Celotex U-Value Comparisons: Which Board Meets Your Specification?

Why U-Values Matter When Specifying Celotex

U-value is the single most important number in any insulation specification. It tells you how much heat escapes through a given structure per square metre, per degree of temperature difference. The lower the U-value, the better the thermal performance — and the lower your energy bills.

UK Building Regulations (Approved Document L) set maximum U-values for different building elements. Choosing the wrong Celotex product or thickness can mean failing an inspection, retrofitting at cost, or simply leaving money on the table.

UK Building Regulations U-Value Targets (2026)

Building Element New Build Max Extension / Renovation Max
Pitched roof (between rafters) 0.15 W/m²K 0.18 W/m²K
Flat roof 0.15 W/m²K 0.18 W/m²K
External wall 0.18 W/m²K 0.30 W/m²K
Ground floor 0.13 W/m²K 0.25 W/m²K
Internal wall (existing solid) N/A 0.30 W/m²K

Always verify against the latest Approved Document L for your project type. Targets can vary for extensions depending on floor area ratio.

The Celotex Range: Products and Lambda Values

All Celotex boards use PIR foam, but different facings, densities, and applications affect both lambda value and suitability. Here’s a summary of the core range:

Product Lambda Value Primary Application Key Feature
GA4000 0.022 W/mK General purpose (roofs, floors, walls) Low lambda, versatile, double foil-faced
FR5000 0.022 W/mK Flat roofs (above deck) High compressive strength for foot traffic
TB4000 0.022 W/mK Timber frame walls Sized for stud bays, vapour-check facing
RS5000 0.022 W/mK Pitched roofs (between rafters) Rafter depth optimised, taped joints
PL4000 0.022 W/mK Internal walls (dot-and-dab) Factory-laminated 12.5mm plasterboard
EL3000 0.023 W/mK Floors (below screed) High compressive strength, load-bearing
TD4000 0.022 W/mK Warm flat roofs (tapered) Tapered cut for falls, bespoke to project

U-Value Performance by Thickness: GA4000 (Most Popular)

The GA4000 is Celotex’s best-selling general-purpose board. The table below shows the approximate U-value contribution of the board in isolation (i.e., ignoring other construction layers). Real-world U-values will be lower (better) once you factor in plasterboard, masonry, decking, etc.

GA4000 Thickness U-Value (Board Only) Typical Combined U-Value*
50mm 0.44 W/m²K ~0.35–0.40 W/m²K
75mm 0.29 W/m²K ~0.22–0.26 W/m²K
100mm 0.22 W/m²K ~0.17–0.20 W/m²K
120mm 0.18 W/m²K ~0.15–0.17 W/m²K
150mm 0.15 W/m²K ~0.12–0.14 W/m²K

*Combined U-values are indicative only and depend on construction type. Always use a full elemental U-value calculation for specification purposes.

Celotex vs Mineral Wool: U-Value Comparison

PIR boards like Celotex outperform mineral wool (Rockwool, Knauf) by roughly 2:1 on thermal conductivity. This means you can achieve the same U-value target with approximately half the thickness — critical where space is limited.

Insulation Type Lambda Value Thickness Needed for 0.18 W/m²K*
Celotex GA4000 (PIR) 0.022 W/mK ~120mm
Knauf Earthwool (mineral wool) 0.044 W/mK ~240mm
Rockwool RWA45 (mineral wool) 0.045 W/mK ~250mm

*Board in isolation. Actual project thickness will vary depending on full build-up and thermal bridging calculations.

The trade-off: mineral wool is typically cheaper per m2 and performs better acoustically and in fire scenarios. PIR wins on space efficiency and moisture resistance.

How to Choose the Right Celotex Product for Your U-Value Target

  1. Identify your building element — roof, wall, or floor.
  2. Find your regulatory U-value target from Approved Document L (new build vs. renovation).
  3. Select the appropriate Celotex product for your application using the table above.
  4. Run a U-value calculation using all layers of your construction — not just the insulation board. Free calculators are available from Celotex and NHBC.
  5. Verify with your building control officer before ordering in bulk.

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