Work out the U-value of a wall, roof or floor build-up in about two minutes, and see whether it meets the Building Regulations limit for a new extension in England.
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In England, new walls, roofs and floors in an existing home, including extensions and conversions, should meet the limiting U-values in Table 4.2 of Approved Document L, Volume 1: 0.18 W/m²K for walls, 0.15 W/m²K for roofs and 0.18 W/m²K for floors. Windows and doors should be 1.4 W/m²K or better, and rooflights 2.2 W/m²K. A lower U-value means less heat escapes. This calculator adds up the resistance of each layer you enter and compares the result with the limit for that element.
Each layer resists heat flow. Its thermal resistance (R) is its thickness in metres divided by its thermal conductivity (λ). The total resistance is the sum of every layer plus the inside and outside surface resistances, and the U-value is 1 divided by that total:
U = 1 ÷ (Rsi + R1 + R2 + … + Rse)
We use the standard surface resistances for each direction of heat flow: 0.13 inside and 0.04 outside for walls, 0.10 and 0.04 for roofs, and 0.17 and 0.04 for floors. A small unventilated cavity is given a resistance of 0.18 m²K/W.
| Material | Typical λ (W/mK) |
|---|---|
| PIR insulation board | 0.022 |
| Phenolic insulation board | 0.020 |
| Mineral wool cavity batt | 0.035 |
| Mineral wool loft roll | 0.044 |
| Aircrete block | 0.15 |
| Brick outer leaf | 0.77 |
| Dense concrete block | 1.13 |
These are typical figures for comparison only. Products vary, so use the declared value on your product's datasheet wherever you can.
For more help, read our guides to householder planning applications and lawful development certificates. Planning the size of your extension? Try our permitted development rear extension calculator or loft conversion volume calculator.
0.18 W/m²K or better, under Table 4.2 of Approved Document L, Volume 1. For a cavity wall this usually means around 100 mm of PIR board or 150 mm of mineral wool in the cavity, with an aircrete inner leaf.
0.15 W/m²K or better. The same limit applies to pitched roofs. A warm deck flat roof usually needs around 140 to 150 mm of PIR board above the deck.
Yes. The U-value is the rate heat passes through one square metre of the element for each degree of temperature difference, so a lower figure means less heat lost and lower heating bills.
Yes. Walls, roofs and floors that are newly built or newly insulated as part of a conversion are new fabric elements and should meet the same Table 4.2 limits. Renovating an existing element that you are keeping uses different, less strict values.
Sometimes. Approved Document L allows a trade-off where you show, by calculation, that the extension as a whole performs no worse than one built to the standard. This normally needs a SAP assessor.
No. Your U-value and whether it meets the limit are free with no sign-up. Your details are only needed for the layer-by-layer breakdown, the extra insulation figures and a free planning report from our team.
Last checked 11th November 2026. This tool gives a guide only, based on Approved Document L, Volume 1 (2021 edition incorporating 2023 amendments). It is not a formal U-value calculation, and Building Control has the final say.
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