25mm Insulation Board

25mm Insulation Board Types and Applications

A 25mm insulation board is one of the most versatile and widely used thicknesses in UK construction. At 25mm, these boards are the preferred choice wherever build-up depth is severely restricted, where a second layer of insulation is being added over existing material, or where a thin thermal break is required to eliminate cold bridging. At Insulation UK, we stock 25mm insulation boards from all the leading manufacturers, with trade prices and fast UK-wide delivery on every order.

25mm PIR Insulation Boards

25mm PIR insulation board (polyisocyanurate) is the most popular choice at this thickness, delivering a thermal conductivity of 0.022 W/mK. Most 25mm PIR boards are foil-faced, lightweight, easy to cut, and suitable for roofs, walls, floors, and loft applications.

At 25mm, PIR boards are particularly effective as a thermal break layer in timber and steel framed walls, as an additional layer over existing insulation to uplift thermal performance, and in underfloor heating build-ups where minimal thickness beneath the screed or floor finish is essential.

Popular choices include Tekwarm, Celotex, Kingspan Therma, Recticel Eurothane GP, Ecotherm Eco-Versal, and Unilin.

25mm Phenolic Insulation Boards

For applications where every millimetre counts, 25mm phenolic insulation board 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 at the same 25mm thickness and making phenolic boards the preferred choice for passive house projects, heritage buildings, and any application where the absolute thinnest insulation solution is required without compromise on performance.

25mm Polystyrene Insulation Boards

25mm polystyrene insulation board is available in expanded polystyrene (EPS) form and is a cost-effective option for applications where high thermal performance is not the primary requirement. 25mm EPS boards are well suited to external wall insulation base layers and below-ground applications. For those looking for cheap 25mm insulation board options without compromising on reliability, expanded polystyrene insulation board is one of the most competitively priced choices available.

Where is a 25mm Insulation Board 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 an additional layer over existing insulation to increase thermal performance in roofs, walls, and floors without significant disruption to the existing build-up.

Underfloor Heating

In underfloor heating systems, 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 this thickness 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 Boards?

Competitive Trade Pricing Including some of the cheapest 25mm insulation board prices available online, with no minimum order value

All the Leading Brands Tekwarm, Celotex, Kingspan, Recticel, Ecotherm, and Unilin, all stocked at 25mm and available at trade prices

Fast UK-Wide Delivery Reliable delivery across the UK with free shipping on orders over £500 (ex VAT)

Half Boards Available 25mm insulation boards available as half boards for smaller areas and last-minute orders

Expert Technical Support Our team has over 50 years of experience in the insulation industry. Whether you need help selecting the right board for your project or guidance on achieving a specific U-value, get in touch and we will point you in the right direction.