100mm Celotex GA4100 is a rigid polyisocyanurate (PIR) insulation board with a thermal conductivity of 0.022 W/mK, manufactured in 2400x1200mm sheets. This thickness achieves U-values between 0.17-0.22 W/m²K depending on the build-up, making it suitable for meeting Part L 2021 requirements across roofs, walls, and floors. Trade professionals specify 100mm PIR when projects require solid thermal performance without eating up too much space.

What is 100mm Celotex GA4100?

Celotex GA4100 is a general application PIR insulation board with aluminium foil facings on both sides. The GA4000 series means it works for most building applications, unlike specialist products like TB4000 (for thin applications) or CW4000 (cavity wall specific).

The board is made from rigid PIR foam bonded between reflective aluminium foil layers. Those foil facings do double duty – they provide vapour resistance and cut down radiant heat transfer, which improves overall thermal performance.

What you need to know about 100mm Celotex GA4100:

• Lambda value: 0.022 W/mK
• Board size: 2400mm x 1200mm
• Coverage: 2.88m² per board
• Compressive strength: 120 kPa minimum
• Fire classification: Class E (to BS EN 13501-1)
• Operating temperature: -50°C to +80°C

The 100mm thickness is one of the most commonly specified options for Part L compliance. It gives you proper thermal performance without requiring massive build-up depths that eat into ceiling heights or floor-to-ceiling space.

Technical Specifications and Performance Data

Here’s what matters when you’re assessing whether 100mm Celotex works for your project.

Thermal Performance

The declared lambda value of 0.022 W/mK is what you use for U-value calculations. This stays consistent across the operating temperature range, assuming you install properly without compression, gaps, or moisture getting in.

Physical Properties

• Thickness tolerance: ±2mm
• Density: 30-35 kg/m³
• Water vapour resistivity: 150 MNs/gm
• Dimensional stability: <2% (at 70°C, 90% RH)

Compressive Strength

At 120 kPa compressive strength, 100mm GA4100 handles most residential and light commercial floor applications. To put that in perspective, 120 kPa equals 12 tonnes per square metre. That’s plenty for standard floor loading – screed weight, floor finishes, furniture, foot traffic.

If you’re working on applications with heavier loading like commercial kitchens, plant rooms, or anywhere vehicles might access, you’ll want boards with enhanced compressive strength ratings of 150-200 kPa.

Fire Performance

The Class E fire rating meets Building Regulations for most applications when you use it as part of a complete build-up. That rating applies to the insulation board itself. The overall construction’s fire performance depends on everything in the assembly.

Worth remembering – cavities containing combustible insulation need cavity barriers at appropriate intervals per Approved Document B. Buildings over 18m have additional fire safety requirements for external wall constructions with combustible insulation.

U-Value Performance for 100mm Celotex

U-value performance depends on your complete build-up, not just the insulation thickness. Here’s what you can achieve with 100mm Celotex in typical constructions.

Pitched Roof Between Rafters

Flat Roof (Warm Roof Construction)

Solid Floor (Above Slab)

Cavity Wall (Partial Fill)

These numbers assume good workmanship – tight-fitting boards, properly sealed joints, decent edge detailing. Sloppy installation can bump U-values up by 0.02-0.05 W/m²K through thermal bridging and air gaps.

Part L Compliance with 100mm PIR Insulation

Part L 2021 sets maximum U-value targets for new builds and extensions. The 100mm thickness meets or exceeds most requirements in properly designed constructions.

Part L 2021 U-Value Requirements

• Roofs: 0.16 W/m²K maximum
• Walls: 0.26 W/m²K maximum
• Floors: 0.18 W/m²K maximum

Where 100mm Celotex Achieves Compliance

100mm PIR typically gets you there for:

Solid floors above ground bearing slabs – The 0.17-0.18 W/m²K you achieve with 100mm PIR beneath screed comfortably meets the 0.18 W/m²K floor requirement. This is probably the most straightforward application for 100mm thickness.

Cavity walls (partial fill) – 100mm PIR in a partial fill cavity construction hits 0.16-0.17 W/m²K, well below the 0.26 W/m²K wall requirement. You’ve got plenty of headroom here.

Warm flat roofs – 100mm PIR in a warm roof build-up achieves 0.18-0.20 W/m²K. While this technically meets basic compliance, many specifiers go with 120-150mm for flat roofs to hit the 0.16 W/m²K target more comfortably. Flat roofs take more weather exposure, so extra performance makes sense.

Pitched roofs between rafters – 100mm between rafters gets you 0.19-0.21 W/m²K. This meets the 0.16 W/m²K requirement in some constructions but might need additional insulation layers in others, depending on your specific build-up.

