The specification instinct on a traditional wall is to reach for the highest vapour permeability class available and treat the number as the answer. Historic England's own guidance is more careful than that, and so is the evidence for the product this page previously recommended: the Atlas silicone-silicate render has no readable datasheet, and every classification published against it here came from somewhere other than a document. What follows is the honest version — the conservation principle from the authority that sets it, and the product facts from the only certificate that names the render.
What Actually Governs Moisture in a Traditional Wall
Heritage render specification turns on liquid water rather than vapour class alone, and Historic England states the priority plainly: although vapour permeability receives much attention in conservation and product literature, the movement of water as a liquid is of greater significance. Vapour diffusion through porous materials is relatively slow by comparison, while liquid water moves quickly through connected pores towards an evaporative surface, driven by capillary forces strong enough to raise it against gravity.
That single passage should change how a render is chosen for an old building. A vapour class describes one transport mechanism; the wall is dealing with several at once. Historic England describes how traditional construction actually manages moisture, and the mechanisms are largely architectural rather than chemical.
- Deflection before absorption. Roof coverings, overhangs, plinths, eaves, cills and hoods with drip details move rainwater into disposal systems before the wall has to deal with it — the principle summarised in conservation practice as giving a building a good hat and boots.
- Thickness and construction type. Solid masonry walls generally rely on their thickness to prevent moisture reaching the interior face, with render increasing resistance to penetration rather than replacing that thickness.
- Absorb, store, transport, release. Porous hygroscopic materials buffer moisture, taking it up and releasing it again as conditions change, which is a storage function no thin coating provides.
- Surface area aids evaporation. In exposed locations, roughcast — wet dash in the north, harling in Scotland — or aggregate in lime coatings increased surface area specifically to help the wall dry.
Historic England is also explicit about the risk at the other end: modern methods using materials highly resistant to the passage of water vapour or liquid water are not usually appropriate for traditionally constructed buildings, because they risk trapping moisture, increasing fabric decay and affecting occupant health. Note the framing — the caution is against resistant materials, not an endorsement of any particular permeable product.
What the Certificate Says About the Silicone-Silicate Render
One document names this render, and it is BBA Agrément Certificate 13/5018. It states that Atlas Silicone-Silicate Render is a finishing coat of the Atlas/Aval EPS external wall insulation system, in 1.5 to 2.0 mm grain, at a coverage of 2.5 to 3.0 kg/m², applied over an Atlas Silkon ANX key coat. That is the full extent of what is documented.
No vapour permeability class, no water absorption class, no adhesion figure, no reaction-to-fire class and no Declaration of Performance number appears in any source that could be retrieved. Two Atlas pointers for this product were attempted this session and both refused, as they have on every previous attempt. The practical consequence is worth stating directly, because the previously published version of this page did the opposite: this render cannot currently be specified to a conservation officer on documented breathability, because no breathability figure for it exists in a readable document.
Where a certified permeability figure is genuinely required in a submission, the route is a product whose sheet has been read. Ceresit CT 74 declares V1 at Sd ≤ 0.14 m with W3 absorption, w ≤ 0.1 kg/m²h⁰·⁵, and 0.6 MPa adhesion under EN 15824 — the most vapour-open classification in the silicone render range, and a documented one. Atlas Silicone Render declares V2 at 0.14 ≤ Sd < 1.4 m, which remains a high permeability band, and is named in the same certificate as a finishing coat at 2.5 to 3.5 kg/m².
Key Takeaway: the silicone-silicate render has no readable datasheet, so no vapour class, fire class or DoP number can be published for it — only the BBA-certified coverage of 2.5 to 3.0 kg/m² and its role as a finishing coat over a Silkon ANX key coat. Where a documented permeability figure is needed for consent, CT 74 at V1 is the render whose sheet supports the claim.
Specifying Honestly Where Consent Is Involved
Listed building consent and conservation area work put a specification in front of someone who may check it, which makes an unsupported figure a practical risk rather than only an editorial one. Four points hold on the documents available.
- Lead the submission on the wall, not the product. Historic England's own emphasis is liquid water movement, rainwater deflection and the fabric's capacity to absorb and release moisture. A specification that engages with those mechanisms reads as competent; one that cites a single vapour class does not.
- Cite only figures you can produce on request. For the silicone-silicate render that means the BBA coverage figure and its system role. Claiming V1, A2-s1,d0 or a DoP number for it, as this page previously did, invites a request for the document that supports them.
- Check whether the certificate's scope matches your wall. BBA 13/5018 covers this render as a finishing coat of an EPS insulation system. A traditional solid wall being rendered without insulation is outside that build-up, so the certificate describes the product's role rather than certifying your application of it.
