PIR (polyisocyanurate) rigid insulation boards are the go-to material when space is limited but thermal performance can’t be compromised. They deliver roughly twice the insulating value per millimetre compared to mineral wool, which is why they dominate applications like flat roofs, slimline wall build-ups, and floor-slab insulation where every millimetre counts.
Kingspan and Celotex are the two dominant brands in the UK, and while there are differences between specific product ranges, the competition between them drives pricing and means the technical specifications are closely matched. This guide covers sizes, performance data, and current pricing across the most common applications.
What Is PIR Insulation?
PIR is a closed-cell rigid foam, manufactured by reacting polyols and isocyanates. The closed-cell structure traps gas within the foam matrix, giving it a thermal conductivity (λ) of 0.022–0.028 W/mK — significantly better than the 0.032–0.044 W/mK typical of mineral wool. The boards are faced on both sides with a foil or glass-tissue facing that acts as a vapour control layer and gives the product its characteristic silver or cream appearance.
PIR is not the same as EPS (expanded polystyrene) or XPS (extruded polystyrene). EPS/XPS are cheaper but have higher λ values (0.030–0.040 W/mK) and lower compressive strength. PIR is the right choice where thin profiles and high performance are needed; EPS/XPS suit applications where bulk insulation is acceptable and cost is the priority.
Standard Sizes and Available Thicknesses
The standard sheet format for both Kingspan and Celotex is 1200 × 2400 mm, occasionally available in 1200 × 1200 mm half-sheets. Thicknesses available vary by product range:
| Thickness | Common applications |
|---|---|
| 25 mm | Below screed (supplementary layer), sarking |
| 40 mm | Floor (lower performance), cavity closer |
| 50 mm | Floor insulation, wall sarking, secondary layer |
| 60 mm | Flat roof decks, floor slab over ground |
| 70 mm | Floor, warm roof (supplementary) |
| 75 mm | Rafter insulation (between+below), floor |
| 80 mm | Warm flat roof (main layer) |
| 90 mm | High-performance floor/roof build-up |
| 100 mm | Full warm roof, rafter insulation |
| 120 mm | High-performance warm roof |
| 150 mm | Maximum standard stock depth |
Not all thicknesses are stocked at all merchants — 25, 50, 75, and 100 mm are the most reliably available. Intermediate sizes often need ordering.
Kingspan vs Celotex: Performance Comparison
Both brands offer multiple product ranges targeting different applications. The key differentiator is λ value and compressive strength:
| Product | Brand | λ (W/mK) | Compressive strength | Primary use |
|---|---|---|---|---|
| Kooltherm K7 | Kingspan | 0.020 | Low | Pitched roof (between rafter) |
| Kooltherm K12 | Kingspan | 0.021 | Low | Framed partition (wall) |
| Kooltherm K103 | Kingspan | 0.022 | 70 kPa | Below screed flooring |
| Kooltherm K5 | Kingspan | 0.023 | Low | Flat roof (warm) |
| Therma TW57 | Kingspan | 0.023 | Low | External wall cavity |
| GA4000 | Celotex | 0.022 | 70 kPa | General purpose floors/roofs |
| TB4000 | Celotex | 0.022 | 70 kPa | Inverted and warm roof |
| RS5000 | Celotex | 0.022 | 70 kPa | Pitched roof sarking |
| PL4000 | Celotex | 0.022 | 35 kPa | Below screed (light duty) |
The Kooltherm phenolic-core products (K7, K12) achieve λ = 0.020–0.021 W/mK by using a phenolic foam core rather than pure PIR. This is meaningfully better in thin applications — at 50 mm, you gain approximately the same performance as 60 mm of standard PIR.
