What a SIP actually is
A structural insulated panel is a sandwich: two rigid facing boards with an insulating core bonded between them. The result carries structural load and insulates at the same time, which is why the panel can replace both the frame and the insulation in a wall or roof.
What they are good at
- Thermal performance. Continuous insulation with fewer gaps and fewer cold bridges than a frame built and then filled.
- Airtightness. Fewer joints and a more controlled build-up, which matters because heat loss through air leakage is easy to underestimate.
- Speed on site. Panels are manufactured to size and erected quickly, so the structure is weathertight sooner.
- Consistency. Factory manufacture means less variation than site-built framing.
The trade-offs worth knowing
- Design has to be finalised earlier. Panels are cut to a design; changing your mind after manufacture is expensive in a way that changing a stud wall is not.
- Ventilation becomes essential, not optional. A very airtight building needs designed ventilation, or you trade a heat problem for a condensation problem.
- Services need planning. Routes for cables and pipes must be thought through in advance rather than chased in later.
- Cost is not automatically lower. The material cost is often higher; the saving comes from programme time and running costs, which is a different argument.
The honest summary: SIPs buy you performance and speed, at the cost of flexibility. If your design is settled, that is a good trade. If it is still moving, it is not.
Where they make sense on a domestic job
They suit new-build and larger extensions where the design is fixed, garden rooms and studios where you want a small building to be genuinely usable year round, and roof build-ups where you want high performance without enormous depth.
They are usually less compelling on small, irregular extensions tying into an old building with uneven existing structure, where site-built framing can be adjusted as things are found.
How this fits with insulation generally
Panels are one option among several. For most homeowners the higher-value work is less exciting: getting loft insulation to current depth, dealing with draughts, and making sure the roof is ventilated properly, particularly if spray foam has previously been installed and removed.
The best insulation decision is the one that suits the building you have, not the one with the best marketing.