25mm Insulation Board

Why 25mm Insulation?

25mm insulation is one of the most specified rigid board thicknesses in the UK wherever build-up depth is tight. At this thickness, boards earn their place as a thermal break in framed walls, a top-up layer over existing insulation, or a slim solution beneath underfloor heating systems where every millimetre below the finished floor level counts. Here is what the range covers:

• PIR boards at 25mm achieve 0.022 W/mK thermal conductivity in one of the thinnest practical board formats.
• Phenolic boards at 25mm outperform PIR with a lambda as low as 0.019 W/mK for the tightest U-value targets.
• EPS options available from 0.030 to 0.038 W/mK for cost-effective floor and below-ground applications.
• Purpose-built thermal bridging strips such as the Celotex TB4000 range, designed specifically for stud and frame junctions.
• Available as half boards for smaller areas, awkward cuts and last-minute top-ups.

25mm PIR Insulation Boards

PIR is the most commonly specified material for 25mm rigid insulation in the UK. Foil-faced, lightweight and easy to cut, 25mm PIR boards are suitable for thermal break applications in timber and steel framed walls, as a top-up layer over existing insulation and beneath underfloor heating systems where minimal thickness is essential. All boards achieve a thermal conductivity of 0.022 W/mK and are non-water absorbent, meaning performance is maintained across damp or humid environments.

Celotex TB4025

The Celotex TB4025 is one of the most frequently ordered 25mm insulation boards in the UK. Purpose-built for thermal bridging applications, it is fitted between studs and the internal lining board in timber and steel framed walls to reduce heat loss at junctions. BBA certified, lightweight and simple to install, it is the standard specification wherever a slim thermal break is required. Browse the full Celotex insulation range for all board types and thicknesses.

Ecotherm and Unilin

Ecotherm Eco-Versal and Unilin PIR complete the 25mm PIR range, sharing the same 0.022 W/mK thermal conductivity and foil-faced construction as the Celotex equivalent. Both are practical alternatives where price or availability is a consideration and both suit the same thermal break, top-up layer and underfloor heating applications. Browse all PIR insulation boards to compare brands and specifications.

25mm Phenolic Insulation Boards

For applications where every millimetre counts, 25mm phenolic insulation offers exceptional thermal performance over PIR at the same thickness. Kingspan Kooltherm K103 achieves a thermal conductivity of 0.019 W/mK, outperforming standard PIR and making it the preferred choice for passive house projects, heritage buildings and any application where the absolute thinnest insulation solution is required without compromise on performance. Browse the full Kingspan Kooltherm range for all Kooltherm board types and thicknesses.

25mm EPS Insulation Boards

Expanded polystyrene is one of the most competitively priced options for 25mm floor insulation where the very highest thermal efficiency is not the primary concern. Jablite Jabfloor EPS100 achieves a thermal conductivity of 0.036 W/mK and is BBA certified for office and school floor applications, while the graphite-enhanced Jablite HP+ improves on standard EPS with a lambda of 0.030 W/mK in the same footprint. Higher compressive strength grades are available for heavier commercial and industrial floor applications. Browse the full 25mm polystyrene insulation range to compare all available grades.

Where Is 25mm Insulation Used?

Thermal Bridging

These boards are widely used to prevent thermal bridging in timber and steel framed wall constructions, fitted between the frame and the internal lining board to reduce heat loss at junctions. The Celotex TB4000 range is specifically designed for this application.

Additional Layer

At just 25mm, PIR boards are a practical choice as a top-up layer over existing insulation to increase thermal performance in roofs, walls and floors without significant disruption to the existing build-up.

Underfloor Heating

A slim 25mm board is often installed beneath the heating pipes or electric mat to direct heat upwards and reduce downward losses. PIR is a popular choice for this application due to its slim profile and compatibility with most underfloor heating systems.

Roof and Loft Applications

PIR and phenolic boards at 25mm are well suited to between and below rafter applications in pitched roof and loft conversion projects where rafter depth limits how much insulation can be installed.

Why Choose Insulation UK for 25mm Insulation?

Competitive Trade Pricing: Competitively priced 25mm insulation board prices available online, with no minimum order value. Whether you need one board or a full pallet, you will get a fair price.

All the Leading Brands: Tekwarm, Celotex, Kingspan, Recticel, Ecotherm and Unilin, all stocked at 25mm and available at trade prices across PIR, phenolic and EPS.

Fast UK-Wide Delivery: Reliable delivery across the UK with free shipping on orders over £500 (ex VAT). Get the boards you need, when you need them.

Half Boards Available: 25mm insulation boards available as half boards for smaller areas, awkward spaces and last-minute top-ups, with express and next-day shipping options available.

Expert Technical Support: Our team has over 50 years of combined experience in the insulation industry and can help you choose the right board for your project.

Not sure which 25mm insulation board suits your project? Call us on 03003 034 578 or get in touch online and our team will help you find the right specification.