Getting insulation thickness right on an external wall is an arithmetic job with one trap in it, and the trap is the number most people reach for first. The declared thermal conductivity of a board is not what a compliant calculation uses at thickness level — the declared thermal resistance is. This guide sets out the method, works with figures read from the GENDERKA sheets behind the Renders World graphite-enhanced EPS insulation boards, and is explicit about which numbers a specifier has to bring from elsewhere.
How to Calculate a U-Value for an Insulated Wall
A U-value calculation for wall insulation thickness starts with the declared thermal resistance of the board, not its lambda: GENDERKA declares λD 0,032 W/mK for EPS 032 Fasada Extra, and BS EN ISO 6946 provides the summing method. Get the order of operations right and the rest is addition.
The method itself is short. Each layer of the assembly contributes a thermal resistance in m²K/W, the surface films at the inner and outer faces contribute two more, and the U-value is the reciprocal of the total. Where a layer is homogeneous, its resistance is thickness in metres divided by design conductivity; where the manufacturer declares a resistance against a thickness, as GENDERKA does, that declared value is the one to use.
- Take the declared resistance for the insulation at the thickness you intend to fit, from the row matching that thickness on the manufacturer's sheet or Declaration of Performance.
- Calculate the resistance of every other layer — plaster, masonry, adhesive bed, reinforced basecoat, render — as thickness divided by the design conductivity for that material.
- Add the internal and external surface resistances at the values the convention fixes for walls with horizontal heat flow. These are set by the calculation conventions, not chosen by the assessor, and leaving them out is the most common informal-calculation error.
- Sum every resistance and invert it. U = 1 ÷ Rtotal, in W/m²K.
Two inputs in that list do not come from a render supplier and are not published here: the design conductivities of the existing substrate and the fixed surface resistances. Both belong to the conventions document and to published thermal property tables, and a specifier working to Building Control takes them from there rather than from a merchant's page. That boundary is deliberate — a substrate conductivity guessed generously is the fastest route to an under-specified board.
Declared Thermal Resistance, and Why Thickness ÷ Lambda Falls Short
Dividing thickness by lambda and declaring resistance are not the same operation, and the gap is large enough to matter on a marginal specification. At 100 mm, division by 0,032 returns 3.125 m²K/W; the GENDERKA sheet and Declaration of Performance both declare 3.10 m²K/W. The declared figure is the one that appears on the DoP, so it is the one a Retrofit Coordinator or Building Control officer can trace.
The table below carries the declared resistance for each thickness Renders World stocks in this range, read from the technical sheet and confirmed against the DoP for the Bydgoszcz plant. Both documents state the same table, which is a useful cross-check in itself.
| Board thickness | Declared thermal resistance RD (m²K/W) | Typical position in the build-up |
|---|---|---|
| 10 mm | 0.30 | Reveals and junctions |
| 20 mm | 0.60 | Soffits and thin returns |
| 30 mm | 0.90 | Detailing and infill |
| 50 mm | 1.55 | Light upgrade, restricted depth |
| 80 mm | 2.50 | Common domestic retrofit depth |
| 90 mm | 2.80 | Domestic retrofit |
| 100 mm | 3.10 | Domestic retrofit with margin |
| 150 mm | 4.65 | Deep retrofit |
| 160 mm | 5.00 | Deep retrofit |
| 180 mm | 5.60 | Low-energy work |
| 200 mm | 6.25 | Low-energy work |
Stepping from 100 mm to 200 mm adds 6.25 − 3.10 = 3.15 m²K/W to the assembly, which is more resistance than most solid masonry walls contribute in total. That is the arithmetic reason insulation thickness dominates the result, and the reason the board decision is worth making on declared figures rather than on a rounded lambda.
One Row on the Sheet Is Not Publishable
Both GENDERKA documents state 3.40 m²K/W at 110 mm and 3.40 m²K/W again at 120 mm, in a table that otherwise rises steadily, then jump to 4.05 at 130 mm. A repeated value inside a monotonic series is a transcription fault rather than a physical result, so neither figure appears in the table above and no resistance is published here for either thickness. Neither is a stocked thickness, so nothing on the shelf is affected — but anyone reading that sheet directly should treat the 120 mm cell as unresolved and ask GENDERKA to confirm it before using it in a submission.
Which Documents Govern the Calculation
Three documents sit behind a UK wall U-value, and knowing what each one does prevents citing the wrong one at the wrong scope. Two are calculation standards and one is statutory guidance, and only the last of them sets a target.
- BS EN ISO 6946 — the calculation method. The current edition is edition 3, published June 2017 with a corrected version issued December 2021 and confirmed as current in 2022. It covers components of thermally homogeneous layers and explicitly excludes windows, doors, curtain walling, ground-coupled components and elements designed to let air through, so a ventilated rainscreen is not simply an ISO 6946 sum.
- BR 443, Conventions for U-value Calculations — the UK conventions layered on top of the standard, fixing surface resistances and correction factors. Worth one caution: BRE's own companion volume of worked examples, U-value conventions in practice, is based on the 2006 edition and BRE states it is not consistent with the later edition. Working through those examples without checking the convention against the current edition is a real and easily made error.
- Approved Document L, Volume 1: Dwellings — the statutory guidance that sets limiting standards. A 2026 edition now exists and takes effect in England on 24 March 2027 for building work not connected with higher-risk building work, and on 24 September 2027 where it is. Its Section 3 is where the limiting U-values for new and existing dwellings live, and it names both the Home Energy Model and SAP as approved calculation methodologies.
