Part L compliance on a rendered wall is settled by a calculation across the whole build-up, not by a board depth chosen in advance — and the first question on any UK project is which edition of Approved Document L the work actually falls under. Renders World supplies the boards those specifications are built from, and the sections below set out what the regulations ask for before a depth is fixed. The external wall insulation systems hub carries the component ranges referred to throughout.
How Part L Governs Insulation Thickness on UK Walls
Every thickness calculation for Part L UK work begins with Regulation 23 of the Building Regulations 2010, which requires a renovated thermal element to be brought up to standard once more than 50% of its surface area is affected. That single threshold decides whether a facade upgrade is a cosmetic job or a controlled one, and a full-perimeter external insulation scheme crosses it by definition.
The regulation gives two separate triggers, and either one is enough on its own. It also carries a feasibility qualifier that specifiers rely on more often than they expect, because it is what allows a sensible outcome on a wall that cannot physically take the depth the standard would imply.
- Major renovation of an individual thermal element brings the whole element into scope, regardless of the proportion worked on.
- More than 50% of the element's surface area renovated brings the whole element into scope under Regulation 23(1).
- Replacement of more than 50% of a thermal element's surface area triggers the same duty under Regulation 23(2), applied to the whole element.
- Feasibility is written into the regulation itself: the upgrade applies so far as it is technically, functionally and economically feasible.
The wording matters because the duty attaches to the whole element, not to the part being worked on. A wall insulated on three elevations is still assessed as one element, which is why the specification stage is worth settling before scaffolding goes up. The precise text sits in Regulation 23 on legislation.gov.uk, and it is the clause a building control body will work from.
Which Approved Document L Edition Applies to Your Project
There are currently two published editions of Approved Document L Volume 1, and only one of them is in effect. Getting this right before any Part L thickness requirements are discussed saves specifying against a standard that does not yet govern the work — a live risk in 2026, because the newer edition is published and freely downloadable while the older one still applies.
| Edition of Approved Document L, Volume 1 | Status as at August 2026 | Date it takes effect |
|---|---|---|
| 2021 edition incorporating 2023 amendments | The edition in effect for current building work | 15 June 2022 |
| 2026 edition, work not connected with higher-risk building work | Published, not yet in effect | 24 March 2027 |
| 2026 edition, work connected with higher-risk building work | Published, not yet in effect | 24 September 2027 |
Both editions, and the transitional wording that separates them, are published on the Approved Document L page on gov.uk. The practical reading for a retrofit being designed now is that the 2021 edition governs, with the 2026 edition worth reading early on any scheme whose construction will run past its commencement dates. Under current guidance, earlier editions continue to apply to buildings subject to the previous standards, so a long programme can legitimately sit under the older document from start to finish.
One figure from the 2021 edition is worth quoting precisely, because it is routinely repeated out of scope. Its notional dwelling specification — the theoretical dwelling used to set targets for a new dwelling — assumes external walls at U = 0.18 W/(m²·K). That is a reference value for target setting in a new-build calculation, not a limiting value for a renovated solid wall, and the two are not interchangeable. Where a renovation target is needed, it is read from the renovation tables in the same document for the specific project route.
Why Board Thickness Alone Never Demonstrates Compliance
Insulation thickness for building regulations purposes is an output, never an input. Compliance is demonstrated on the U-value of the completed element, which counts the existing masonry, the adhesive bed, the board, the reinforced basecoat, the render finish and the internal and external surface resistances together. The insulation dominates the result, and it still cannot be assessed alone.
Key Takeaway: No board depth satisfies Part L by itself. The regulation sets a performance standard for the whole wall, the assessment produces a required thermal resistance, and the board depth is whatever delivers it on that specific substrate — which is why two houses on the same street can need different thicknesses.
This is also why a declared figure beats a derived one. Thickness divided by lambda and the manufacturer's declared thermal resistance for the same board differ by rounding, and only the declared value carries forward into a Declaration of Performance. Working from the declared row keeps the submission consistent with the document behind it, which is the version a building control body can check. A qualified U-value assessment remains the step that turns those declared figures into a compliant specification.
