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Celotex Board Thickness Guide: How to Choose the Right Insulation Depth
Why Board Thickness Matters
Celotex PIR (polyisocyanurate) insulation boards deliver high thermal performance in a slim profile — but choosing the wrong thickness can mean a failed Building Regulations inspection or wasted spend. This guide helps you match the right board depth to your project type and U-value target.
Understanding U-Values
A U-value measures how quickly heat passes through a building element (walls, roofs, floors). The lower the U-value, the better the insulation. UK Building Regulations set maximum U-values for new builds and extensions:
- Pitched roofs (between rafters): 0.18 W/m²K
- Flat roofs: 0.18 W/m²K
- External walls: 0.30 W/m²K (0.18 for new builds)
- Ground floors: 0.25 W/m²K (0.15 for new builds)
Celotex PIR has a thermal conductivity (lambda value) of typically 0.022–0.023 W/mK, one of the best available for rigid board insulation.
Celotex Thickness by Application
Pitched Roof — Between Rafters (Cold Loft / Warm Roof)
For a warm roof with 150mm rafters, a 100mm Celotex GA4000 board between the rafters combined with a 40mm board below (counter-batten layer) is a common approach to achieve 0.18 W/m²K. Thicker rafters allow full-fill solutions with a single layer of 150mm board.
- Shallow rafters (100mm): Use 70mm + 25mm counter-batten layer
- Deep rafters (175mm+): Single 150mm board may suffice
Flat Roof (Above Deck)
Flat roofs typically require 120mm–150mm of PIR above the deck to hit 0.18 W/m²K, depending on the deck construction. Celotex FR5000 and GA4000 are both suitable for above-deck applications.
Solid or Suspended Timber Floors
For ground floors, 70mm–100mm of Celotex GA4000 between joists, or a continuous layer of 50mm–75mm below a screed, is typically required to achieve 0.25–0.15 W/m²K targets depending on floor dimensions.
Internal Wall Insulation (Dot-and-Dab)
Slim-profile applications on solid walls typically use 37.5mm or 52.5mm Celotex PL4000 plasterboard-faced boards. These are factory-laminated with 12.5mm plasterboard for a finished wall ready for decoration.
Quick Thickness Reference Table
| Application | Target U-Value | Typical Celotex Thickness | Product |
|---|---|---|---|
| Pitched roof (between rafters) | 0.18 W/m²K | 100mm + 40mm | GA4000 |
| Flat roof (above deck) | 0.18 W/m²K | 120mm–150mm | GA4000 / FR5000 |
| Ground floor (between joists) | 0.25 W/m²K | 70mm–100mm | GA4000 |
| Solid wall (internal) | 0.30 W/m²K | 37.5mm–52.5mm | PL4000 |
| Timber frame wall (cavity) | 0.18 W/m²K | 90mm–140mm | GA4000 |
How to Calculate the Thickness You Need
Use this simplified formula as a starting point:
Required thickness (m) = Lambda value ÷ Target U-value
For Celotex GA4000 (lambda = 0.022 W/mK) targeting 0.18 W/m²K in isolation:
0.022 ÷ 0.18 = 0.122m (122mm)
In practice, existing construction layers contribute to the overall U-value, so you may need less. Always use a full U-value calculator or consult Celotex's technical support for a precise specification.
Common Mistakes to Avoid
- Mixing lambda values: Not all PIR boards have identical lambda values. Always check the product datasheet — GA4000 is 0.022 W/mK, while some boards are 0.023 W/mK.
- Ignoring thermal bridging: Rafters, joists, and studs create cold bridges that reduce the effective U-value. Account for these in your calculation.
- Under-ordering: Always add 5–10% for cutting waste, especially on roofs with hips, valleys, or rooflights.
- Skipping vapour control: PIR boards are vapour-resistant but not vapour-proof. Confirm whether a separate VCL is required for your build-up.
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