One of the most common questions we get is simple: which SIP panel thickness should I use? The answer depends on what you are building, how the space will be used, whether Building Control is involved, and what thermal performance you are aiming for.
This is a practical starting point, not engineering approval. If the project is habitable, structural, attached to a dwelling, or subject to Building Control, the final specification should be checked properly.
122mm SIP panels
122mm SIP panels are commonly used for garden rooms, offices, studios and non-habitable outbuildings where a slim, warm and practical shell is needed. They are popular because they balance cost, handling, wall thickness and thermal performance.
For many garden room projects, 122mm may be suitable where Building Control is not involved. If the room will be used in a more demanding way, or if the local authority or designer needs a stronger thermal target, a thicker panel may be better.
172mm SIP panels
172mm SIP panels are a common step up for extensions, annex-style projects, more heavily used spaces and builds where Building Control or habitable use is more likely. They provide a thicker insulation core and can make it easier to work towards stronger U-value targets.
If a customer tells us the building could become habitable accommodation, an extension, annexe, dwelling-style space or Building Control project, 172mm is often the safer starting point for quoting.
222mm SIP panels
222mm SIP panels are for higher-performance builds, thicker roof build-ups or projects where the thermal target is more demanding. They are not needed for every garden room, but they can be useful where the specification needs to be closer to high-performance residential standards.
For roof U-values or specific Part L targets, use the UltraSIPs U-value calculator as a starting point and confirm the final specification with your designer, SAP assessor or Building Control route.
Walls, roofs and floors do not always need the same thickness
Some builds use the same thickness throughout. Others use a thicker roof or floor than the walls. This depends on span, thermal target, detailing, floor construction and the intended use of the space.
The UltraSIPs configurator lets you test different wall, roof and floor thicknesses so you can see how the estimate changes before you commit.
Quick rule of thumb
- Garden room or office: 122mm may be enough for many non-habitable uses.
- Extension, annexe or habitable-style space: start by reviewing 172mm.
- Higher thermal performance: consider 222mm, especially for roofs or demanding targets.
If you are not sure, send us your drawing, sketch or dimensions through the contact page. We can help you choose a sensible starting point and explain what is included in the quote.