- Treat consent requirements as a matter for the authority. Whether a particular re-rendering needs listed building consent, and what a conservation officer will accept, is determined by the local planning authority. This page previously asserted approval patterns and rejection tendencies with no source behind them; those claims are withdrawn.
Where the project combines heritage fabric with a thermal upgrade, the solid wall insulation approach for Victorian and Edwardian properties covers the survey and build-up decisions that come before the finish coat.
Preparation, Application Window and Quantities
The certificate specifies Atlas Silkon ANX as the key coat beneath this render, which makes the priming sequence part of the documented specification rather than a matter of preference. Suitable products sit in the exterior render primers range, and the preparation sequence for different substrates covers high-suction and friable surfaces, which is what most traditional masonry presents.
Application conditions differ by product and are not interchangeable. Atlas allows +5 to +30 °C with relative humidity below 80%, extended above 0 °C using ATLAS ESKIMO setting accelerator on the Atlas renders. Ceresit sets +5 to +25 °C below 80% humidity for CT 74. Heritage elevations are frequently sheltered, shaded and slow to dry, so the timing guidance for silicone render application is worth reading before committing to a shoulder-season programme.
Quantities follow the certificate figure. For a 100 m² elevation at 2.5 to 3.0 kg/m²: 100 × 2.5–3.0 = 250–300 kg, divided by 25 kg, gives 10 to 12 buckets. On the same wall the Atlas Silicone Render at its certified 2.5 to 3.5 kg/m² gives 250–350 kg, or 10 to 14 buckets. Add 5 to 10% for reveals and detailing on an irregular traditional facade, where the allowance genuinely earns its place, and order each elevation in a single batch.
Colour matters more on heritage work than the arithmetic suggests, since a shade has to sit against surviving original fabric rather than against a colour card. The render colour charts hold the physical samples, and confirming a shade at the specified grain in natural daylight is the step that prevents a mismatch no specification can correct.
Where This Leaves a Heritage Specification
The useful conclusion is not the one this page previously reached. A silicone-silicate hybrid remains a reasonable material proposition on traditional fabric, and the mineral binder argument is genuine — but at Renders World the specific SKU cannot presently be documented, and a conservation submission built on undocumented figures is weaker than one built on fewer, verifiable ones. Three routes follow from that.
- Where a documented permeability figure is required: specify CT 74 at V1, Sd ≤ 0.14 m, with the sheet available for submission.
- Where the certified system role is what matters: the silicone-silicate render at BBA-stated coverage over a Silkon ANX key coat, with the certificate produced as the supporting document and no further claims attached.
- Where the fabric is genuinely sensitive: follow Historic England's emphasis and address rainwater deflection, wall thickness and evaporative capacity first, treating the finish coat as one part of that system rather than the solution to it.
Renders World can supply the certificate and the datasheets that exist, and will say plainly which figures are documented and which are not. Bring the substrate, the consent position and the shade target to the technical desk and the specification can be assembled from sources that will survive a conservation officer reading them.
Written by Mariusz Saja. Technically reviewed by Rafał Wyrzykowski. Last reviewed Aug 2026.
Heritage Render Questions
Is the silicone-silicate render V1 classified?
No document available to this site states a vapour class for it. Two manufacturer pointers were attempted again this session and both refused, so the claim cannot be published. If a V1 classification is required for a submission, CT 74 declares V1 at Sd ≤ 0.14 m on a sheet that can be produced.
How much render does a 100 m² heritage elevation take?
Ten to twelve 25 kg buckets at the BBA-certified rate of 2.5 to 3.0 kg/m², before waste. Traditional facades are rarely flat, so add 5 to 10% for detailing and irregularity, and keep the whole elevation on one batch to avoid a visible shade break.
Do I need listed building consent to re-render?
That is a determination for your local planning authority, and it depends on the building and the work proposed. Historic England publishes technical guidance on traditional construction, and the Planning Portal covers the consent process itself. This page makes no claim about what any authority will require or accept.
Is this an alternative to lime render?
They are different materials and the comparison needs care. Historic England notes that traditional walls were rendered in lime and earth, with roughcast or aggregate increasing surface area to aid evaporation. A thin-coat finish does not replicate the moisture storage a thick lime coat provides, so on sensitive fabric the question is which mechanisms the wall depends on rather than which product has the better number.
What goes underneath it?
BBA 13/5018 specifies an Atlas Silkon ANX key coat beneath this render within the certified system. Traditional substrates are usually high-suction and often friable, so a consolidating primer beneath the key coat is common practice, and the substrate preparation guide covers the sequence by wall type.