The complete guide
Breathable paint for old Scottish houses.
Half the peeling walls we are asked to look at in Fife were painted with the right intentions and the wrong product. This is the guide we wish every homeowner read before the first tin was opened: what breathable means, which of the five systems belongs on your wall, how each one fails, what it costs and how to test it on a square metre before you commit to a gable.
By Aethos Studio, St Andrews · Published
Written from our decorating practice on sandstone, harl and lime plaster along the east coast, and checked against manufacturer data sheets and the permeability classes in EN ISO 7783. Where we give figures they are ranges you can verify, not promises.
01
Why old walls need to breathe
A house built before about 1919 in Scotland is almost certainly solid walled. Stone or brick, bedded and pointed in lime mortar, plastered inside with lime, harled outside with lime. There is no cavity and no damp proof course. The wall gets wet when it rains and dries out when it stops. That drying happens through the surface, in both directions, as water vapour.
Lime is porous enough to let that happen. Lime plaster, lime harl and lime paint are, in the language of the data sheets, highly vapour permeable. The wall works as a system: moisture that gets in gets out again before it does harm.
Modern paints were not designed for that. A standard vinyl emulsion or acrylic masonry paint forms a continuous plastic film. It is excellent at keeping rain off a cavity wall and useless at letting a solid wall dry. Moisture that enters through a crack, a failed cill or simply from the ground has nowhere to go. It builds up behind the film, dissolves salts out of the stone, freezes in winter, and eventually pushes the paint off along with the surface of the plaster it was supposed to protect.
Breathable paint, properly chosen, restores the arrangement the building was designed around. That is the whole argument. Everything else in this guide is about the word properly.
02
First, which wall have you got?
Ten minutes with a scraper, a spray bottle and a bottle of vinegar answers most of it. Do this on every elevation and in more than one room. Old houses have been altered.
The scrape test
Pick a hidden spot low on the wall. Scrape with a stiff blade until you reach the substrate. Lime plaster and harl are soft, pale and sandy and come away as powder. Cement is hard, grey and chips. Gypsum, inside, is pink or grey, smooth and slightly softer than cement.
The vinegar test
A drop of white vinegar on the exposed material. Lime fizzes visibly. Cement barely reacts. Gypsum does nothing.
The water test
Spray clean water on the existing painted surface. If it beads and runs, you are looking at a plastic film. If it darkens and soaks in within a minute, the coating is permeable or there is no coating at all.
The layer count
Look at the scraped edge under a torch. Count the paint layers and note their character. Chalky white layers that come away as dust are limewash or distemper. Flexible skins that peel are modern. Most Fife houses have both.
The moisture check
A cheap pin meter is not accurate on masonry but it is good at comparison. Read the wall at knee height and at head height, on the north side and the south. A big difference low down points to ground moisture that no paint will fix.
03
The five systems, compared
Breathable is a marketing word. These five things are what people actually mean by it, and they behave very differently. Costs are indicative 2026 material prices per square metre for the full number of coats, excluding primer and labour. Lifespans assume the right substrate and a competent job.
| System | What it is | How it breathes | Belongs on | Never on | Coats | Life | Look | Material per m² | Skill |
|---|---|---|---|---|---|---|---|---|---|
| Limewash | Slaked lime and water, sometimes with earth pigment and a little casein or linseed. | Fully. It is the wall, in paint form. | Bare lime plaster, lime harl, soft sandstone, old brick. Inside and out. | Anything already coated in modern paint, cement render, gypsum plaster, plasterboard. | 4 to 6 thin coats, misted between | 3 to 7 years outside, far longer inside | Soft, chalky, slightly uneven. Colour shifts as it dries and with the weather. | £3 to £6 | High. Thin coats, damp wall, patience, the right day. |
| Silicate or mineral paint | Potassium silicate binder that chemically bonds with mineral substrates. | Very well. Sd values around 0.01 to 0.05 m on most data sheets. | Bare masonry, lime or cement render, previously mineral painted walls. Excellent outside. | Sound old vinyl or acrylic coatings it cannot bond to. Timber. Plasterboard without the right primer. | 2, over a system primer | 15 to 25 years outside when the substrate is right | Dead flat, very even, mineral rather than chalky. | £7 to £12 | Moderate. Follow the system, protect glass and stone, mind the weather. |
| Clay paint | Clay and chalk with a small plant or acrylic binder. | Well for an interior paint. Absorbs and releases room moisture. | Interior lime or gypsum plaster, plasterboard, over sound existing matt paint. | Exteriors, bathrooms with poor ventilation, anywhere that needs scrubbing. | 2 | Indefinite inside if not abused | Deep matt, velvety, very forgiving of uneven plaster. | £5 to £9 | Low to moderate. Dries fast, so keep a wet edge. |
| Soft distemper | Chalk, water and animal or plant glue. | Fully. | Ceilings and ornate plaster where you may one day want to wash it off. | Walls that get touched, kitchens, anywhere damp. | 2 | Until it is washed off | The softest white there is. Matt beyond matt. | £2 to £4 | Moderate. Mixed warm, applied quickly, not recoated over itself without removal. |
| Microporous masonry or wall paint | Acrylic or silicone resin formulated for higher vapour permeability than standard paint. | Somewhat. Sd values vary enormously; read the number, not the word on the tin. | Previously painted sound render where stripping is unrealistic. | Bare lime work. A wall that is already damp. | 2 | 8 to 15 years outside | Like a good modern paint, because it is one. | £4 to £8 | Low. Normal decorating technique. |
For comparison, a standard vinyl matt or acrylic masonry paint costs £1.50 to £4 per square metre in materials, lasts 5 to 10 years outside and has an Sd value typically between 0.3 and 1.5 m. On a cavity wall or a cement render that is a perfectly good choice. On lime it is the beginning of the peeling story.
