How to Calculate a U-Value: Worked PIR Insulation Examples
Quick answer
A U-value is 1 divided by the total thermal resistance of a wall, floor or roof. Work out each layer's R-value (thickness in metres ÷ λ), add the surface resistances, add everything up, then take 1 ÷ the total. For PIR at λ 0.022 W/mK, every 10mm adds about R 0.45. To skip the sums, use our PIR U-value calculator - this guide shows what it is doing behind the scenes.
The formula in three lines
- R-value of a layer = thickness (m) ÷ thermal conductivity λ (W/mK). 100mm of PIR at 0.022 gives 0.1 ÷ 0.022 = R 4.55; manufacturers declare R 4.50 because they round down.
- Total resistance = inside surface resistance (Rsi) + the R-value of every layer + outside surface resistance (Rse).
- U-value = 1 ÷ total resistance, in W/m²K.
Under BS EN ISO 6946 the standard surface resistances are Rsi 0.10 (heat flowing up, as in a roof or ceiling), 0.13 (horizontal, as in a wall) and 0.17 (heat flowing down, as in a floor), with Rse 0.04 for an external surface. Our guide to understanding R-values explains the building blocks in more depth.
Declared R-values for common PIR thicknesses
These are the declared values for Recticel Eurothane GP (λ 0.022 W/mK), one of the boards in our PIR insulation range:
| Board thickness | Declared R-value (m²K/W) |
|---|---|
| 50mm | 2.25 |
| 75mm | 3.40 |
| 100mm | 4.50 |
| 120mm | 5.45 |
| 150mm | 6.80 |
Worked example 1: adding PIR to an existing wall
Say a survey (or an assumption from the age and type of wall) gives an existing U-value of 1.7 W/m²K. The total resistance is 1 ÷ 1.7 = R 0.59, and that already includes the surface resistances. This is an illustration only, not a measured figure for any real wall.
- Add 50mm PIR (R 2.25): 0.59 + 2.25 = 2.84, so U = 1 ÷ 2.84 = 0.35.
- Add 75mm PIR (R 3.40): 0.59 + 3.40 = 3.99, so U = 1 ÷ 3.99 = 0.25.
We have ignored the plasterboard and fixings, which add a little resistance or a little bridging. Real calculations include them, which is why a designer's figure can differ slightly.
Worked example 2: working back from a target
Now flip it round. If you need 0.30 W/m²K (the improved standard for internal or external wall insulation in Table 4.3 of Approved Document L, see Part L in England for the full set of targets):
- Total resistance needed = 1 ÷ 0.30 = 3.33.
- Take away the existing wall: 3.33 − 0.59 = 2.74 needed from the insulation.
- Minimum thickness = 2.74 × 0.022 = 0.0603 m, or about 60mm.
As a formula: minimum thickness (m) = λ × (1 ÷ target U − R of existing construction). A 60mm board declares R 2.70, giving 0.59 + 2.70 = 3.29 and U = 0.304, a near miss that rounds to 0.30 but does not meet it. A 65mm board (R 2.95) gives 3.54 and U = 0.28. In practice you pick from stocked thicknesses, so you round up, which is why the calculator suggests the next board size rather than the exact figure.
Why timber and ties change the answer
The simple method assumes each layer is uniform. When insulation sits between timber joists or rafters, heat takes the easier route through the timber. BS EN ISO 6946 handles this with the combined method, which averages the insulated and timber paths by area. Manufacturers state their assumptions: the Recticel BBA table for Eurothane GP between joists assumes 47mm joists at 400mm centres, which is about 11% timber. In cavity walls, metal wall ties through the insulation and mortar joints around blocks are corrected for in the same way. Bridging is the main reason that the same board gives different U-values in different buildups.
Worked example 3: a ground floor and the perimeter-to-area ratio
Ground floors are different. Heat escapes mainly around the edge, so the standard BS EN ISO 13370 method uses the exposed perimeter divided by the floor area, written P/A. Take a 4m × 5m extension attached to the house along one 4m side. The exposed perimeter is 4 + 5 + 5 = 14m, the area is 20m², so P/A = 14 ÷ 20 = 0.7.
Kingspan's TF70 brochure (a PIR floor board with the same λ 0.022) lists 100mm under a floor screed at 0.14 W/m²K for P/A 0.2, 0.15 for 0.3 and 0.18 for 0.7. So that small extension, with a high P/A, needs 100mm of PIR to land at 0.18, whereas a large open-plan house floor with P/A 0.2 gets to 0.14 with the same board. See our thickness guide and, for timber floors, PIR for suspended timber floors.
Why manufacturers and calculators can differ
- Different soil assumptions: the Kingspan tables assume sand or gravel, while our calculator defaults to clay or silt, which is the more cautious choice.
- Different build-ups: slab, screed and block densities vary between tables.
- Rounding: declared R-values are rounded down and U-values quoted to two decimal places.
Whatever tool you use, Building Control will want a calculation tied to your actual construction. Manufacturers' technical teams will usually run these for you.
Use the calculator
Pick your element, enter the existing construction and a target U-value, and the PIR U-value calculator does the sums above in a few seconds. We have more tools on our calculators page.
Frequently asked questions
Do I include plasterboard in a U-value calculation?
Yes in a full calculation, because it adds a small amount of resistance. For a quick estimate you can leave it out, which errs slightly on the cautious side.
Is a higher R-value or a higher U-value better?
A higher R-value is better, because it means more resistance to heat flow. A lower U-value is better, since U is the reciprocal of total R.
Why does my calculated U-value not match the datasheet?
Datasheets assume a specific build-up, bridging and soil type. If yours differs, so will the answer. Ask the manufacturer to run your construction if Building Control needs a formal figure.
Order PIR insulation
Once you know the thickness, browse PIR insulation boards, such as Recticel Eurothane GP 100mm and 75mm. We deliver to England only. Call 03330 908479 if you would like help with choosing a board.
Sources: Recticel Eurothane GP Underfloor BBA certificate; Recticel Eurothane GP product page; Kingspan Thermafloor TF70 brochure; Approved Document L (GOV.UK). Figures are indicative and correct at the time of writing; they are not a substitute for a calculation by your designer or the manufacturer's technical team, and Building Control has the final say.