Cost per m² by Thickness (2026 UK Merchant Prices)
Prices below are for standard PIR boards (Celotex GA4000 or Kingspan equivalent) at typical merchant rates. Premium phenolic products (Kooltherm) are 20–35% higher.
| Thickness | Cost per m² (supply only) |
|---|---|
| 25 mm | £8 – £12 |
| 40 mm | £10 – £15 |
| 50 mm | £12 – £16 |
| 60 mm | £13 – £18 |
| 75 mm | £15 – £20 |
| 100 mm | £18 – £24 |
| 120 mm | £22 – £28 |
| 150 mm | £26 – £34 |
A standard 1200 × 2400 mm board at 100 mm costs approximately £52–£70 per sheet (2.88 m² per sheet). Prices from builders’ merchants are typically 10–20% lower than DIY store prices; trade accounts offer further discount on larger orders.
Application-Specific Guidance
Warm Flat Roof
The most demanding application. The insulation sits above the structural deck (typically plywood or concrete) and below the waterproofing membrane. It must be dimensionally stable, have adequate compressive strength (≥70 kPa to support traffic during installation), and provide a consistent substrate for the membrane.
Minimum thickness to comply with Building Regs Part L (U ≤ 0.18 W/m²K) using 100 mm PIR (λ = 0.022): U ≈ 0.21 W/m²K — not quite enough on its own. You need 120 mm to achieve 0.18, or 150 mm to achieve 0.14. In practice, most flat roof refurbs spec 120–150 mm as the single main layer.
Pitched Roof (Between Rafter)
Between-rafter insulation must leave a 50 mm clear ventilation gap above the insulation (on a cold roof) or use a breathable membrane with reduced gap. For a 175 mm rafter depth with 50 mm ventilation gap, you have 125 mm to insulate — 100 mm PIR (λ = 0.022) + 25 mm below-rafter layer achieves approximately U = 0.16 W/m²K, satisfying Part L for new dwellings.
Solid Ground Floor
PIR below a screed needs a compressive strength of ≥70 kPa to resist the screed and live load. Both Celotex GA4000 and Kingspan K103 at 70 kPa are standard choices. For 100 mm PIR on a solid ground floor, U ≈ 0.20 W/m²K assuming a concrete slab — increase to 120 mm to reach 0.17 W/m²K.
Cavity Wall (Partial Fill)
Standard cavity partial-fill boards are 50 mm or 75 mm. The clear residual cavity must be at least 50 mm to maintain resistance to moisture bridging — check local exposure zone. Kingspan’s TW57 and Celotex’s CW4000 are designed for this application with one foil face and one roughened face to shed moisture.
Fire Performance
PIR insulation is combustible and carries a Euroclass E or D rating in standard tests — it will ignite and burn when exposed to direct flame, unlike mineral wool (A1). This has significant implications:
- PIR must not be left exposed in habitable rooms — it must be protected by at least 12.5 mm plasterboard (which achieves 30 minutes’ fire protection)
- In cavity wall applications, the residual air cavity provides some separation but does not remove the combustibility risk
- Building Regulations Part B (fire safety) includes specific guidance on using combustible materials in cavity walls and on the external envelope of buildings over 18 m — PIR is generally not appropriate for external insulation on taller residential buildings
For residential renovation below 18 m this is rarely a practical barrier, but always confirm with Building Control if you’re unsure about your specific application.
Cutting and Installation
PIR boards cut cleanly with a fine-tooth handsaw or a Stanley knife and straightedge. The foil facing scores and the board snaps cleanly for straight cuts. A reciprocating saw speeds up cutting thicker boards. Key installation points:
- Tape all joints with PIR foil tape — untaped joints significantly degrade the effective U-value and create cold bridges
- Stagger joints in multi-layer build-ups so no joint aligns through the full depth
- Mechanically fix where the board is vertical (wall application) — adhesive alone is insufficient
- Handle carefully in frost — PIR becomes more brittle below -5°C and faces can delaminate if boards are stored on a frozen surface
Kingspan and Celotex both publish free BBA-certified technical datasheets and U-value calculation tools on their websites — always use the manufacturer’s figure for the specific product and application rather than generic PIR values, as λ varies between product lines.
