What you enter stays on this device.
Method and sources checked on by the Brikalo editorial team · Our method
The R-value of insulation, in m²K/W, is its thickness in metres divided by its thermal conductivity λ. Approved Document L limits the U-value of the element: if the insulation alone reaches R = 1 ÷ U, it complies; for a renovated roof, 250 mm at λ 0.040.
R-value: thickness divided by lambda
The thermal resistance, or R-value, of a layer of insulation is its thickness in metres divided by its thermal conductivity λ in W/mK. The result is in m²K/W. Building Regulations do not set an R-value, though. Approved Document L sets a limiting U-value for each element, in W/m²K, and the U-value is the inverse of the resistance of all the layers together.
Example: 100 mm of insulation with λ 0.035 gives 0.100 ÷ 0.035 = 2.86 m²K/W. On its own, that layer would have a U-value of 1 ÷ 2.86 = 0.35 W/m²K. Work in metres: 100 ÷ 0.035 would give 2,857, a thousand times too much.
Limiting U-values in Approved Document L
Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments) applies in England. Table 4.2 covers new elements in existing dwellings, such as an extension. Table 4.3 covers existing elements being renovated or retained, for example in a loft conversion:
| Element | U-value (W/m²K) |
|---|---|
| New roof, wall or floor in an existing dwelling (Table 4.2) | 0.15 / 0.18 / 0.18 |
| Renovated roof (Table 4.3, improved) | 0.16 |
| Wall, cavity insulation (Table 4.3, improved) | 0.55 |
| Wall, internal or external insulation (Table 4.3, improved) | 0.30 |
| Renovated floor (Table 4.3, improved) | 0.25 |
An existing element whose U-value is worse than the threshold in Table 4.3 (0.35 for a roof, 0.70 for walls and floors) should be upgraded when it is renovated or retained. If the improved value is not technically feasible or would not pay back within 15 years, the element should reach the lowest U-value that is, and generally no worse than 0.7 W/m²K. Wales and Scotland publish their own guidance, which this calculator does not cover.
Insulation thickness for each Part L U-value
Limiting U-value from Approved Document L, Volume 1 (England), the R-value the insulation must reach on its own (1 divided by U), and the thickness, rounded up to the millimetre, for three thermal conductivities read from a product label. Example: a renovated roof at 0.16 W/m²K needs 250 mm of insulation at λ 0.040.
| Element | U-value (W/m²K) | R of insulation | λ 0.022 | λ 0.032 | λ 0.040 |
|---|---|---|---|---|---|
| New roof | 0.15 | 6.67 | 147 mm | 214 mm | 267 mm |
| New wall | 0.18 | 5.56 | 123 mm | 178 mm | 223 mm |
| New floor | 0.18 | 5.56 | 123 mm | 178 mm | 223 mm |
| Renovated roof | 0.16 | 6.25 | 138 mm | 200 mm | 250 mm |
| Cavity wall fill | 0.55 | 1.82 | 40 mm | 59 mm | 73 mm |
| Solid wall, IWI or EWI | 0.30 | 3.33 | 74 mm | 107 mm | 134 mm |
| Renovated floor | 0.25 | 4.00 | 88 mm | 128 mm | 160 mm |
The thickness that is sure to meet the limit
The real U-value of a wall is 1 divided by the sum of all its resistances: the inside and outside surfaces, the brick or block, the plaster and the insulation. It is always lower than 1 divided by the resistance of the insulation alone. So if the insulation on its own reaches R = 1 ÷ U, the element meets the limit whatever else is there, and the thickness needed is λ ÷ U.
A renovated roof at 0.16 needs R = 6.25: 250 mm at λ 0.040 or 200 mm at λ 0.032. A solid wall insulated internally or externally to 0.30 needs R = 3.33, which is 74 mm at λ 0.022 or 107 mm at λ 0.032. A new wall in an extension (0.18) needs 178 mm at λ 0.032.
The result is on the safe side. A full U-value calculation that includes every layer and the surface resistances (BS EN ISO 6946) usually allows a little less. When the insulation falls short of 1 ÷ U, the calculator does not say the element fails: it says the full calculation is needed.
Special cases
- Walls: if meeting the standard would reduce the floor area of the room by more than 5%, Approved Document L accepts a lesser provision.
- Roofs with limited headroom: the insulation plus any air gap should at least fill the depth of the rafters, using the insulant that gives the lowest practicable U-value.
- Two layers: their R-values add up. 100 mm at λ 0.032 (3.13) plus 100 mm at λ 0.040 (2.50) gives 5.63 m²K/W.
- Windows and doors: they have their own limits in Table 4.2 (1.4 W/m²K or a Window Energy Rating of Band B).
Common mistakes
- Mixing millimetres and metres in the formula.
- Using a lambda from memory: take the declared value on the label or in the declaration of performance of the product you fit.
- Confusing U and R: a low U-value is good, a low R-value is not.
- Using the new-element figure (0.15 for a roof) when renovating, or the reverse.
Measure the area to insulate with the room area calculator. Size the extractor fans that a more airtight home needs with the Part F ventilation rates, and estimate running costs with the electricity usage calculator. Internal wall insulation means redecorating: the paint calculator gives the litres needed.
Method and sources
References used:
This tool is checked by automated tests against known cases. Spotted a mistake? Report it on the contact page.
Last updated