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:
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 insulation) – 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.
At Insulation Wholesale, you can purchase 115mm Insulation at low wholesale prices with fast delivery for most of items within 2-5 days. Competitive rates guaranteed.
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?
Measure the cavity and select boards that fit the specified residual cavity (typically around 10mm) as required by Approved Document L and manufacturer guidance.
Cut to size using a straight edge and a sharp board knife.
Push the board into place between the inner and outer leaves of the cavity wall.
Use fixings or wall ties as required by the construction method. Cavity boards are often held in place by wall ties.
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.
115mm Celotex Thermaclass Cavity Wall 21 is a PIR T&G insulation board designed for full-fill cavity wall applications. Low-emissivity foil facing provides versatility during installation, allowing for a 10mm residual cavity.
£137.98£165.58Ex VATInc VAT£51.68£62.02Per M2Add to cart
115mm Kingspan Kooltherm K106 Cavity Board is a premium performance insulation with a fibre-free rigid thermoset phenolic core with a thermal conductivity of just 0.019 W/mK across all thicknesses.
£113.03£135.64Ex VATInc VAT£52.33£62.80Per M2Add to cart
115mm Recticel Eurowall Full Fill boards snap together with interlocking edges, forming a continuous thermal barrier that optimises performance of cavity wall insulation.
£116.48£139.78Ex VATInc VAT£26.41£31.69Per M2Add to cart
Eco-Cavity Full Fill is suitable for full fill cavity wall applications while maintaining a residual 10 mm cavity. The residual cavity helps resist moisture transfer, aids installation, and accommodates mortar squeeze. The tongue and groove edges to ensure a continuous layer of insulation, increasing protection from wind driven rain and aiding installation.
