When comparing Structural Insulated Panels (SIPs) for garden rooms or granny annexes, most discussions focus on thickness.

But thickness is only part of the equation.

The insulation core type inside the SIP panel plays an equally critical role in long-term thermal and structural performance.

The most common core materials are:

• EPS (Expanded Polystyrene)
• Foam-injected PUR or PIR (Polyurethane / Polyisocyanurate)

Understanding the difference is essential when specifying a SIP building for year-round residential use.

What Is EPS in SIP Panels?

EPS (Expanded Polystyrene) is widely used in SIP construction.

It is:

• Stable
• Cost-effective
• Lightweight
• Adequate for many non-residential garden structures

EPS-based SIP panels are commonly found in entry-level garden rooms designed to meet a price point.

For seasonal buildings, EPS can perform sufficiently.

But performance depends on application.

What Makes Foam-Injected PUR/PIR Different?

Foam-injected PUR or PIR cores are denser and thermally more efficient per millimetre due to lower thermal conductivity (lambda value).

Lower lambda value = lower heat transfer.

In practical terms:

• For the same panel thickness, PUR/PIR typically achieves a better U-value than EPS
• For the same target U-value, PUR/PIR allows a thinner structural build-up

That means more thermal resistance per millimetre.

In colder climates — including North London, Hertfordshire and the surrounding counties — that difference becomes significant during winter.

EPS vs PUR/PIR: Thermal Performance Comparison

While exact performance depends on full build-up and specification, typical thermal conductivity ranges are:

• EPS: ~0.032–0.038 W/mK
• PUR/PIR: ~0.021–0.028 W/mK

This reduction in lambda value directly improves U-values and overall thermal efficiency in SIP buildings.

The result?

• Lower heat loss
• Reduced heating demand
• Improved internal temperature stability

For granny annexes designed for full-time residential use, that performance gap matters.

Structural Advantages Beyond Insulation

Thermal performance is only one aspect.

Foam-injected PUR/PIR SIP systems often provide:

• Higher compressive strength
• Improved dimensional stability
• More consistent bonding between core and OSB skins
• Enhanced fire classification depending on specification

Because a SIP panel acts as a composite structural element, the bonding integrity between core and skins is critical.

Higher-density cores can contribute to improved long-term rigidity and reduced deflection.

In annexes and extensions intended for permanent living, that structural consistency becomes part of long-term durability.

Why Core Choice Matters for Granny Annexes

When designing granny annexes in Hertfordshire or North London, insulation decisions are not academic.

They directly influence:

• Winter comfort
• Heating system sizing
• Running costs
• Condensation control
• Long-term structural behaviour

A building used occasionally in summer can tolerate compromises.

A building used as full-time accommodation cannot.

How We Specify SIP Core Types at TRJ Construction Ltd

At TRJ Construction Ltd, core specification is determined by intended use — not by minimum cost.

For annexes intended for:

• Full residential occupation
• Multi-generational living
• Therapy rooms or home offices used daily

We prioritise higher-performance insulation cores where appropriate.

Because the envelope determines comfort.

And the envelope is defined not just by thickness — but by core material, bonding quality and installation precision.

Performance Is Engineered, Not Assumed

All SIP panels may look similar externally.

But:

Core type matters.
Lambda value matters.
Bonding consistency matters.
Application matters.

Performance is not automatic.

It is specified.

And that specification defines whether a building is:

A structure placed in a garden

Or

An engineered living space designed for year-round use.