Where You Might Need More Thickness

Some applications benefit from thicker PIR to hit target U-values more reliably:

• Pitched roofs often use 100mm between rafters plus 50-75mm over rafters to achieve 0.16 W/m²K comfortably
• Flat roofs in Scotland (stricter requirements up there) typically need 120-150mm
• Retrofit applications with limited build-up space sometimes use higher-performance insulation like phenolic boards

Thermal Bridging Matters

Part L compliance requires you to account for thermal bridging through wall ties, lintels, floor edges, and other junctions. The calculated U-value has to include these bridging effects, which can add 0.02-0.04 W/m²K to your centre-of-panel U-value.

Use low-conductivity wall ties, insulated lintels, and proper edge detailing to keep thermal bridging impact down.

Application Guide: Roofs, Walls, and Floors

The 100mm thickness works across multiple applications. Installation approach varies for each.

Pitched Roof Applications

Between Rafters: Install 100mm PIR boards friction-fit between rafters. Make sure boards bear on the full rafter depth. Cut boards 5-10mm oversized to create a tight friction fit that stops them slumping over time. Seal joints with aluminium tape to prevent air infiltration.

If your rafter depth exceeds board thickness, support boards on battens fixed to rafter sides. Never compress boards to force them into shallow rafters – you’ll kill the thermal performance.

Above Rafters: For warm pitched roofs, install 100mm PIR boards above the structural rafters, beneath counter battens and roof covering. This approach cuts out thermal bridging through rafters completely and gives you better airtightness.

Lay boards with staggered joints, sealed with tape. Fix through boards into rafters using appropriate fixings that don’t compromise the vapour control layer beneath.

Flat Roof Applications

100mm Celotex works well in warm flat roof constructions where insulation sits above the structural deck:

1. Install vapour control layer over structural deck, lapping joints and sealing at penetrations
2. Lay 100mm PIR boards with staggered joints, starting from one edge
3. Tape all board joints with aluminium foil tape
4. Apply waterproofing layer (torch-on felt, single-ply membrane, or liquid-applied system)

For flat roofs needing falls, use tapered PIR systems rather than creating falls in the structure. Creating falls in the deck gives you varying insulation thicknesses and thermal weak points.

Wall Applications

Cavity Wall Partial Fill: Install 100mm PIR boards against the inner leaf blockwork, keeping a minimum 50mm residual cavity. Use cavity wall ties designed for partial fill insulation. Standard ties can create thermal bridges if you’re not careful.

Fit boards tightly with no gaps. Stagger vertical joints between courses. Protect boards from mortar droppings during outer leaf construction using cavity trays or other protective measures. Mortar on the boards creates thermal bridges and looks sloppy.

Internal Wall Insulation: Fix 100mm PIR boards to internal wall surfaces using mechanical fixings or adhesive dabs. Seal all joints with tape before you put up plasterboard finish on battens or direct-fixed depending on your system specification.

Remember you’re losing 100mm+ of internal space when planning internal insulation retrofits. Check the impact on room sizes, window reveals, and door positions before you start.

Floor Applications

Solid Floors: For new solid floors or floor renovations, install 100mm Celotex GA4100 above the damp-proof membrane, beneath the screed:

1. Lay damp-proof membrane over concrete slab, lapping at walls to tie into wall DPC
2. Lay 100mm PIR boards with staggered joints across the floor area
3. Tape all joints with aluminium foil tape
4. Install perimeter edge insulation strip (10-20mm compressible foam)
5. Lay minimum 65mm unbonded screed over insulation

The 120 kPa compressive strength handles standard residential floor loading. For underfloor heating installations, the same build-up works with heating pipes or electric mats embedded in the screed above insulation.

Suspended Timber Floors: For suspended floors in new builds, install 100mm PIR between floor joists, supported on timber battens fixed to joist sides. You need 150mm joist depth minimum to fit 100mm insulation plus supporting battens and airspace.

Installation Requirements for Trade Professionals

Proper installation determines whether your theoretical U-values actually happen in the real world.

Cutting and Fitting

Cut PIR boards using a fine-toothed saw or sharp knife. Score deeply with a knife, then snap the board over a straight edge for clean breaks. Don’t crush the board edges during cutting – crushed edges mean poor edge-to-edge contact and air gaps.

For friction-fit applications between rafters or joists, cut boards 5-10mm oversized. The compression creates a tight fit that prevents movement and air infiltration.

Joint Treatment

All board joints need sealing with aluminium foil tape to stop air movement through the insulation layer. Air infiltration can reduce effective R-value by 20-30% even with physically continuous insulation. That’s a massive performance hit.

Overlap tape by at least 50mm at tape joints. Press tape firmly onto both board surfaces for good adhesion. Clean any dust or moisture from board edges before taping – tape won’t stick to dirty or wet surfaces.

