Silicone, sand and cement, or lime: which render for your wall?
Breathable is a marketing word. Two different measured properties hide behind it, and only one of them is usually published.
On a solid wall built before about 1919, lime. On sound modern blockwork, or a sand and cement basecoat still in good order, a silicone thin coat gives you a colour you never repaint. If the wall is damp and nobody has worked out why, none of the three. Render is a finish, not a damp treatment.
The short version
- “Breathable” has no definition. Two different measured properties get sold under it, and a render can score brilliantly on one while the other is not published.
- Ask for the vapour permeability figure in writing, the mu value or the sd value. A W class is a water repellency figure and it does not answer the question.
- Check the compressive strength class against the wall. A CS III render on soft pre-1919 brick is the cement pointing mistake at elevation scale.
- Lime is the third option and on a solid wall it is usually the right one. No render manufacturer will tell you that, because none of them sells it.
- Thin coat has a narrow weather window with a fortnight of waiting built into it. A quote that has you rendering in late November is planned wrong.
The word to stop using
“Breathable” is marketing, not a specification
Every render sold in this country is described as breathable, including renders that behave in opposite ways, because the word has no defined meaning. Underneath it sit two measured properties answering two different questions.
Capillary water absorption is liquid water going in. BS EN 998-1, the specification for rendering and plastering mortar, sorts it into classes: W0 has no requirement, W1 takes up no more than 0.40 kg/(m²·min^0.5) and W2 no more than 0.20. Lower is drier, so W2 is the most water repellent class on the scale.
Water vapour permeability is moisture going out. It is a different test to a different standard, BS EN 1015-19, which determines the steady state water vapour permeability of a hardened render, and the answer comes back as a mu value (how much more resistant the material is than still air) or an sd value, the thickness of still air that would resist vapour as much. The two are independent, so a render can be highly water repellent and that tells you nothing about whether the wall behind it can dry.
The clearest illustration is on a data sheet you can download yourself. The K Rend Mineral TC product data sheet lists its Kitemark classifications as compressive strength CS III, capillary water absorption W2, and a thermal conductivity figure. There is no mu value and no sd value anywhere on the sheet, while the description at the top of the same page says the product exhibits superior water resistance while maintaining water vapour permeability. The first half of that is a declared class. The second is an adjective.
The question to ask, in writing. Ask the renderer or the supplier for the vapour permeability of the whole build-up, basecoat and topcoat together, as a mu or sd figure. If what comes back is a W class or the word breathable, they have not answered you. On a solid wall that number decides whether the specification is safe.
Strength
A render can be too strong for the wall it is put on
The second half of the same problem is hardness. BS EN 998-1 also classes render by compressive strength: CS I is 0.4 to 2.5 N/mm², CS II 1.5 to 5.0, CS III 3.5 to 7.5, and CS IV 6 and above. A traditional lime render sits at CS I or CS II. K Rend Mineral TC declares CS III.
Put a CS III coating on soft solid wall masonry and you have repeated the cement pointing error across a whole elevation, and lime mortar against cement mortar for repointing sets out that principle in full. When the coating is stronger and less permeable than what it is stuck to, the wall takes the movement and the moisture. Old brick and soft stone lose their faces.
One gap in the numbers is worth knowing about. BS EN 998-1 covers factory-made render, which is why a bagged thin coat product has classes on the sheet at all. It does not cover site-made mortar, so a sand and cement render gauged in a mixer on your drive has no declared class of any kind. Nothing is being hidden, there is nothing to publish, and that is why the mix ratio and the coat thicknesses belong on the quote.
Side by side
Three systems, not two
Nearly every page written on this compares two. The third column answers the solid wall and it is missing everywhere, because no manufacturer sells it.