04
The decision path we use
Answer in order. Stop at the first line that describes your wall.
- 01
Is the wall wet, or is there an unfixed source of water?
Stop. Fix gutters, cills, pointing, ground levels and ventilation first. Paint nothing for a season and let it dry. Any product applied now will fail and take the evidence with it.
- 02
Is the substrate cement render, gypsum or plasterboard?
Breathability is not your problem. Use a good modern paint, or clay paint inside if you like the finish. Save the money for the rooms where it matters.
- 03
Is it bare lime, or lime under nothing but old limewash and distemper?
Limewash. Wet the wall, thin coats, four or more, in dull settled weather. Outside, expect to recoat the exposed elevations every few years. It is cheap, it is honest and it is what the wall expects.
- 04
Is it lime or stone with a failing modern coating on top?
Remove the coating. Steam, poultice or careful mechanical removal, never aggressive blasting on soft sandstone. Then limewash or silicate. Removal is most of the cost and there is no product that lets you skip it.
- 05
Is it lime or stone under a sound, well bonded modern coating?
The hardest case. Removal may do more harm than good. A silicate system designed for previously painted mineral surfaces, after a three panel adhesion test, is usually the pragmatic answer. Accept that you are improving the wall, not restoring it.
- 06
Is it interior lime plaster in a lived in room?
Clay paint for walls that get touched, limewash or soft distemper for ceilings and ornate work. Both let the plaster keep managing humidity.
05
How breathable paint fails
These are the six failures we are called to most often. Every one of them was a good product used in the wrong place or the wrong way. Recognising them saves you from repeating them.
Peeling in sheets within two years
Breathable paint applied over an old plastic coating. The new paint breathes, the layer under it does not. Moisture collects at the boundary and pushes the lot off together. The cure is removal, not another product.
Powdering and chalking to the touch
Limewash applied to a dry, dusty wall without wetting it down first, or too thick, or on a hot day. The lime carbonates on the surface before it bonds. Brush off, wet the wall properly and apply thin coats in dull weather.
A white bloom on silicate paint
Efflorescence from salts in the masonry, or the paint applied in cold damp conditions. Often cosmetic and brushes off after a season, but persistent salt tells you water is moving through the wall for a reason worth finding.
Patchy colour and tide marks
Different suction across the wall: old repairs in cement, filled cracks, areas that stayed wet. On limewash it can be part of the charm. On silicate it usually means the primer was skipped or the wall was not evenly damp.
Green and black growth on the north wall
A breathable paint keeps the surface slightly more open, and a damp shaded elevation grows things. Check gutters, ground levels and overhanging trees before blaming the paint. Some systems include a biocide; it delays growth, it does not prevent it.
Paint failing only around the windows and skews
Water getting in at the detail, not a coating problem. Failed pointing, cracked cills, missing lead. Fix the detail first or you will paint this corner every spring.
06
Run a three panel test
This is how we decide on a real job. A square metre and a month tells you more than any brochure. Do it on the worst elevation, not the best.
- 01
Mark out three panels
Each about one metre square, side by side on the same wall, at the same height, ideally on the north or the weather side. Photograph them with a date.
- 02
Prepare each one exactly as you would the whole wall
Panel one: your first choice system with its own primer. Panel two: the runner up. Panel three: the same as panel one but without the primer, or over the existing coating, whichever shortcut you are tempted by. The third panel is the important one.
- 03
Record what you used
Product, batch, the weather that day, how wet the wall was, how many coats and how long between them. Write it on the back of the photograph or in a note on your phone.
- 04
Wait through weather
A minimum of four weeks including proper rain and at least one cold night. Longer is better. A summer test on a sheltered wall proves very little.
