IDEAS & GUIDANCE
SIP Panel Thickness: Where Performance Begins
Learn why SIP panel thickness affects insulation, comfort, condensation control and year-round performance in garden rooms and annexes.
SIP Panel Thickness – Where Performance Begins
When comparing SIP panels for garden rooms and granny annexes, one of the most misunderstood factors is thickness.
You will often see:
- 100mm SIP wall panels
- 125mm SIP wall panels
- 150mm SIP wall panels
Many entry-level garden rooms across North London, Hertfordshire and the Home Counties are built using 100mm walls and 100mm roofs.
Technically, that qualifies as insulated construction.
But insulation is not a yes-or-no characteristic.
It is a measurable level of thermal resistance.
Understanding SIP Panel Thickness and U-Values
Thermal performance in Structural Insulated Panels (SIPs) is commonly measured in U-values (W/m²K). The lower the number, the better the insulation performance.
Typical performance ranges:
- 100mm EPS SIP wall: ~0.30–0.35 W/m²K
- 125mm SIP wall: ~0.23–0.28 W/m²K
- 150mm SIP panel: ~0.18–0.22 W/m²K
If the insulation core is upgraded to foam-injected PUR or PIR, thermal conductivity improves further per millimetre, often achieving:
- ~0.13–0.17 W/m²K depending on full wall build-up
That numerical difference may look small.
In real-world performance, it is not.
What SIP Thickness Means for Garden Rooms and Granny Annexes
The difference between 100mm and 150mm SIP panels translates into:
- A space that heats quickly but loses heat quickly
- Versus
- A space that maintains stable internal temperatures through winter
For garden offices, therapy rooms, music rooms or full residential granny annexes, temperature stability matters.
Greater thickness means:
- Reduced heat loss
- Improved surface temperature consistency
- Lower condensation risk
- Reduced energy demand
Performance is cumulative. Roof, wall and base thickness work together as a complete thermal envelope.
Why 100mm Panels Are Common in Budget Garden Rooms
Many entry-level garden buildings are designed around a price point.
Reducing wall thickness reduces material cost.
Reducing roof depth reduces material cost.
But reducing thickness also reduces:
- Thermal resistance
- Long-term comfort
- Winter usability
In milder seasons, the difference may not be immediately obvious.
In January, it is.
Our SIP Specification at MyGrannyAnnexe
At MyGrannyAnnexe, we specify SIP panels for year-round performance — not seasonal use.
Typical specification:
- 125mm structural SIP wall panels
- 150mm insulated roof panels
- 150mm insulated base systems
Because our focus is not simply building a structure in a garden.
It is creating an engineered living space suitable for full-time use across North London and the Home Counties — where winter performance genuinely matters.
Thickness Is an Engineering Decision
In SIP construction, thickness is not cosmetic.
It directly affects:
- Structural rigidity
- Thermal stability
- Energy efficiency
- Long-term durability
We do not reduce thickness to reduce cost.
We engineer performance to increase long-term value.
Because when the envelope is right, everything else follows.
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