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 5–30mm more PIR, depending on thickness (see the table).
- At the prices below, the matching PIR board costs roughly 40–45% 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 | £5.73 | £7.18 |
| 40mm | 2.10 | 50mm | 2.25 | +10mm | £17.08 | £7.38 | £9.70 |
| 50mm | 2.60 | 60mm | 2.70 | +10mm | £19.99 | £8.97 | £11.02 |
| 60mm | 3.15 | 70mm | 3.15 | +10mm | £23.74 | £10.12 | £13.62 |
| 70mm | 3.65 | 90mm | 4.05 | +20mm | £27.49 | £12.30 | £15.20 |
| 75mm | 3.90 | 90mm | 4.05 | +15mm | £29.57 | £12.30 | £17.28 |
| 90mm | 4.70 | 120mm | 5.45 | +30mm | £34.58 | £15.33 | £19.24 |
| 100mm | 5.25 | 120mm | 5.45 | +20mm | £37.91 | £15.33 | £22.58 |
| 120mm | 6.30 | 150mm | 6.80 | +30mm | £44.57 | £17.94 | £26.64 |
| 140mm | 7.35 | 75mm + 90mm (2 layers) | 7.45 | +25mm | £51.24 | £22.57 | £28.68 |
| 150mm | 7.85 | 25mm + 150mm (2 layers) | 7.90 | +25mm | £54.57 | £22.71 | £31.87 |
Prices correct at 27 September 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 £795 of PIR (18 sheets), a saving of about £1,170 ex VAT.
What the extra depth means on site
- Solid floors under screed: 5–30mm 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.
Shop floor insulation boards, PIR insulation boards or Kingspan insulation, including Kooltherm phenolic boards, or call us on 03330 908479 (Mon–Thu 8am–5pm, Fri 8am–12pm) for a price on a full load.