What a U-value is
A U-value is how much heat passes through 1 m² of wall, roof or floor for each degree of temperature difference, in W/m²K. Lower is better. Building Regulations set maximum U-values for each element, so the question is never really "how many millimetres" but "what thickness gets this build-up below the target".
How it's calculated
Each layer resists heat. Its resistance is its thickness in metres divided by its thermal conductivity (lambda, λ, in W/mK):
R = thickness (m) ÷ λ
Add up the layers, add the inside and outside surface resistances (for a roof, 0.10 inside and 0.04 outside), and the U-value is 1 divided by the total. Insulation has a tiny λ (mineral wool about 0.038, PIR board about 0.024), so it does almost all the work. Brick (0.68) and plasterboard (0.21) barely count.
Worked example: a loft
| Build-up (ceiling up) | Total R (m²K/W) | U-value | Against 0.15 |
|---|---|---|---|
| 12.5 mm plasterboard + 200 mm mineral wool | 5.46 | 0.183 | Fail |
| 12.5 mm plasterboard + 270 mm mineral wool | 7.30 | 0.137 | Pass |
| 12.5 mm plasterboard + 100 mm between joists + 200 mm over | 8.09 | 0.124 | Pass |
For the 270 mm loft: plasterboard 0.0125 ÷ 0.21 = 0.060, mineral wool 0.270 ÷ 0.038 = 7.105, plus surfaces 0.10 + 0.04. Total R = 7.30, so U = 1 ÷ 7.30 = 0.14 W/m²K. This simple method ignores the timber joists bridging the lower layer, which pushes the real figure up a little, so a 300 mm build-up (100 mm between the joists and 200 mm across them) is the safer choice.
The Part L 2021 targets (England)
| Element | Extension or new element in an existing home | New home (limiting value) |
|---|---|---|
| Pitched or flat roof | 0.15 | 0.16 |
| External wall | 0.18 | 0.26 |
| Ground floor | 0.18 | 0.18 |
Figures are W/m²K, from Approved Document L 2021 Volume 1, Tables 4.2 and 4.1. Wales, Scotland and Northern Ireland have their own figures. Building control or a SAP assessor has the final say, and a new home has to meet its overall energy target as well, which usually needs better than the limiting values.
From U-value to how much to buy
Once the thickness is set, the quantity is simple: area × thickness gives the volume in m³, and for rolls or boards you divide the area by the coverage printed on the pack for that thickness, rounding up. A 40 m² loft at 300 mm is 12 m³ of insulation. Allow a little extra for cutting around joists, hatches and eaves.