115mm Insulation

115mm insulation is a cavity wall–specific thickness, manufactured to suit full-fill or partial-fill masonry cavities. It is not used for floors, roofs, or ceilings.

What is 115mm Thick Insulation?

115mm thick insulation is precisely 115mm deep and is usually sold as rigid boards. Used for thermal performance and dimensional stability, it typically offers better thermal performance per millimetre than soft rolls.

In UK construction, 115mm insulation boards are designed specifically for cavity wall use. This thickness aligns with common masonry cavity widths and approved wall tie systems.

These boards are made from rigid foam materials such as PIR or phenolic. They typically have foil or fleece facings to help with moisture control.

Inside the boards are tiny closed air pockets that resist heat transfer through the material. Since thermal energy transfer through the wall is slow, the indoor space stays warmer.

What is 115mm Insulation Thickness Used For?

115mm insulation thickness is used for cavity wall insulation. It is installed within:

• Cavity walls – 115mm boards are designed to fully fill suitable masonry cavities or work as partial-fill systems with a residual air gap. Suitability depends on cavity width, exposure zone, wall tie specification, and third-party certification.
• New-build external walls in modern masonry construction – where cavity widths are designed around thermal targets.
• Cavity wall retrofits (where applicable) – To upgrade walls that have sufficient cavity width and meet current moisture-risk requirements.

Thick 115mm insulation is a practical choice that delivers consistent thermal resistance within masonry cavity wall build-ups.

Types of 115mm Insulation

Because 115mm is a cavity-matching thickness, few materials are supplied at this depth:

PIR (Polyisocyanurate)

PIR is the most common at 115mm thickness.

• PIR’s closed cells effectively trap heat.
• Better thermal performance than many alternatives of similar thickness.
• Ideal for wall cavities and retrofit jobs.
• Achieves about 0.022 W/m.K in thermal conductivity.

Phenolic Foam

Phenolic foam panels are equally rigid and have a closed-cell structure.

• Often better at stopping heat flow than PIR.
• Works well in cavity walls where space is limited.
• Phenolic boards – for example, 115mm Kingspan Kooltherm K106 – achieve 0.019 W/m.K.

Phenolic boards can be marginally better when you want maximum performance in minimal thickness.

Other Materials

Most 115mm insulation thickness sold in the UK is either PIR or phenolic. However, insulation can also be made from:

Polystyrene foams (EPS or XPS) – EPS / XPS is widely used in floors and foundations at other thicknesses, but 115mm variants are uncommon and not typically specified for cavity wall systems.

Natural boards like wood-fibre or cork exist, but are uncommon at 115mm in mainstream cavity wall designs.

FAQ

Where to buy 115mm thick insulation?

You can buy 115mm thick insulation online from Insulation Wholesale. We stock a wide range of 115mm PIR and phenolic cavity boards. Our team can help you pick the right board for the job. We offer competitive pricing and deliver right to your site.

How to fix 115mm insulation?

1. Measure the cavity and select boards that fit the specified residual cavity (typically around 10mm) as required by Approved Document L and manufacturer guidance.
2. Cut to size using a straight edge and a sharp board knife.
3. Push the board into place between the inner and outer leaves of the cavity wall.
4. Use fixings or wall ties as required by the construction method. Cavity boards are often held in place by wall ties.
5. Seal all gaps and check alignment to prevent thermal bridges (cold spots).

How good is 115mm insulation?

115mm insulation boards deliver strong thermal resistance without being bulky. Product thermal conductivities (lambda values) are typically:

• 0.019 W/mK for high-end phenolic boards
• 0.022 W/mK or so for PIR boards.

Lower values mean less heat escaping and high thermal resistance from a manageable board depth. 115mm insulation can help you meet the target thermal resistance in external walls if properly installed.

How to cut 115mm insulation?

• For straight cuts – Use a long straightedge and a sharp blade or board knife.
• For curved shapes – Mark the cut line and use a fine-toothed saw or handsaw.
• When cutting foil-faced boards, score the foil carefully to prevent excessive ripping.

Cutting generates dust and small particles, so use appropriate personal protective equipment (PPE) to protect the lungs, eyes, and hands.

Planning a larger order of 115mm insulation? Get a fast quote from our team and price up exactly what you need for your project. We will help you secure the right boards in the right quantities, with clear pricing and quick delivery in the UK.