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 TRJ Construction Ltd

At TRJ Construction Ltd, 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.