Expert guideDry-line an outside wall, re-roof, or strip a floor, and the Building Regulations have something to say about how well it is insulated afterwards. This is what England, Wales and Scotland ask for, in plain English, with the tables and the exceptions. Checked against the official documents in October 2026.
By Miles Sharples and the Energy Giants installation team · Updated October 2026When the rules apply
The energy rules for existing homes kick in when you renovate a thermal element: an outside wall, a roof or a ground floor. In England, Approved Document L says renovation means any of these:
If more than 50% of that wall, roof or floor is being renovated, or the job is a major renovation (more than 25% of the whole home’s outer surface), the whole element should be brought up to the standard in the tables below. Wales uses the same 50% and 25% triggers. When you renovate from inside, the area is measured as the outside wall seen from inside the room.
Based on the Building Regulations 2010 regulation 23 and Approved Document L Volume 1 paragraphs 11.2 to 11.4 (England), Approved Document L Volume 1 paragraphs 11.14 to 11.21 (Wales), and the Domestic Technical Handbook clause 6.2.10 (Scotland). A plain-English guide, not legal advice.
All three nations use U-values: how many watts escape through each square metre for each degree of difference between inside and outside. Lower is better. Pick a nation.
| Element | Threshold (a) | Improved (b) |
|---|---|---|
| Roof | 0.35 | 0.16 |
| Wall, cavity insulation | 0.70 | 0.55 |
| Wall, internal or external insulation | 0.70 | 0.30 |
| Floor | 0.70 | 0.25 |
U-values in W/m²K, area-weighted averages. Column (b) is what a renovated element should reach. An existing element worse than column (a), for example in a loft conversion, should also be upgraded to column (b). Source: Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, Table 4.3.
The rules accept that old houses are not new ones. In England, if the improved U-value is not technically or functionally feasible, or would not pay for itself through energy savings within 15 years, the element should be upgraded to the best U-value that is feasible and does pay back within 15 years. Once upgraded, it should generally be no worse than 0.7 W/m²K. Wales uses the same 15 year payback test.
Both England and Wales tie these targets to moisture. England’s guidance on solid walls says condensation and moisture risk must be assessed when insulated board goes on the inside, and that this will usually mean vapour control and damp protection. Wales says that where internal wall insulation is intended, the wall should be assessed to make sure it is suitable, including a moisture risk assessment, by someone appropriately trained such as a PAS 2035 retrofit coordinator.
Kingspan publishes U-values for its K118 insulated plasterboard on a 215 mm solid brick wall. Against England and Wales’ 0.30 target for an internally insulated wall:
| K118 board, fixing | U-value | 0.30 target |
|---|---|---|
| 62.5 mm on timber battens | 0.30 | Meets it |
| 62.5 mm bonded over an existing lining | 0.31 | Just short |
| 72.5 mm bonded over an existing lining | 0.27 | Meets it |
| 92.5 mm on timber battens | 0.21 | Meets it |
Source: Kingspan Kooltherm K118 brochure, Eighth Issue (03/2026), Figures 1 and 2. Thicknesses include the 12.5 mm plasterboard. Scotland’s wall benchmark of 0.17 is beyond the K118 range on this wall, which is where “as far as is reasonably practicable” comes in.
On solid walls we fit the 72.5 mm board: it clears the target with some margin, while keeping the lining slim. More on the product in our Kingspan Kooltherm guide.
New editions are already published. In England, the 2026 edition of Approved Document L Volume 1 takes effect on 24 March 2027 (24 September 2027 for higher-risk building work) and repeats the same figures for existing elements as its Table 3.3. In Wales, the 2026 version takes effect on 4 March 2027 and keeps the same values, with the three roof rows merged into one.
FAQs
It depends how much of that wall is being renovated. Dry-lining counts as renovating the wall. If more than 50% of an individual wall is renovated, or more than 25% of the whole home’s outer surface, the whole wall should be brought up to the target where that is feasible.
In England and Wales, 0.30 W/m²K, where that is technically, functionally and economically feasible. Scotland’s benchmark is 0.17 as far as is reasonably practicable, with 0.70 as the usual minimum after an alteration.
In England, a lesser standard may be appropriate if meeting the target would cut a room’s floor area by more than 5%. Wales treats more than 5% as not functionally feasible. Either way, the aim becomes the best that is feasible.
England and Wales both use a 15 year simple payback test. If the full target would not pay back within 15 years through energy savings, the element should be upgraded to the best U-value that does, generally no worse than 0.7 W/m²K.
New editions of Approved Document L take effect in England on 24 March 2027 and in Wales on 4 March 2027. Both keep the same U-values for upgrading existing walls, roofs and floors.
Yes. Scotland uses the Building Standards Technical Handbook rather than Approved Documents. The April 2026 edition gives benchmarks of 0.17 for walls, 0.15 for floors and 0.12 for roofs, to be met as far as is reasonably practicable.
Go deeper
We survey the wall, measure the room, and tell you what U-value is realistic and what the rules expect, before anything is ordered.
Sources: Approved Document L Volume 1 (England, 2021 edition incorporating 2023 amendments, and 2026 edition); Approved Document L Volume 1 (Wales, 2022 edition incorporating 2024 amendments, and 2026 edition); Building Standards Technical Handbook: Domestic (Scotland, April 2026); Kingspan Kooltherm K118 brochure (03/2026). Checked October 2026. A guide, not legal advice.
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