The short answer
The main disadvantages of structural insulated panels are quotes that are rarely much cheaper than brick and block once both are finished, little room for design changes once panels are ordered, and a need to keep the timber skins dry. They also demand planned ventilation and an accurate base. All are manageable, but only if they are dealt with at design stage.
On this page
- 1. They are rarely the cheap option
- 2. Late changes are expensive
- 3. The skins must not stay wet
- 4. Airtight means you have to ventilate
- 5. Light walls store little heat
- 6. Fixings, services and noise need forethought
- 7. Not every builder, lender or insurer knows them
- How much do these matter for an extension?
Most of what is written about SIPs is written by people selling them. This is the other side: seven real drawbacks, how much each one matters on a house extension, and what a competent designer does about it. If the panels themselves are new to you, start with what SIPs are.
1. They are rarely the cheap option
Typical national ranges for 2026 put a finished single-storey extension in brick and block at £1,800 to £3,000 per m² and a finished SIPs extension at £1,300 to £2,400, both including VAT. On paper SIPs can look cheaper, but the ranges overlap, and a low SIPs figure is often a shell price (£1,200 to £1,800 per m² for the panels alone), not a finished one. Always ask which you are being quoted.
Other savings add up. A shorter build means fewer weeks of site costs and less time living around builders. Thinner walls give a slightly bigger room inside the same footprint. Lower heat loss trims the heating bill. But on the day you compare like-for-like quotes, SIPs are rarely far cheaper, and a big gap usually means the two quotes cover different specifications. The cost calculator shows the range for your floor area.
2. Late changes are expensive
A bricklayer can move a window on Tuesday morning. A SIP panel is cut by machine weeks before it reaches you, with the opening already in it. Change your mind after sign-off and the panel is remade, the lorry is rebooked and the crew waits.
The cure is dull: finish the design before ordering. Decide the kitchen layout, the door sizes, the roof light positions and the boiler flue route on paper. People who enjoy designing as they go are better served by another method.
3. The skins must not stay wet
Oriented strand board is tough, but it is still wood. Wet for a few days during the build, it dries without harm. Wet for months behind a leaking flashing or a blocked cavity, it swells and eventually rots, and the panel loses the strength that comes from its skins.
Every sound SIP detail exists to prevent this: a breather membrane over the outside, a drained and ventilated gap behind the cladding, a sole plate lifted at least 150 mm above the ground, and a vapour control layer on the warm side where the design calls for one. Flat roofs need particular care. Leaks that would be a nuisance in masonry are a structural matter here, so gutters and flashings deserve a look every autumn.
4. Airtight means you have to ventilate
SIP buildings leak very little air. That saves heat and it also traps moisture from cooking, showers and breathing. An extension that is far tighter than the house it joins can end up with condensation on its windows if nobody planned for it.
Building Regulations already require background ventilation and extraction in new rooms. With SIPs, treat that as a design item: trickle vents sized properly, a ducted extractor over the hob, and continuous extract or heat-recovery ventilation if the extension holds a kitchen and a utility.
5. Light walls store little heat
Brick and block soak up warmth through a sunny afternoon and release it overnight. A SIP wall has almost no thermal mass, so a heavily glazed, south-facing extension can warm up quickly in July.
The insulation is not the cause; the glass is. External shading, a roof overhang, solar-control glazing and windows that open high and low on opposite sides all work. A concrete floor slab or stone worktops add some mass back. Ask the designer how the room has been assessed for summer overheating.
6. Fixings, services and noise need forethought
You should not chase a cable into a SIP or cut a hole wherever a plumber finds convenient. Services run in a battened void on the room side, and any hole through the panel is drilled where the manufacturer allows. Heavy items such as wall-hung kitchen units fix well into OSB, but very heavy loads want a timber noggin planned in advance.
Lightweight walls also pass more low-frequency sound than masonry. Beside a quiet garden this is irrelevant. Beside a main road, ask for a denser lining board or an extra layer.
7. Not every builder, lender or insurer knows them
A general builder who has never stood SIPs can do it, with the manufacturer’s drawings and some care, but the first job is where joints get missed. Paying the supplier’s own crew to erect the shell removes most of that risk.
Lenders and insurers class SIPs as non-standard construction. For an extension to a conventionally built house this seldom causes trouble, but keep the panel system’s third-party certificate, the engineer’s calculations and the building control completion certificate together. A future buyer’s surveyor will ask for them.
How much do these matter for an extension?
| Drawback | On a typical extension | What deals with it |
|---|---|---|
| Higher price per m² | Real, and the main one | Compare like for like; value the time saved |
| Late changes | Real | Freeze the design before ordering |
| Moisture | Serious if ignored | Membrane, ventilated cladding, raised sole plate |
| Ventilation | Moderate | Extraction and trickle vents designed in |
| Overheating | Depends on glazing | Shading and openable windows |
| Fixings and noise | Minor | Service void; denser linings if needed |
| Finance and familiarity | Minor for extensions | Certificates kept; trained crew |
None of these is a reason to rule panels out. They are reasons to choose a firm that raises them before you do. If you are weighing up a SIPs extension against masonry, a priced design from a specialist will show where the money goes.