Kooltherm K103 vs Celotex PIR Floor Insulation: How Much Thicker, and How Much Cheaper?
Kingspan Kooltherm K103 and Celotex PIR boards are both rigid, foil-faced insulation boards used under screeds and between floor joists. The difference is the core. Kooltherm K103 is phenolic, with a thermal conductivity (λ) of 0.019 W/mK, while Celotex PIR boards (TB4000, GA4000 and XR4000, now sold as Sopratherm) are 0.022 W/mK. That means K103 insulates better per millimetre, so the question is how much thicker Celotex has to be, and whether the extra depth is worth the saving. (We also have a K103 vs Recticel Eurothane GP comparison.)
The short answer
- To match a K103 board at the lowest cost you need 5–35mm more Celotex PIR, depending on thickness (see the table).
- At the prices below, the matching Celotex board costs roughly 39–45% of the K103 price per m², a saving of £7.25–£32.70 per m² ex VAT. On a typical 50m² floor that is several hundred pounds saved, and over £1,000 at 100mm K103 and above.
- Choose K103 when floor height is tight, for example in a renovation with fixed door thresholds, stairs or an existing floor level to meet. Choose Celotex when you have a few extra millimetres to spare.
Free printable download: K103 vs Celotex comparison, one A4 page (PDF). Handy to keep in the van or on site.
K103 vs Celotex: thickness for the same R-value
R-value (thermal resistance) is thickness ÷ λ. If you use a board with the same or higher R-value, the rest of the floor build-up stays the same and the U-value will be the same or better. The table shows the cheapest Celotex board per m² that does that. Popular sizes such as 75mm, 100mm and 120mm often cost less per m² than the in-between sizes, so the best-value match is sometimes a little thicker.
| K103 | K103 R (m²K/W) | Cheapest Celotex match | Celotex R | Extra depth | K103 £/m² | Celotex £/m² | Saving £/m² |
|---|---|---|---|---|---|---|---|
| 25mm | 1.30 | 30mm | 1.35 | +5mm | £12.91 | £5.66 | £7.25 |
| 40mm | 2.10 | 50mm | 2.25 | +10mm | £17.08 | £7.30 | £9.77 |
| 50mm | 2.60 | 60mm | 2.70 | +10mm | £19.99 | £8.93 | £11.06 |
| 60mm | 3.15 | 75mm | 3.40 | +15mm | £23.74 | £9.85 | £13.89 |
| 70mm | 3.65 | 100mm | 4.50 | +30mm | £27.49 | £12.03 | £15.46 |
| 75mm | 3.90 | 100mm | 4.50 | +25mm | £29.57 | £12.03 | £17.54 |
| 90mm | 4.70 | 120mm | 5.45 | +30mm | £34.58 | £14.95 | £19.62 |
| 100mm | 5.25 | 120mm | 5.45 | +20mm | £37.91 | £14.95 | £22.95 |
| 120mm | 6.30 | 150mm | 6.80 | +30mm | £44.57 | £18.01 | £26.56 |
| 140mm | 7.35 | 75mm + 100mm (2 layers) | 7.90 | +35mm | £51.24 | £21.88 | £29.36 |
| 150mm | 7.85 | 75mm + 100mm (2 layers) | 7.90 | +25mm | £54.57 | £21.88 | £32.70 |
Prices correct at 16 October 2026, ex VAT; check the product pages for current prices. K103 is sold by the full pack, Celotex per 2.88m² sheet. The 30mm board is Celotex TB4000, 50mm to 100mm are GA4000 and 120mm and 150mm are XR4000. R-values are rounded down to the nearest 0.05 m²K/W. At 140mm and 150mm the Celotex match is laid in two layers (75mm + 100mm), because the thickest single board is 150mm.
Worked example: a 50m² floor
Say your floor needs K103 100mm (R 5.25). The cheapest Celotex equivalent is XR4000 120mm (R 5.45), which is 20mm thicker. Bought in whole packs or sheets for 50m², that is about £1,965 of K103 (6 packs of 3) against £775 of Celotex (18 sheets), a saving of about £1,190 ex VAT.
Celotex is delivered separately from our other products: £40 ex VAT under £550 ex VAT and free from £550 ex VAT, with a minimum Celotex order of £400 ex VAT. The example above is over £550, so delivery is free. We deliver to England only.
What the extra depth means on site
- Solid floors under screed: 5–35mm more insulation means digging deeper, using a thinner screed (within the screed’s minimum thickness) or accepting a higher finished floor.
- Suspended timber floors: boards fitted between joists usually have depth to spare, so Celotex is often the easy win here.
- Renovations: if raising the floor means trimming doors, altering thresholds or changing a stair riser, the extra work can outweigh the saving. That’s where K103 earns its price.
Check before you buy
- Work out the U-value for your actual floor with our free PIR U-value calculator, and see what thickness of PIR insulation do I need?. Confirm the final specification with your building control body or designer.
- Check each board’s datasheet and the Sopratherm BBA certificate (linked on the product pages) for compressive strength and loading, especially for heavily loaded floors.
- Use the insulation calculator to work out sheets or packs. Remember K103 comes in full packs only.
- Celotex is now sold as Sopratherm; the boards and codes are unchanged. Comparing Celotex with other PIR? See Kingspan TP10 vs Celotex XR4000.
Shop Celotex insulation boards (now Sopratherm), floor insulation boards or Kingspan insulation boards, including Kooltherm phenolic, or call us on 03330 908479 (Mon–Thu 8am–5pm, Fri 8am–12pm) for a price on a full load.