Maximising Energy Efficiency: Why 100mm Celotex Insulation is a Game-Changer for UK Homes

Posted on October 8, 2025 by EcoBuild Insights

As the UK continues its push towards net-zero emissions, homeowners are increasingly turning to high-performance insulation to slash energy bills and meet stricter building regulations. With rising energy costs and a chilly autumn already upon us, upgrading your home's thermal envelope has never been more urgent. Enter 100mm Celotex, a versatile PIR (polyisocyanurate) insulation board that's become a staple in renovations and new builds alike. In this post, we'll explore why this thickness strikes the perfect balance for UK properties, from cosy lofts to draughty walls, and how it can transform your home's efficiency.

What is 100mm Celotex and Why Choose It?

Celotex, a trusted name under the Saint-Gobain banner since 1925, produces GA4100 – a rigid, foil-faced PIR board designed for multi-purpose use. At 100mm thick, it's substantial enough to deliver robust thermal resistance without the bulk of thicker alternatives, making it ideal for space-conscious applications.

Key specs that make it stand out:

• Dimensions: 2400mm x 1200mm sheets, covering 2.88m² per board – easy to handle and transport.
• Thermal Conductivity: An impressive 0.022 W/m.K, meaning it conducts heat exceptionally poorly (a good thing!).
• R-Value: 4.5 m²K/W, providing strong insulation in a slim profile.
• Certifications: BBA-approved (Cert no. 17/5405) and UKCA marked to BS EN 13165, ensuring compliance with UK building regs.

Unlike fluffy mineral wool rolls, Celotex boards are rigid and low-emissivity foil-faced, which reflects heat back into your home and minimises thermal bridging – those sneaky heat-loss spots around joists and studs. Plus, it's eco-friendly: zero ozone-depleting potential and low global warming impact, aligning with BRE Global Green Guide standards.

Top Applications for 100mm Celotex in UK Builds

This board's versatility is its superpower. Whether you're a DIY enthusiast insulating a loft or a builder retrofitting a Victorian terrace, 100mm Celotex fits seamlessly into various scenarios.

1. Pitched and Flat Roofs

In lofts or under rafters, 100mm Celotex helps achieve U-values as low as 0.13 W/m²K when combined with other layers – well below the 0.18 target for new builds. It's perfect for pitched roofs where space is at a premium, and the foil facing enhances performance in ventilated cavities.

2. Walls: Internal and External

For cavity walls or internal dry-lining, it reduces cold bridging without eating into room space. In timber- or steel-framed systems, it's a go-to for meeting Part L regs. Homeowners love it for quick retrofits – just cut to size and secure with adhesive or fixings.

3. Floors: Solid and Suspended

Laying it underfloor in suspended setups or over concrete slabs? At 100mm, it's equivalent to about 200mm of mineral wool in thermal performance, ideal for ground floors. Reddit DIYers rave about its ease: lift boards, slot in, relay – done in a day, no loft legs needed for basic jobs.

4. Bonus: Overcoming Thermal Bridging

Its precision-cut edges and low-emissivity properties make it ace at sealing gaps, boosting overall efficiency by up to 20% in mixed builds.

Benefits That'll Warm Your Heart (and Wallet)

Investing in 100mm Celotex isn't just about ticking boxes – it's about real, tangible wins:

• Energy Savings: Expect 15-25% reductions in heating bills, potentially paying for itself in 3-5 years.
• Comfort Boost: No more chilly draughts; even temperatures year-round.
• Durability: Rigid and moisture-resistant, it lasts decades without sagging.
• Fire Safety: Euroclass F rating – safe for most domestic uses (check specifics for your project).
• Ease of Install: Lightweight (about 10kg per board), cuts with a fine-tooth saw or knife, and no PPE dramas beyond gloves and a mask.

Environmentally, it's a winner too – helping UK homes cut CO₂ emissions in line with the 2050 net-zero goal.

Installation Tips: DIY-Friendly Guide

Fancy tackling it yourself? Here's a quick rundown (always consult a pro for structural work):

1. Prep: Measure twice, ensure surfaces are clean and dry. Wear gloves to avoid foil scratches.
2. Cut: Mark with a straight edge, score with a utility knife, and snap – or saw for thicker bits.
3. Fix: Use expanding foam or dabs for walls; mechanical fixings for roofs/floors. Butt joints tightly and tape seams with foil tape.
4. Seal: The low-e foil does the heavy lifting, but vapour barriers may be needed in damp-prone areas.
5. Pro Tip: For floors, layer over a DPM and top with ply for a sturdy finish.

Tools needed: Tape measure, knife/saw, straight edge, and insulation tape. Total time for a 20m² room? Under four hours.

Wrapping Up: Insulate Smarter, Not Harder

In a UK climate where winters bite and summers surprise, 100mm Celotex GA4100 is your efficient, no-fuss ally for warmer, greener living. Whether retrofitting an older home or future-proofing a new one, its high R-value and adaptability make it a smart choice. Ready to beat the bills? Source boards from trusted suppliers like Insulation Superstore or Travis Perkins – and check for bulk deals (some offer £25 off orders over £500).

Got a project in mind? Drop a comment below – have you used Celotex before? Share your tips!

EcoBuild Insights is committed to sustainable building advice. All specs based on manufacturer data; always verify with current regs.