The difference between 140mm and 150mm insulation boards

If you’re specifying insulation for a roof, wall, or floor build-up, you may be deciding between two very similar options: 140mm PIR and 150mm celotex insulation. At first glance, a 10mm difference might seem insignificant — but it can affect thermal performance, compliance margins, cost, and long-term energy efficiency.

Celotex is one of the UK’s most widely used rigid insulation manufacturers, offering high-performance PIR (polyisocyanurate) boards for residential and commercial construction. Understanding how small thickness changes impact performance can help you make a more informed choice.

1. What Is Celotex PIR Insulation?

Celotex insulation boards are manufactured using PIR foam, a closed-cell rigid material known for its excellent thermal efficiency. PIR delivers strong insulation performance relative to its thickness, making it ideal where space is limited but thermal targets are demanding.

Both 140mm PIR boards and 150mm PIR boards use the same core material and foil facings — the difference lies purely in thickness and the resulting thermal resistance.

2. Thermal Performance: 140mm vs 150mm

The main technical difference between 140mm and 150mm insulation boards is their thermal resistance (R-value). Thicker boards slow heat transfer more effectively.

• 140mm Celotex – delivers very high thermal performance and is often sufficient for standard Building Regulations.
• 150mm Celotex – provides a modest but meaningful improvement in insulation performance.

In practical terms, the additional 10mm can help reduce U-values slightly, offering extra compliance headroom — particularly useful in roofs and high-efficiency wall build-ups.

3. Building Regulations and Compliance Margins

UK Building Regulations (Part L) set minimum U-value targets for walls, floors, and roofs. While both thicknesses can meet regulations depending on the construction type, 150mm boards give designers more margin.

• 140mm may meet requirements exactly in some roof or wall systems.
• 150mm helps exceed minimum standards and future-proofs the build.

For projects aiming for higher EPC ratings or long-term energy efficiency, that extra thickness can make compliance easier without changing the build-up.

4. Cost Considerations

Cost differences between 140mm and 150mm Celotex are usually modest, but they can add up on large projects.

• 140mm boards typically cost slightly less per board and per square metre.
• 150mm boards have a higher upfront cost but can reduce long-term heating expenses.

When viewed over the lifetime of the building, improved insulation performance can often outweigh the small increase in material cost.

5. Installation and Practical Considerations

From an installation perspective, both thicknesses behave almost identically:

• Both are rigid, lightweight, and easy to cut.
• Fixings, taping, and detailing methods remain the same.
• Performance depends more on correct fitting than thickness alone.

However, cavity depth or rafter height can be decisive. If your structural depth is limited to 140mm, using 150mm boards may require adjustments such as additional battens.

6. Moisture and Condensation Performance

Celotex PIR boards have a closed-cell structure with foil facings, helping to resist moisture ingress and vapour diffusion. Thickness alone does not significantly change moisture behaviour.

Correct installation — including vapour control layers and airtight detailing — plays a much bigger role in preventing condensation than the choice between thicknesses.

7. Which Thickness Should You Choose?

• Choose 140mm PIR if space is fixed, budgets are tight, and U-values are already compliant.
• Choose 150mm PIR if you want improved thermal performance, future-proofing, or extra compliance margin.

Both options deliver excellent insulation performance when properly installed. The best choice depends on your target U-value, available depth, regulatory goals, and long-term energy priorities.

Whether you choose 140mm PIR or 150mm PIR, investing in high-quality insulation is one of the most effective ways to improve comfort, reduce heat loss, and lower energy bills.