A typical SIPs wall panel costs £80–£150/m² supplied and erected, compared to £60–£100/m² for equivalent masonry or timber frame. However, the labour saving from the fast erection can offset the material cost premium for the overall project.

SIPs Roof Construction

SIPs panels are particularly effective as a roof structure for flat or low-pitched roof extensions. A SIPs roof panel combines the structural deck, insulation and the basis for the waterproofing membrane in a single element — eliminating the complex build-up of rafters, insulation quilt, vapour barrier and decking typically required for a timber-framed flat roof. SIPs roofs achieve excellent U-values (0.12–0.15 W/m²K) and high airtightness, while the speed of erection is even more pronounced than for walls.

Design and Aesthetic Considerations

SIPs construction can accommodate any external finish — brick, render, timber cladding, metal — in exactly the same way as timber frame. The OSB facing boards form the substrate to which external finishes are fixed. Internally, the OSB inner face forms the direct substrate for plasterboard lining.

One aesthetic opportunity offered by SIPs is the thin wall profile — a 165mm SIPs wall achieves higher thermal performance than a 300mm insulated masonry wall, maximising internal floor area within the planning envelope.

SIPs Extension Costs UK 2025

Typical SIPs extension costs in 2025:

• SIPs shell only (walls and roof, supply and erect): £200–£350/m² of floor area
• Complete SIPs extension to plastered shell (including foundations, slab, external finishes): £1,800–£2,500/m²
• Premium spec SIPs extension with full fit-out: £2,500–£3,500/m²

These costs are broadly comparable to high-specification timber frame construction. The premium over masonry construction reflects the higher material cost but is partially offset by faster labour.

Finding SIPs Contractors

Not all builders are experienced with SIPs construction. Specialist SIPs erectors work alongside or are employed by the main contractor. Key SIPs manufacturers in the UK (including Kingspan TEK, SIPs UK and others) provide approved erector programmes and can recommend local approved installers. Always use an experienced SIPs erector rather than a general builder unfamiliar with the system.

Frequently Asked Questions

Do SIPs extensions need special Building Regulations treatment?

SIPs extensions are assessed under the standard Building Regulations for extensions. The structural design (typically a proprietary structural system with manufacturer’s technical documentation) is submitted to Building Control for approval. Third-party certification from the BBA (British Board of Agrément) or similar provides Building Control with confidence in the system.

Are SIPs extensions suitable for conservation areas?

The structural system (SIPs) is invisible — the external appearance of a SIPs extension is identical to any other construction method once clad in the specified material. Planning authorities assess the appearance, not the structure. SIPs are entirely appropriate for conservation area applications.

Can I insulate the roof and walls at the same thickness with SIPs?

Yes — one of the advantages of SIPs is the ability to achieve consistent insulation levels across roof and walls, minimising thermal bridges at the junctions. This consistency is more difficult to achieve with conventional construction where roof and wall specifications differ.

Can Crown Architecture design a SIPs extension?

Yes. Crown Architecture & Structural Engineering designs extensions using SIPs, timber frame and masonry construction and can advise on the most appropriate system for your project, specification and budget. Call 07443 804841 to discuss your extension.

Explore SIPs for Your Extension with Crown Architecture

SIPs construction offers outstanding thermal performance, fast build and excellent quality for house extensions. Crown Architecture & Structural Engineering can design and specify a SIPs extension that meets your brief and budget.

Call 07443 804841 or use the form above to get started.