Fixing Methods

Fixing method depends on where you’re installing:

• Roofs above deck: Mechanical fixings through boards into deck, with appropriate fixings for the deck material
• Walls: Mechanical fixings, adhesive dabs, or cavity wall ties depending on system
• Floors: Usually loose-laid with screed weight providing restraint
• Between rafters/joists: Friction fit with no additional fixings needed

Follow fixing manufacturer guidance on fixing density and placement. You need wind uplift resistance without creating excessive thermal bridging through too many fixings.

Vapour Control

The aluminium foil facings give you some vapour resistance (approximately 200 MNs/gm), but this might not be enough for all applications. For warm roofs, walls, and floors, install a separate vapour control layer on the warm side of the insulation where dewpoint calculations show risk of interstitial condensation.

Always position any additional vapour control layer on the warm side of the insulation. This prevents moisture migration into the insulation layer.

Working in Cold Weather

You can install PIR boards at temperatures down to -5°C. Below that, adhesives and tapes might not bond properly. Store boards flat and protected from weather before installation. Wet boards need to dry before installation – moisture on facings stops effective tape sealing.

Board Dimensions and Coverage Calculations

Planning material quantities means understanding board dimensions and calculating coverage with appropriate wastage allowances.

Standard Board Dimensions

• Length: 2400mm
• Width: 1200mm
• Coverage per board: 2.88m²
• Weight per board (100mm): approximately 9-10kg

Coverage Calculations

To calculate boards needed for a project:

1. Measure the total area requiring insulation
2. Divide by 2.88m² per board
3. Add 5-10% wastage depending on area shape and complexity

Example calculation for a 50m² floor:

50m² ÷ 2.88m² per board = 17.36 boards 17.36 × 1.05 (5% wastage) = 18.23 boards Order 19 boards

Wastage Factors by Application

• Rectangular floors with minimal cuts: 5% wastage
• Pitched roofs between rafters: 10% wastage
• Complex shapes with numerous penetrations: 15% wastage
• Cut and fit between irregular studs: 20% wastage

Cutting Patterns

Minimise waste by planning cutting patterns before you start. For areas requiring board sections (like between rafters), measure accurately and cut all pieces before installation rather than cutting one at a time. This lets you use offcuts from one cut elsewhere.

For floors, start laying from the longest straight wall and work towards the more complex edge. This minimises difficult cuts.

Ordering 100mm Celotex GA4100

When ordering 100mm Celotex for projects, several factors affect quantity and delivery requirements.

Ordering Quantities

Celotex GA4100 typically comes in packs of 4 boards (11.52m² per pack) or 6 boards (17.28m² per pack) depending on supplier. Some merchants sell individual boards for small projects, though per-board pricing is higher than pack pricing.

For large projects, full pallet quantities give you the best pricing. A standard pallet typically holds 20-24 boards depending on configuration.

Lead Times

Stock items usually ship within 1-3 working days. For large quantities or during busy periods (spring/summer building season), allow longer lead times. Confirm stock availability before you commit to project start dates. Nothing worse than crews standing around waiting for materials.

Delivery Considerations

PIR board deliveries need suitable vehicle access and unloading facilities. Boards measuring 2400x1200mm need careful handling to prevent damage. Arrange delivery to coincide with storage availability or immediate installation – site storage of large quantities can be challenging.

Most suppliers offer kerbside delivery as standard. For restricted access sites, confirm delivery vehicle size and discuss any access limitations when ordering.

Storage Requirements

Store boards flat and protected from weather. Stack maximum 10 boards high to prevent bottom boards compressing. Protect from direct sunlight – UV exposure can degrade foil facings over extended periods.

If temporary site storage is necessary, cover stacks with tarpaulins or store in containers. Moisture ingress between boards can cause facer delamination even though the PIR foam itself resists water absorption.

Price Factors

100mm Celotex pricing varies by:

• Order quantity (bulk pricing reduces per-board cost)
• Supplier and regional availability
• Time of year (prices often increase during busy building season)
• Fuel surcharges and delivery distance
• Collection vs delivery

Quality Checks on Delivery

Inspect boards on delivery before signing acceptance:

• Check for physical damage (crushed corners, broken boards)
• Verify correct thickness (100mm, not 90mm or 120mm)
• Confirm board count matches delivery note
• Check for moisture damage or facer delamination

Report any issues immediately to the supplier. Once you sign for delivery, returning damaged boards becomes more difficult.Need reliable PIR insulation that meets Part L requirements? Online Insulation stocks a comprehensive range of insulation products including 100mm Celotex boards, with fast delivery across the UK for trade professionals.