Kooltherm K103 vs PIR Floor Insulation: How Much Thicker, and How Much Cheaper?
Kingspan Kooltherm K103 and PIR boards such as Recticel Eurothane GP 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 the PIR boards we sell, such as Recticel Eurothane GP, are 0.022 W/mK. That means K103 insulates better per millimetre, so the question is how much thicker PIR has to be, and whether the extra depth is worth the saving.
The short answer
- To match a K103 board at the lowest cost you need 5–35mm more PIR, depending on thickness (see the table).
- At the prices below, the matching PIR board costs roughly 40–47% of the K103 price per m². On a typical 50m² floor that is several hundred pounds saved, and over £1,000 at 100mm 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 PIR when you have a few extra millimetres to spare.
Free printable download: K103 vs PIR comparison, one A4 page (PDF). Handy to keep in the van or on site.
K103 vs PIR: 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 PIR board per m² that does that. Popular sizes such as 120mm and 150mm are often cheaper than the in-between sizes, so the best-value match is sometimes a little thicker.
| K103 | K103 R (m²K/W) | Cheapest PIR match (Eurothane GP) | PIR R | Extra depth | K103 £/m² | PIR £/m² | Saving £/m² |
|---|---|---|---|---|---|---|---|
| 25mm | 1.30 | 30mm | 1.35 | +5mm | £12.91 | £6.07 | £6.84 |
| 40mm | 2.10 | 50mm | 2.25 | +10mm | £17.08 | £7.30 | £9.77 |
| 50mm | 2.60 | 60mm | 2.70 | +10mm | £19.99 | £9.32 | £10.68 |
| 60mm | 3.15 | 70mm | 3.15 | +10mm | £23.74 | £10.47 | £13.27 |
| 70mm | 3.65 | 90mm | 4.05 | +20mm | £27.49 | £12.64 | £14.85 |
| 75mm | 3.90 | 90mm | 4.05 | +15mm | £29.57 | £12.64 | £16.93 |
| 90mm | 4.70 | 120mm | 5.45 | +30mm | £34.58 | £15.68 | £18.89 |
| 100mm | 5.25 | 120mm | 5.45 | +20mm | £37.91 | £15.68 | £22.23 |
| 120mm | 6.30 | 150mm | 6.80 | +30mm | £44.57 | £18.02 | £26.55 |
| 140mm | 7.35 | 25mm + 150mm (2 layers) | 7.90 | +35mm | £51.24 | £22.75 | £28.49 |
| 150mm | 7.85 | 25mm + 150mm (2 layers) | 7.90 | +25mm | £54.57 | £22.75 | £31.83 |
Prices correct at 15 October 2026, ex VAT; check the product pages for current prices. K103 is sold by the full pack, PIR per 2.88m² sheet. R-values are rounded down to the nearest 0.05 m²K/W. At 140mm and 150mm the PIR is laid in two layers, because the thickest single board is 150mm.
Worked example: a 50m² floor
Say your floor needs K103 100mm (R 5.25). The cheapest PIR equivalent is Eurothane GP 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 £813 of PIR (18 sheets), a saving of about £1,152 ex VAT.
K103 vs Celotex PIR (now Sopratherm)
If your specification names Celotex, the same approach applies. Celotex PIR boards (TB4000, GA4000 and XR4000, now sold as Sopratherm; the boards and codes are unchanged) are also λ 0.022 W/mK. Because Celotex sizes and prices differ slightly from Eurothane GP, the cheapest Celotex match is sometimes a different thickness: at the prices below it costs roughly 40–45% of the K103 price per m², a saving of £7.13–£32.70 per m² ex VAT.
Free printable download: K103 vs Celotex comparison, one A4 page (PDF).
| 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 | £5.78 | £7.13 |
| 40mm | 2.10 | 50mm | 2.25 | +10mm | £17.08 | £7.30 | £9.77 |
| 50mm | 2.60 | 60mm | 2.70 | +10mm | £19.99 | £9.09 | £10.90 |
| 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 | £15.34 | £19.24 |
| 100mm | 5.25 | 120mm | 5.45 | +20mm | £37.91 | £15.34 | £22.57 |
| 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 19 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.
Celotex worked example (50m²): 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 £795 of Celotex (18 sheets), a saving of about £1,170 ex VAT.
Celotex is delivered separately from our other products: free delivery and, with a minimum Celotex order of £400 ex VAT. The example above is over £550, so delivery is free. Most orders over £400 + VAT are delivered free across England, with delivery to mainland Wales and Scotland too. Mainland Wales and mainland Scotland: Celotex insulation and Knauf loft roll have £40 ex VAT delivery on every order; ceilings have a £400 ex VAT minimum plus £40 ex VAT delivery; British Gypsum plaster and plasterboard have a £400 ex VAT minimum plus £40 ex VAT delivery. We do not deliver to Northern Ireland or the Scottish islands.
Comparing Celotex with other PIR? See Kingspan TP10 vs Celotex XR4000.
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 PIR 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 (linked on the product page) 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.
- If your specification names Celotex, we also stock Celotex GA4000 and XR4000 floor boards (now sold as Sopratherm). They are λ 0.022 W/mK like Eurothane GP, so the PIR thicknesses in the table apply to them too. For a Celotex-only table with Celotex prices, see K103 vs Celotex PIR above.
Shop floor insulation boards, PIR insulation boards, Celotex insulation boards (now Sopratherm) 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.