| Dimension | Silicone thin coat system | Sand and cement | Lime render |
|---|---|---|---|
| Standard | BS EN 998-1, or BS EN 15824 where the topcoat has an organic binder | Site made, so outside BS EN 998-1 | BS EN 459-1 binder |
| Typical thickness | A few millimetres over a basecoat | 15 to 20 mm in two or three coats | 15 to 25 mm in three coats |
| Compressive strength | CS III for K Rend Mineral TC | Commonly CS III to CS IV by mix | CS I to CS II |
| Water absorption | W2 for Mineral TC, the most repellent class | Not usually declared | Higher by design, and it dries out again |
| Vapour permeability | Often not on the data sheet. Ask for mu or sd | Low | High |
| How it cracks | Mesh in the basecoat controls it. Hairline cracks resist invisible repair | Shrinkage cracking is the classic failure, and water behind it lifts the render off | Fine cracking largely closes itself |
| Colour | Through coloured, never painted. Patch repairs show | Painted. Repainting is the maintenance cycle and hides repairs | Limewash, recoated every few years |
| Weather window | Tight at both ends, with a fortnight of curing in it | Frost restricted. Needs protecting from drying too fast | Narrowest. Spring to early autumn |
| Right substrate | Blockwork, a sound basecoat, or an insulation system built for it | Modern walls, or matching existing cement render | Solid pre-1919 brick, soft stone, cob |
| Relative cost | Highest | Middle | Highest labour, lowest material |
Timing
Thin coat has a narrower season than the sales pitch suggests
Thin coat is sold as the quick, low maintenance option. The application conditions on the same K Rend Mineral TC sheet tell a slower story.
- The temperature of the substrate and the surrounding air should be “1 °C and rising”. Not 1 °C. Rising.
- It should be used between 1 °C and 20 °C, so there is a ceiling as well as a floor.
- Do not apply in conditions of strong sunshine or wind, and do not apply onto damp or frozen substrates.
- Allow freshly applied render basecoats a minimum of 14 days before further work commences.
- The fresh render must be protected from rain and from drying too fast.
Read those together and a good part of the British calendar disappears. A bright, breezy day in March fails two at once. A basecoat put on in the first week of November cannot be topcoated until the third, by which time the temperature may not come back up. Render caught by a frost before it has set does not recover, the same mechanism as frost damage to brickwork and paving after a thaw.
That does not make thin coat a bad product. It makes the programme part of the specification. Ask what happens to a half finished elevation if the weather turns. What a quote should include covers how to get that answered in writing.
Exposure
There is a national map, and some elevations sit at the severe end of it
How much water a wall deals with is a measured quantity, not a guess. Approved Document C divides the country into zones for exposure to wind driven rain, then tells you to adjust the zone for the actual site.
For a site specific answer the reference is BS 8104, the code of practice for assessing exposure of walls to wind driven rain, which gives a local spell index for rain penetration and a local annual index for durability and weathering.
What the zone changes is the specification. A south west facing gable on the Dorset coast takes several times the wetting of a sheltered elevation on the same house, so a build-up that is comfortable on the second is marginal on the first. It is also where a strong, low permeability coating does the most harm, because it takes in the most water and has the same single route out. Water coming in only during driving rain is worth reading first if the damp is weather dependent.
Cracking
Render cracks are usually a specification error, not a trowel error
The standard explanation for cracked render is bad workmanship and it is usually wrong. Cracks appear where a coating cannot accommodate as much movement as the wall makes, and every wall moves: it wets and dries, it warms and cools, it settles seasonally over clay.
A soft coating absorbs that in fine cracks nobody notices. A hard, brittle one concentrates it into a few big ones. So the person who specified a CS III render for a wall that needed CS I caused the crack, and the person on the trowel gets the blame two winters later.
So re-rendering to the same specification buys a repeat. Which render cracks matter sets out how to tell a cosmetic crack from a structural one. A crack letting water in behind a low permeability render is the worst combination going: water gets in through the crack and cannot get back out through the render.
The verdict
So which render does your wall need?
Silicone thin coat suits a modern, sound substrate, sand and cement suits repairing or matching a modern render, and lime suits a solid pre-1919 wall. The two cases where none is right, a damp wall with no known cause and perished brick, cover a lot of the houses quoted for rendering every year.
Silicone thin coat is right when
The substrate is modern and sound: blockwork, a sand and cement basecoat in good order, or an insulation system built to receive it. You want a through coloured finish you will not repaint and you accept that patch repairs show. Get the vapour figure anyway, because a modern wall still has to dry.
Sand and cement is right when
You are repairing or matching an existing cement render on a modern wall, or the wall will be painted regardless. Anybody can repair it and masonry paint hides the join. Ask for the mix ratio and coat thicknesses, because there is no declared class here.
Lime is right when
The wall is solid, with no cavity, and the building is pre-1919. Neither of the other two belongs on it. A solid wall handles water by absorbing it and giving it back, and lime render is the only one of the three that lets it. This is the column the manufacturer pages leave out, for a commercial reason rather than a technical one.
