York rendering guide
What “breathable” means in a render specification
Breathable is used to sell everything from lime render to silicone paint, and it means different things on different data sheets. For an old York wall the useful question is simple: once rain or ground moisture gets into the wall, can it get out again through the layers you are putting on? Answering it means separating vapour from liquid water and the render from what goes over and under it.
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Three ways moisture moves
SPAB's guidance on breathability describes moisture moving through traditional materials in three ways: as water vapour, held as moisture in the pores of the material, and as liquid water drawn along by capillarity. A breathable material, in SPAB's terms, is permeable enough for absorbed and surface moisture to evaporate in ordinary conditions, so that it does not build up and cause deterioration. It has nothing to do with draughts.
That is why a lime-rendering specification for an old wall covers more than the render. The mortar, the render coats and the finish all have to let moisture leave. The same applies when insulation and a new coating are proposed for an older wall: it is the whole build-up, not the most prominent product, that decides whether the wall can dry.
Reading the numbers on data sheets
Permeability is quoted in different units, which makes comparison hard. SPAB gives a vapour resistance of about 1.0 MNs/g for a 19mm non-hydraulic lime render and suggests, as a rough guide, that suitable breathable materials for old buildings should be up to about 2.5 MNs/g. Manufacturers often quote a resistance factor instead: K Rend's K Mono monocouche, for example, is listed with a water vapour permeability figure of µ ≤ 15.
Paints and thin coats are often given an Sd value, the thickness of still air with the same resistance, where smaller means more open. K Rend's Silicone Masonry Paint is listed at Sd ≤ 0.06m, Class I, high water vapour permeability. These figures describe vapour only, for one layer at one thickness. They say nothing about liquid water, and they cannot simply be added to a figure for a different layer in different units.
Water repellent is not the same as vapour open
A silicone finish is designed to stop liquid water getting in while letting vapour out. K Rend describes its Silicone TC as water repellent while allowing water vapour to pass freely, and lists the same paint mentioned above as having low water absorption. On a sound modern wall that is a sensible combination.
On an old solid wall the picture is different. Rain does get in, through hairline cracks, joints and the masonry itself, and it has to leave again as liquid as well as vapour. SPAB advises avoiding colourless water-repellent treatments on old walls, whose long-term effects it describes as unknown, and favours limewash.
Cement render and masonry paint on old walls
SPAB's guidance on lime and cement renders explains the common failure. An impervious Portland cement render restricts evaporation; being more rigid than the wall, it develops hairline cracks; those cracks draw water in, and it becomes trapped in the fabric. Timber-framed and earth walls are most at risk of serious damage, but brick and stone suffer too.
Paint can undo a good render. SPAB notes that standard modern paint systems usually have lower permeability than limewash, with a risk of moisture becoming trapped, so a lime render finished in an ordinary masonry paint loses much of its point. When a breathable render is proposed, ask what will go over it, and whether any existing paint is coming off first.
Breathability and insulation
Insulating an old solid wall changes how it handles moisture. SPAB notes that insulation on such walls may need to buffer moisture or encourage capillary movement, especially at the base of the wall or where it is particularly exposed. Approved Document L says historic and traditional buildings of vapour-permeable construction, including stone walls and lime render, should only be improved where it will not cause long-term deterioration of the fabric.
So a proposal to insulate and render an older York wall needs more than a product described as breathable. It needs an explanation of the whole build-up, how liquid water will be managed and how the base and openings are detailed. Historic England's guidance on external wall insulation in older buildings sets out the questions.
Questions to ask about a breathable claim
Ask which layer the claim refers to: the render, the finish or the whole system. Ask which property: vapour permeability, liquid water absorption or both. Ask for the figure, its unit and the thickness it applies to. Ask what is already on the wall, such as old paint or cement patches, and whether it is staying.
Then ask what goes on top and how it will be maintained. A breathable lime render needs limewash or a compatible mineral finish, renewed as it wears. A silicone system needs a sound background and gentle cleaning. If the answers are missing, the word breathable is not doing any work in the quotation.
Separate the material from the defect
If a wall is wet, look first for the source. SPAB's advice on rainwater fittings notes that downpipes leak at joints, gutters crack where their fixings rust, and render fails locally where water concentrates. A new breathable render will not cure a leaking gutter, a sill without a drip or ground built up over the damp-proof course.
Keep a record of what was retained, removed and added, with product names and the finish, so the next person to work on the wall knows what it is made of. The rendering options for York properties cover repairs, replacement finishes and insulation if the survey points to more than a change of material.
Common questions
Can I apply the same advice to an extension?
Assess it separately. A modern block extension and an old solid wall handle moisture differently, so a silicone system may suit the first and lime the second. The junction between them needs its own detail.
Is lime the only breathable render?
No. Modern silicone and some cement-based renders are described as vapour permeable, and K Rend publishes figures for its products. The difference on an old wall is how lime handles liquid water and moves with the masonry, which is why SPAB recommends it for traditional buildings.
See housing and planning information for York and nearby places, including the source and geography behind each figure.
