The timber is a hole in your insulation
Put 100mm of mineral wool quilt between 100mm studs and it is tempting to call that 100mm of insulation. It is not. Roughly fifteen per cent of that wall, by area, is timber: the studs themselves at 400 or 600 centres, plus noggins, the sole plate, the head, and the doubled studs either side of every opening.
Softwood conducts at about 0.13 W/mK. Mineral wool quilt is around 0.044. So the timber is more than three times the heat path of the stuff packed beside it, and heat goes where it is easiest. Work the wall as a plain series sum, layer after layer, and it calculates about a fifth better than it performs.
The correct treatment is parallel paths: work out the resistance through the insulation and the resistance through the timber separately, then combine them by area. That is what this tool does when you tick a layer as running through timber, and it shows you both numbers so you can see what the bridging costs.
The layers with no thickness
There are three resistances in every calculation that are not materials at all, and leaving them out is the other common error. Air films on each face resist heat: 0.13 on the inside of a wall, 0.04 on the outside. A roof gets 0.10 on the inside rather than 0.13, because heat rising off a ceiling moves more freely than heat crossing a wall, which is why a roof is not a wall lying down. A floor gets 0.17. An unventilated cavity of 25mm or more is worth another 0.18, and that is the same whether it is 25mm or 100mm.
An extension is held to a tighter standard than a new house
This catches people out, so know that before you price a job. A wall in a brand new dwelling has a limiting U-value of 0.26. A new wall in an existing dwelling, which is what an extension is, has to reach 0.18. Roof goes from 0.16 to 0.15 the same way.
The logic is that a whole new house is assessed on its total emissions, so a slightly weaker wall can be traded against something else, and an extension has no such calculation to sit inside. In practice it means an extension wall needs meaningfully more insulation than the house it is being stuck onto.
Renovating something: two numbers, not one
For an element you are keeping rather than building, Part L works differently and this is the part most often quoted wrongly. Table 4.3 gives a threshold and an improved value. The duty to upgrade only applies if what is there now is worse than the threshold. If it is, the target is the improved value.
- Roof: threshold 0.35, improved 0.16
- Wall, cavity fill: threshold 0.70, improved 0.55
- Wall, internal or external insulation: threshold 0.70, improved 0.30
- Floor: threshold 0.70, improved 0.25
So a wall at 0.65 triggers nothing, and a wall at 0.75 triggers a jump all the way to 0.30 if you are lining it internally. That is a large cost difference sitting on a small difference in the starting point, and it is exactly the sort of thing worth checking before you price the job rather than after.
Which edition, which is not the obvious answer
A 2026 edition of Approved Document L is published, and because it is published it is easy to assume it already applies. It does not, yet. It takes effect on 24 March 2027 for ordinary building work, and 24 September 2027 for work connected with higher-risk buildings. Until then the 2021 edition incorporating the 2023 amendments governs.
The useful part: the limiting U-values are identical in both editions. People assume the numbers have moved and they have not, so anything you size today against these figures still stands when the new edition arrives.
Common questions
How do I calculate a U-value?
Add the thermal resistance of every layer, thickness in metres divided by the material’s conductivity, then add the surface resistances and any cavity. The U-value is one divided by that total. Most calculators skip this: layers interrupted by timber have to be worked as parallel paths, not as a single resistance.
Why is my wall worse than the calculation says?
Almost always thermal bridging through the timber. Softwood conducts at 0.13 W/mK and mineral wool quilt at 0.044, so a stud is over three times the heat path of what is beside it. A stud wall is roughly 15% timber by area once you count noggins, sole plate, head and doubled studs at openings, and that makes the real U-value about a fifth worse than a plain series sum.
What U-value do I need for an extension wall?
0.18 W/(m²·K) for a new wall in an existing dwelling, from table 4.2 of Approved Document L. That is tighter than the 0.26 limit for a wall in a whole new house, which surprises people. Roof is 0.15 and floor is 0.18.
Do I have to insulate a wall I am renovating?
Only if it is already worse than the threshold. Table 4.3 gives two numbers for elements you are keeping: a threshold of 0.70 for a wall, and an improved value of 0.55 for cavity fill or 0.30 for internal or external insulation. If the wall is better than 0.70 the duty does not bite. If it is worse, the target is the improved figure, not the threshold.
How much PIR do I need to reach 0.18?
It depends what else is in the build-up, which is why the tool works it out from your actual layers. As a rule PIR gets you there in a bit under half the depth of mineral wool quilt, because its conductivity is about half. That trade is depth against cost, and on a loft with limited rafter depth it is often the only way to hit the number.
Which edition of Part L applies to my job?
The 2021 edition incorporating the 2023 amendments. A 2026 edition is published and it is easy to assume it governs, but it does not take effect until 24 March 2027, and 24 September 2027 for higher-risk building work. The good news is the limiting U-values are identical in both, so the numbers here do not change when it does.
Are these conductivity figures exact?
No, they are indicative, and they are labelled that way in the tool. Every manufacturer declares its own lambda and they differ enough to move an answer: PIR runs from about 0.022 to 0.026 depending on the product. For anything being submitted, take the declared value off the actual board’s datasheet. These are for sizing and pricing.