Targets themselves are a compliance question rather than a calculation one, and the honest position is that a limiting value should be read out of the edition of Approved Document L that governs your project on its start date, not lifted from a supplier's guide. One published figure is worth stating with its scope: the government's Future Homes and Buildings Standards consultation set an external wall U-value of 0.18 W/m²K in both proposed notional dwelling specifications. That is a notional building value in a consultation that closed in March 2024, used to set a whole-dwelling target — not an elemental backstop, and not a figure any single board thickness satisfies on its own.
What Shifts the Calculated Result on a Real Wall
A clean layer-by-layer sum describes a wall that does not exist, and the adjustments are where an experienced assessor earns the fee. Each of the following is handled inside the standards rather than by adding a safety margin at the end, and each has a documented home.
- Mechanical fixings bridge the insulation, and ISO 6946 handles metal fasteners through a correction term in its Annex F rather than ignoring them; cases where insulation is bridged by metal more substantially fall outside the standard altogether. Fixing density is a manufacturer and system figure, so take it from the system specification — the fixing pattern and spacing guide covers how the layout is set.
- Condensation risk can legitimately relax the target. Approved Document L's 2026 edition states directly that, to protect against the harmful effects of interstitial and surface condensation, a lesser standard may be acceptable, referring the reader to Approved Document C. That is a designed-in escape route for moisture-sensitive walls, and the dew point and condensation risk guide sets out the physics behind the judgement.
- Historic and traditional construction is treated differently again: the same document says energy efficiency should be improved only where doing so will not cause long-term deterioration of the fabric, naming vapour-permeable constructions such as cob, stone and lime-rendered walls, and it points to Historic England's guidance. Thickness is a secondary question on those walls.
- Substrate conductivity is an input, not a constant. Dense masonry, lightweight block and rubble-filled stone behave very differently, and the same board reaches a different U-value on each. Where the wall is non-standard, the substrate figure needs a proper source before the sum is worth running.
Reading the assembly as a system rather than a stack of layers makes these adjustments easier to place, and the EWI system build-up and its functional layers is the companion piece for that. Renders World supplies the layers as a set for exactly this reason: the calculation assumes continuity, and continuity is a specification decision.
Turning the Answer into a Specification
Once the target resistance is known, the board choice becomes a single lookup against the declared table above, and it is worth carrying one step of headroom where the wall allows it. The 100 mm graphite board at 3.10 m²K/W covers most domestic retrofit work, while the 160 mm board at 5.00 m²K/W is where deep-retrofit specifications tend to land. Both carry the same declared conductivity, so the resistance step is the only variable in play.
Key Takeaway: Use the declared thermal resistance for the thickness you are fitting, not thickness divided by lambda — at 100 mm the GENDERKA sheet declares 3.10 m²K/W where division returns 3.125. Bring substrate conductivities and surface resistances from the conventions document, read the limiting standard from the edition of Approved Document L that governs your start date, and keep the 120 mm row on that sheet out of any submission until the manufacturer confirms it.
Two practical habits keep a calculation defensible. Build it one row per layer in a spreadsheet so a thickness change is a single edit, and keep the manufacturer's sheet and Declaration of Performance filed alongside it, because the DoP is what carries the declared resistance and the plant that declared it. GENDERKA declares this product from five plants, so where a DoP number goes into a submission, the supplying plant is worth confirming at the point of order.
The layer set the calculation assumes — board, adhesive, fixings, reinforced basecoat, mesh, primer and finish — is available from Renders World as a single specification through the complete EWI system bundle, priced per square metre. For the wider silo, the external wall insulation category shows which collection owns which layer of the build-up, and the full range of graphite boards from 10 mm to 200 mm sits behind the declared resistance table on this page.
Written by Mariusz Saja. Technically reviewed by Rafał Wyrzykowski. Last reviewed Aug 2026.
Frequently Asked Questions
Is there a quick way to estimate the thickness needed?
A rough sizing step is reasonable, but it should not be the figure that goes into a submission. Work out how much resistance the assembly is short of, then read the nearest thickness up from the declared table above rather than dividing by lambda, since the declared and derived values diverge slightly at every thickness. The full sum then confirms it, and the substrate figure is the input most likely to move the answer.
How does the calculation change with mineral wool instead of EPS?
The method is identical — declared resistance per thickness, summed with the other layers and the surface films. What changes is the declared figure itself, which comes from that slab's own Declaration of Performance, and the fire and vapour considerations that often drive the choice in the first place. Those sit outside a U-value sum entirely, and the graphite EPS and mineral wool comparison takes them properly.
Does an air cavity go into the calculation?
It depends on the system. A directly bonded external wall insulation build-up has no designed cavity, so the layers are summed as a continuous assembly. A ventilated rainscreen is a different case: BS EN ISO 6946 excludes components through which air is designed to permeate, so that build-up is not a simple ISO 6946 sum and needs the appropriate treatment for the cavity condition.
Why do declared and design conductivity differ?
A declared value is the manufacturer's figure under standard reference conditions and is what appears on the Declaration of Performance; a design value is the figure appropriate to the application, and the conventions document sets out when and how the two differ. For a Building Control submission, work from the declared value on the specific product's documentation and apply the convention rather than a rule of thumb, which is why the sheet and DoP are worth keeping with the calculation.
Does a given thickness make a wall compliant?
No single thickness does, because compliance is a property of the whole build-up and, for a new dwelling, of the whole-dwelling calculation. A thickness contributes a declared resistance; the substrate, the other layers, the surface films and any correction factors complete the sum, and the target itself comes from the governing edition of Approved Document L. Specify the thickness from the calculated shortfall and keep the working.