Celotex 25mm Insulation Board TB4025 Review: A Reliable Choice for UK Builders and DIYers

Posted on October 9, 2025 by Insulation Warehouse Direct Team

If you're tackling a renovation, new build, or even a simple loft upgrade, choosing the right insulation is key to keeping your home warm, energy bills down, and your project compliant with UK building regs. At Insulation Warehouse Direct, we've stocked and recommended countless insulation products over the years, and the Celotex 25mm PIR Insulation Board (TB4025) consistently stands out as a go-to option. In this review, we'll break down what makes this board from the trusted Saint-Gobain brand tick – from its specs and performance to real-world applications and why it's a smart buy for your next job.

What is Celotex TB4025 25mm Insulation Board?

Celotex TB4025 is a high-performance polyisocyanurate (PIR) rigid insulation board designed to combat thermal bridging – those pesky cold spots in walls, floors, and roofs that lead to heat loss and higher energy costs. Measuring 2400mm x 1200mm x 25mm, each board covers 2.88m², making it easy to transport and handle on-site. It's double-sided with low-emissivity aluminium foil facings, which not only boost thermal efficiency but also add a layer of moisture resistance.

This board is part of the versatile TB4000 range, ideal for multi-purpose use in pitched roofs, flat roofs, solid floors, cavity walls, and even timber or steel-framed systems. It's BBA-certified (certificate 95/3197) and carries a BRE Green Guide Rating of A, so you can rest assured it meets stringent UK standards for sustainability and performance.

Key Specifications and Features

Here's a quick rundown of the essentials:

What stands out? That ultra-low thermal conductivity means you get top-tier insulation without needing bulky thicknesses – perfect for tight spaces like stud walls or rafter gaps. The foil facings enhance performance in cavity air spaces, potentially lowering your U-values and helping you hit energy targets faster.

Performance in Action: How Does It Hold Up?

In our experience (and backed by customer feedback), Celotex TB4025 delivers where it counts. Its lightweight yet rigid construction makes it a breeze to cut and fit – use a fine-tooth saw for clean edges or a utility knife for scores and snaps. We've seen it used successfully in everything from loft conversions to shed upgrades, where it noticeably reduces drafts and condensation.

On thermal performance, the 0.022 W/mK rating translates to real savings. For instance, in a typical cavity wall setup, a single layer can shave significant heat loss, contributing to lower EPC ratings and happier homeowners. Users on trade forums rave about its durability: "Used this to reduce thermal bridging around columns and worked a charm! Very good price too!" And in van conversions – a popular off-label use – it's praised for keeping things "toastie" without moisture or rust issues.

That said, it's not invincible. While the foil offers some protection, it's best paired with a proper vapour barrier in high-humidity areas. And for underfloor heating, we'd steer you toward alternatives like Recticel Eurothane G, as TB4025 isn't certified for that.

Pros and Cons: The Honest Breakdown

Pros:

• Versatile and Multi-Purpose: Fits floors, walls, roofs – no need for multiple products.
• Energy-Efficient: Low thermal conductivity and foil facings mean better U-values in less space.
• Eco-Friendly: Zero ozone depletion and low global warming potential align with green building goals.
• Easy Installation: Lightweight (about 4.5kg per board) and simple to cut/fix mechanically.
• Trusted Brand: Saint-Gobain backing ensures quality and compliance.

Cons:

• Thickness Limitation: At 25mm, it's great for bridging but may need layering for full insulation in larger projects.
• Not for High-Rise External Walls: Building regs restrict it above 11m for fire safety.
• Foil Facing Quirks: Can't glue directly to plaster; mechanical fixings recommended.

Overall, the pros far outweigh the cons for most domestic and light commercial jobs.

Why Choose Celotex 25mm from Insulation Warehouse Direct?

At Insulation Warehouse Direct, we stock Celotex TB4025 because it's reliable, readily available, and priced competitively – often with bulk discounts for trade customers. Whether you're insulating a new extension or upgrading an older property, this board helps you achieve compliance without the hassle. Plus, our nationwide delivery means you get it fast, direct to site.

Rating: 4.8/5 – A solid performer that's earned its spot in our top sellers. If thermal bridging is your nemesis, grab a pack and feel the difference.

Ready to insulate smarter? Shop Celotex TB4025 25mm now or drop us a line for expert advice. What's your next project? Share in the comments below!

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