What Is the U-Value of 100mm PIR Insulation?
"What's the U-value of 100mm PIR?" is one of the questions we're asked most. Strictly speaking, a board on its own doesn't have a U-value. A U-value describes a whole wall, floor or roof, including the timber, the air spaces and the other layers. What a board does have is a thermal resistance (R-value), and that is what goes into the U-value calculation. This guide gives the R-value of 100mm PIR, typical U-values from Kingspan's published tables, and a table for every thickness we stock with a link to our U-value calculator.
Quick answer
- 100mm PIR at λ 0.022 W/mK has an R-value of 4.50 m²K/W. Kingspan (Thermapitch TP10) and Recticel (Eurothane GP) both publish this figure.
- In a real construction, 100mm of PIR typically gives a U-value of roughly 0.12–0.20 W/m²K, depending on the build-up (see the examples below).
- For your own wall, floor or roof, use our free PIR U-value calculator.
R-value vs U-value
- R-value (m²K/W) measures how well one layer resists heat flow. It's the thickness in metres divided by λ, and higher is better. 0.100 ÷ 0.022 = 4.545, which manufacturers round down to 4.50.
- U-value (W/m²K) is the rate of heat loss through the whole element, and lower is better. It takes account of every layer, the timber studs or joists that bridge the insulation, air spaces and the surface resistances.
That's why the same 100mm board gives different U-values in a roof, a wall and a floor. For more background, read why U-values matter and understanding R-values.
Typical U-values with 100mm PIR
These are from the typical construction tables in Kingspan's Thermapitch TP10 brochure. They're calculated to BS EN ISO 6946, and Kingspan notes they are for guidance only:
| Construction (Kingspan TP10 brochure) | U-value (W/m²K) |
|---|---|
| Pitched roof, unventilated: 100mm TP10 between 125mm rafters at 600mm centres, plus 37.5mm Kooltherm K118 under the rafters (Figure 1c) | 0.18 |
| As above with 57.5mm K118 under the rafters | 0.16 |
| As above with 72.5mm K118 under the rafters | 0.14 |
| Pitched roof, unventilated: 100mm TP10 between 100mm rafters plus 100mm over the rafters (Figure 3a) | 0.12 |
| Loft floor: 100mm TP10 between 100mm joists at 400mm centres plus 100mm over them, ventilated (Figure 4b) | 0.12 |
| Loft floor or ceiling: 100mm TP10 between joists at 400mm centres, plus 57.5mm K118 under them, ventilated (Figure 4c) | 0.18 |
| Dwarf wall: 100mm TP10 between 47mm studs at 600mm centres, plus 37.5mm K118, ventilated (Figure 4a) | 0.20 |
| As above with 57.5mm K118 | 0.17 |
Floors depend heavily on the size and shape of the floor (the perimeter-to-area ratio). For suspended timber floors, see what thickness of PIR for a suspended timber floor, which uses Recticel's BBA figures.
R-value and price for every PIR thickness
R-values are from Kingspan's TP10 and Recticel's Eurothane GP datasheets, which publish the same figures. Prices are per 2400 × 1200mm sheet (2.88m²), ex VAT, at the time of writing. A dash means we don't stock that thickness in that brand.
| Thickness | R-value (m²K/W) | Recticel Eurothane GP | Kingspan TP10 | U-value |
|---|---|---|---|---|
| 25mm | 1.10 | £13.74 | £20.41 | Calculate U-value |
| 30mm | 1.35 | £16.49 | £27.08 | Calculate U-value |
| 40mm | 1.80 | £19.58 | £32.49 | Calculate U-value |
| 50mm | 2.25 | £21.24 | £30.83 | Calculate U-value |
| 60mm | 2.70 | £25.83 | £43.74 | Calculate U-value |
| 70mm | 3.15 | £29.16 | £47.49 | Calculate U-value |
| 75mm | 3.40 | £29.58 | £42.91 | Calculate U-value |
| 80mm | 3.60 | £33.74 | £54.99 | Calculate U-value |
| 90mm | 4.05 | £35.41 | £59.99 | Calculate U-value |
| 100mm | 4.50 | £35.83 | £51.66 | Calculate U-value |
| 110mm | 5.00 | £45.83 | – | Calculate U-value |
| 120mm | 5.45 | £44.16 | £63.33 | Calculate U-value |
| 125mm | 5.65 | – | £81.66 | Calculate U-value |
| 130mm | 5.90 | £51.24 | – | Calculate U-value |
| 140mm | 6.35 | £53.33 | £91.66 | Calculate U-value |
| 150mm | 6.80 | £51.66 | £77.49 | Calculate U-value |
The calculator lets you choose a pitched roof, wall or floor build-up, enter your rafter, stud or joist sizes and see the U-value for each board thickness we stock.
How 100mm PIR compares with other materials
R-values at 100mm, from each manufacturer's datasheet:
| 100mm of… | λ (W/mK) | R-value (m²K/W) |
|---|---|---|
| Kingspan Kooltherm K103 (phenolic) | 0.019 | 5.25 |
| PIR (Recticel Eurothane GP, Kingspan TP10) | 0.022 | 4.50 |
| Kingspan GreenGuard GG300 (XPS) | 0.033 | 3.00 |
| URSA ACOUSTIC ROLL (glass mineral wool) | 0.035 | 2.85 |
For floors where every millimetre counts, see our K103 vs PIR comparison, and for floor boards generally, PIR vs EPS vs XPS.
Frequently asked questions
Is 100mm PIR enough?
It depends on the element and the U-value you need. In the pitched-roof examples above, 100mm between the rafters meets 0.18 with 37.5mm insulated plasterboard underneath. For the targets in England, see what thickness of PIR insulation do I need.
Does the brand change the U-value?
Not if the λ and thickness are the same. 100mm Recticel Eurothane GP and 100mm Kingspan TP10 both have an R-value of 4.50 m²K/W.
Is two 50mm boards the same as one 100mm board?
The R-values add up: 2.25 + 2.25 = 4.50 m²K/W. Two layers with staggered joints can also reduce heat loss through the joints.
What's a good U-value?
Lower is better. Building Regulations set maximum U-values for each element. For roofs in England, the renovation target is 0.16 W/m²K. Our thickness guide lists the others.
Order PIR insulation
Browse all our PIR insulation boards, or check your build-up first with the PIR U-value calculator. Prices are shown ex VAT. We are delivery only (no collection) and deliver to mainland England and Wales, with a £400 ex VAT minimum order and free delivery on every order (see delivery information). Call us on 03330 908479 (Mon–Thu 8am–5pm, Fri 8am–12pm), email sales@insulationwarehousedirect.co.uk, or order online.
Sources: Kingspan Thermapitch TP10 brochure, Typical constructions and U-values (Figures 1c, 3a, 4a, 4b, 4c); Recticel Eurothane GP product page; Kingspan Kooltherm K103 brochure; Kingspan GreenGuard brochure; URSA ACOUSTIC ROLL datasheet. Figures are indicative and correct at the time of writing.