Neither, and this is the important one: the wall is damp and nobody has found out why
Render over a damp solid wall traps the moisture and moves the problem inside. The outside looks tidy and the damp comes through on the internal plaster instead, often within a year. Render is not a damp treatment. Find the cause first: damp at the bottom of a wall works through the usual suspects in order of how cheap they are to rule out.
Also neither: rendering over spalled or perished brick to hide it
This gets suggested more often than you would expect and it fails predictably. The render bonds to a face that is already delaminating, so the bond is only as strong as the loose material behind it. It comes off in sheets, taking more of the face with it. Replace the perished units first.
Next steps
What to do next
Before choosing a render, work out whether the wall is solid or cavity, find the cause of any damp, and ask every renderer for the strength class, vapour figure and coats in writing. Check the planning position before changing the finish.
- Work out whether the wall is solid or cavity. Measure the brickwork at a window reveal and look for short header bricks every few courses. That alone rules out two of the three.
- If there is any damp, find the cause first. Photograph it, date it, and see whether it dries back over a dry fortnight.
- Ask every quoting renderer for three things in writing: the compressive strength class, the vapour permeability of the full build-up as a mu or sd value, and the coats and thicknesses.
- Ask what happens if the weather turns mid-job and whether protection is priced.
- Check the planning position before changing the finish. Schedule 2, Part 1 of the General Permitted Development Order takes cladding a wall with render, stone, timber, tiles or pebble dash out of permitted development on article 2(3) land, which includes conservation areas. Listed buildings are separate again, covered in listed building consent for repointing and rendering.
If you want somebody to look at the wall before deciding what goes on it, that is what our plastering and rendering work covers. Thomas Hannen prices the job himself and a free written itemised quote comes out of it. If what he finds is a solid wall that wants lime, or a downpipe that wants fixing and no render at all, that is what the quote will say.
Questions
Common questions
Is silicone render breathable?
Whether silicone render is breathable depends on a figure most data sheets do not publish. Breathability is water vapour permeability, tested to BS EN 1015-19 and expressed as a mu or sd value. The W class you will be shown instead is capillary water absorption, which measures liquid water going in rather than vapour coming out. Ask for the mu or sd figure for the whole build-up.
Can you put silicone render on an old solid wall?
It is usually the wrong choice. A thin coat system such as K Rend Mineral TC declares compressive strength class CS III, while traditional lime render sits at CS I or CS II. A coating harder and less permeable than pre-1919 brick pushes the moisture and the movement back into the masonry, and the brick face is what fails.
Will rendering a wall stop damp?
No, render is a finish, not a damp treatment. Rendering over a damp solid wall traps the moisture in the wall and the problem reappears on the internal plaster, often within a year. Find the cause first, whether that is ground levels above the damp proof course, a failed downpipe, a bridged cavity or a leaking gully.
What time of year can render be applied?
The window is narrower than most people expect. K Rend's Mineral TC data sheet asks for substrate and air temperature at 1C and rising, use between 1C and 20C, no application in strong sunshine or wind, nothing onto damp or frozen substrates, and a minimum of 14 days on a fresh basecoat before further work. Lime render is tighter still, spring to early autumn.
Why has my render cracked?
More often because the render was too strong for the wall than because it was badly applied. Every wall moves as it wets, dries, warms and cools. A soft coating spreads that movement into fine cracks; a hard, brittle one concentrates it into a few large ones. Re-rendering in the same specification tends to produce the same cracks again.
Read next
Related advice
The guides below pick up where this one leaves off, and the plastering and rendering page covers the work itself and the places we cover, if you would rather have the job looked at.
- Cracked render: which cracks matter
- Lime or cement mortar: which your wall actually needs
- Damp at the bottom of a wall
- Blown plaster and the hollow sounding wall
Plastering and rendering, and where we cover All advice articles
Who wrote this
Thomas Hannen
Thomas owns Bournemouth Roofing & Building and prices every job himself instead of sending somebody else. The company is Bournemouth Roofing & Building Ltd, number 15471719, registered in England and Wales. If you want to talk something through before you commit to anything, the site visit and the written itemised quote are both free.
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Not sure what should go on your wall?
Thomas looks at the wall first: solid or cavity, wet or dry, sound or perished. Then he puts the specification in writing, including the vapour figure, so you can compare it against anybody else's.