Reading Declared Thermal Resistance From Board Documents
The graphite EPS boards Renders World stocks are declared to EN 13163:2012+A1:2015 with a thermal conductivity of λD 0,032 W/mK, and the technical sheet publishes a declared thermal resistance against each thickness. Those declared values are the numbers to carry into an assessment, and the table below covers the depths most often specified on UK retrofits.
| Board thickness | Declared thermal resistance RD | Where it typically lands |
|---|---|---|
| 50 mm | 1.55 m²K/W | Reveals, soffits and constrained details |
| 80 mm | 2.50 m²K/W | The common domestic retrofit depth |
| 90 mm | 2.80 m²K/W | Where reveal space allows a step up |
| 100 mm | 3.10 m²K/W | Standard full-facade solid wall work |
| 150 mm | 4.65 m²K/W | Deeper renovation specifications |
| 200 mm | 6.25 m²K/W | Low-energy retrofit territory |
Note what the table does not do: it does not tell you which row your project needs. It supplies one operand, and the existing wall's measured resistance supplies the other. Graphite EPS insulation boards give the thinnest build-up for a given resistance, which is the reason they dominate solid-wall retrofit — a lower conductivity protects reveal depths, sill projections and the finished proportions of the elevation.
Plinth and below-DPC boards behave differently and are worth calculating separately. The hydrophobic foundation board sold for that zone is declared at λD 0,036 W/mK with water absorption of 3% under long-term total immersion, so the same resistance takes more depth there than it does on the field of the wall. Planning that step early keeps the plinth line and the wall line meeting cleanly.
How to Specify Board Depth on a UK Retrofit
The sequence below is the one that produces a defensible thickness calculation for Part L UK projects without guessing at any stage. It is deliberately short, because most of the judgement sits in the first and last steps rather than in the middle.
- Confirm the edition and the route. Establish whether the work is a renovation, a replacement or a new element, and which Approved Document L edition governs the application date.
- Confirm the trigger. Check the proportion of the element being worked on against the Regulation 23 thresholds, since this decides whether the upgrade duty applies at all.
- Get the existing wall assessed. The resistance of the substrate is measured or established by a competent assessor, not assumed from the construction type.
- Take declared resistance from the board document. Use the declared row for the thickness under consideration rather than dividing thickness by conductivity.
- Round up to a stocked depth. Specifying the next depth up gives the finished value margin, and it keeps the order to boards that are actually on the shelf.
- Check the knock-on details. Board depth drives fixing length, reveal returns, sill projection and base track width, and these are cheaper to resolve on paper.
Where a scheme also touches permitted development or listed status, the consenting route is worth confirming alongside the thermal one — the planning permission and permitted development rules for external wall insulation cover that ground. For projects being designed against the standards coming down the track, the Future Homes Standard requirements for 2026 set out where new-build fabric expectations are heading.
Renders World holds the graphite EPS, mineral wool and plinth board ranges together with the adhesives, mesh and fixings that complete the build-up, so a depth settled on paper can be ordered as one specification. The full systems range and its component collections are the practical next step once your target resistance is known, and the technical desk can talk through board depth against reveal and sill constraints on awkward elevations.
Written by Mariusz Saja. Technically reviewed by Rafał Wyrzykowski. Last reviewed Aug 2026.
FAQ — Part L Thickness, Editions and Compliance
Does Part L set a minimum insulation thickness?
No depth appears anywhere in the regulation. The standard is expressed as thermal performance for the completed element, and depth is whatever delivers that performance on the wall in front of you. Two identical boards can therefore be correct on one house and insufficient on its neighbour.
Which edition of Approved Document L applies to my project right now?
Work being carried out in 2026 falls under the 2021 edition incorporating the 2023 amendments, which took effect on 15 June 2022. The 2026 edition is published but takes effect on 24 March 2027, and on 24 September 2027 for higher-risk building work.
How much of a wall can I renovate before the upgrade duty applies?
Regulation 23 sets two independent triggers, and meeting either one brings the whole element into scope:
- The work constitutes a major renovation of the thermal element.
- The work amounts to more than 50% of the element's surface area.
Should I use declared thermal resistance or divide thickness by lambda?
Use the declared resistance published against that thickness. The two methods disagree slightly through rounding, and the declared value is the one carried on the manufacturer's documentation, which keeps your submission consistent with the evidence behind it.
Is a thicker board always the safer specification?
Extra depth adds resistance, and it also pushes out reveals, sills, verges and fixing lengths. The comfortable position is the depth that clears your assessed target with a margin, checked against the detailing it has to work with — which is why the calculation is done before ordering rather than after.