- 05
Test adhesion
Score a small cross hatch through each panel with a sharp knife, press firm tape over it and pull it off in one movement. Count the squares that come away. Anything more than a few on the primed panels is a failing system on that wall.
- 06
Test water
Spray each panel. Watch how fast it darkens and how long it takes to dry back to an even colour compared to the bare wall beside it. A panel that stays dark for hours after the rest has dried is holding water.
- 07
Look at it in different light
Morning, evening, wet, dry. Colour and texture change more on breathable finishes than on modern paint. Decide whether you like it before you buy forty litres.
Our own panels
We publish our panels. Send us yours.
When we run this test on a client wall we photograph it, log it and share the adhesion counts and drying times in the letter, with the owner's permission. If you run your own three panels, send the photographs and your notes and we will add the useful ones to this page with credit. Real walls in real weather beat brochures, and we would rather show you a failure than invent a success.
07
What it costs per square metre
Materials are the small number. Preparation and access are the big ones, and on an old house they are the same whichever paint you choose. These are honest planning ranges for the east of Scotland in 2026, for a professional job on a typical two storey elevation. Your quote will differ; use these to spot one that is wildly out.
Materials, breathable system, all coats
£4 to £12 per m²
Limewash cheapest, silicate dearest. Primers add £1 to £3.
Materials, standard masonry paint
£1.50 to £4 per m²
Cheaper to buy, and often dearer over ten years when it has to come off.
Preparation of a sound lime wall
£8 to £20 per m²
Washing, raking out, small repairs, wetting down.
Removal of a failed modern coating
£25 to £70 per m²
The number that decides most jobs. Steam and hand tools on soft stone; never blast.
Application, breathable system
£10 to £25 per m²
Limewash needs more visits for more thin coats. Silicate needs fewer, but careful masking.
Access
£800 to £3,500 per elevation
Tower or scaffold, depending on height, ground and how long the job runs. Shared with any other work you can do at the same time.
08
When not to bother
Leave it modern when
- The wall is cement rendered and sound, and you are not planning to remove the cement.
- The room is a modern extension or has been dry lined with plasterboard.
- The existing coating is bonded, dry and the wall shows no sign of moisture after a wet winter.
- You need a scrubbable surface in a busy family kitchen or a rental.
- The budget does not stretch to removal and a silicate adhesion test fails.
Go breathable when
- You have bare or lightly coated lime, harl or soft stone.
- Paint has peeled more than once in the same place and the detail above it is sound.
- You are repointing or reharling in lime anyway and the coating can go on with it.
- The building is listed or in a conservation area and the planners expect it.
- You want a finish that ages by softening rather than by peeling.
09
Questions we get asked
Is breathable paint always better for an old house?
No. A breathable paint on an impermeable substrate, such as cement render or a wall already sealed with plastic paint, gains you nothing and may fail faster. Breathability matters where the wall itself is permeable: lime plaster, lime harl, porous stone and old brick. Match the paint to the wall, not to the age of the house.
How do I know if my wall is lime or cement?
Scrape a small hidden patch. Lime is soft, pale and crumbles to a sandy powder that fizzes with a drop of vinegar. Cement is hard, grey, sharp edged and barely reacts. Old Scottish houses often have lime under a later cement skim, so check more than one spot and look at the reveals and behind downpipes.
Can I put limewash over emulsion?
Not successfully. Limewash bonds by soaking into a porous surface and carbonating within it. On emulsion it sits on top, powders and comes off on your sleeve. Strip back to bare lime, or choose a silicate system designed for previously painted mineral surfaces after a proper adhesion test.
What does Sd value mean on a paint data sheet?
Sd is the equivalent air layer thickness in metres. It tells you how much resistance the dried paint film puts up to water vapour. Under 0.14 m is classed as high permeability under the European standard; limewash and silicate paints sit far below that. A standard masonry paint may be 0.5 m or more. It is the one number that turns the word breathable into something you can compare.
How long should new lime plaster dry before painting?
Lime plaster needs to carbonate as well as dry, and it carbonates from the surface inwards. As a working rule allow at least a day per millimetre of thickness in good drying conditions, and longer in a cold Scottish winter. Limewash can go on earlier than most paints because it carbonates alongside the plaster. Silicate and clay paints want the plaster fully cured and the moisture reading within the manufacturer limit.
Will breathable paint cure my damp?
No paint cures damp. A breathable system stops the paint from trapping moisture and making the problem worse, and it lets the wall dry out once the source is fixed. Find the source first: failed gutters, high ground levels, cement pointing, blocked ventilation or a leak. Then decorate.
Keep reading
Related guides from the journal
The Aethos Letter