The short answer
SIPs are structural insulated panels: two sheets of oriented strand board bonded to a core of rigid foam insulation. The bonded sandwich is strong enough to act as the wall or roof on its own, with no separate frame, and it insulates and seals the building at the same time.
On this page
The name does most of the explaining. A structural insulated panel is structure and insulation made as one factory product. It replaces the studs, the insulation stuffed between them and the sheathing board nailed over the top with a single rigid piece.
What a panel is made of
Every SIP has three layers.
Two skins. In the UK these are almost always oriented strand board, the grade known as OSB/3, which is made for load-bearing use in humid conditions. Each skin is usually 11 or 15 mm thick.
One core. A slab of rigid foam fills the space between the skins. It is bonded to both across its whole face, either because the foam is injected as a liquid and sticks as it sets, or because a cut sheet of foam is glued in under pressure.
That bond is the important part. Three loose layers stacked together would bend like a pack of cards. Glued into one piece they behave like a steel I-beam turned on its side: the skins do the work of the flanges and the core holds them apart. This is why a panel only 142 mm thick can hold up a roof.
The two core types
| Urethane core (PUR or PIR) | Expanded polystyrene core (EPS) | |
|---|---|---|
| How it is made | Foam injected between the skins | Foam sheet glued between the skins |
| Insulating power | Higher, so panels are thinner | Lower, so panels are thicker |
| Common wall thickness | 142 mm or 172 mm | 150 mm to 250 mm |
| Weight | Slightly heavier for its size | Lighter |
| Price | Usually the dearer panel | Usually the cheaper panel |
Neither is the “better” panel. A urethane core suits a tight site where every centimetre of wall thickness is floor space lost. An EPS core suits a project where the wall can be a little fatter and the budget a little thinner. Both can reach the same U-value; they just need different thicknesses to do it.
How panels join and seal
Panels are cut in a factory from your drawings, with door and window openings already formed and the edges rebated. On site they stand on a treated timber sole plate that is fixed to the base over a damp-proof course.
Each panel locks to the next with a spline, a strip that slots into the rebates of both. The joint is sealed with expanding foam or mastic, screwed or nailed through the skins, and often taped. Corners and openings are lined with solid timber.
The result has very few gaps. A stud wall has a piece of solid timber every 600 mm, and each one is a path for heat to leak through. In a SIP wall the foam is continuous except at the joints and around openings, so far less of the wall is solid wood. That continuity, together with the sealed joints, is why SIP buildings tend to score well in air-leakage tests.
Where SIPs are used
- Whole houses, commonly two storeys and sometimes three.
- Extensions. A SIPs extension is one of the most common uses, because the shell can be stood in days beside a house people are still living in.
- Roofs. Long panels span from wall to ridge with no trusses, leaving the roof space open. Our page on SIPs roof panels covers spans and build-ups.
- Garden buildings and annexes.
- Infill inside a steel, concrete or oak frame, where the panels insulate and enclose but another structure carries the load.
What they are like to build with
Fast, and unforgiving. The speed comes from doing the thinking early: once the drawings are fixed and the panels are cut, a crew can stand the walls and roof of a single-storey extension in three to five days. The same precision means the base has to be level and square to within a few millimetres, and a window moved after the order is placed means a panel made again.
Panels must also be kept reasonably dry. OSB will shrug off rain during the build, but it should not stay wet, so the shell is wrapped in a breather membrane as soon as it is up and then covered by cladding with a ventilated gap behind it.
Inside, a batten zone and plasterboard go over the panels. The battens give cables and pipes somewhere to run without cutting into the structure, and the plasterboard provides the fire protection.
Do SIPs meet Building Regulations?
The regulations do not approve or ban building methods. They set results: the structure must stand up, heat loss must stay under set limits, fire must be resisted for a set time. A SIP building is shown to comply the same way any other is, through an engineer’s calculations and a U-value calculation for each element.
For a new extension wall in England the limit is 0.18 W/m²K. A 142 mm urethane panel gets close to that alone and passes with an insulated lining; a 172 mm panel has margin to spare.
What building control officers, warranty providers and lenders will also ask for is evidence that the panel system itself has been independently tested. Reputable manufacturers hold a third-party certificate for their system, and many belong to the Structural Timber Association.
What do SIPs cost?
For an extension, a finished SIPs build typically costs £1,300 to £2,400 per m² of floor, including VAT. The SIPs shell on its own is about £1,200 to £1,800 per m², and that is not a finished price. Those are typical national ranges for 2026, not quotes; London and the South East usually cost 20 to 40% more. The cost calculator turns them into a budget for a given floor area, and the page on SIPs extension kits explains what the lower figure leaves out.
If you would like a price for a real design, the quote form below takes about a minute.