If your walls are cob, stone or solid brick, the answer is lime, and using modern plaster instead will cause you real problems.
The rule is simple: old solid walls need to breathe, modern cavity walls do not.
Cob, stone and solid brick walls were built to let moisture move in and out. They get damp, and they dry out again. Lime plaster is breathable, so it lets that happen. Modern gypsum plaster and cement render are far less permeable, put them on an old wall and the moisture that gets in cannot get out.
It has to go somewhere. It ends up in the timbers, in the floors, and pushing your new plaster back off the wall.
Devon has a lot of old buildings. Here is the quick check.
Measure at a window reveal. A solid wall is typically 400mm or more, often much thicker on cob. A modern cavity wall is around 250–300mm.
Broadly, anything built before about 1920 is likely solid wall. Cavity construction became standard after that.
Very thick, slightly uneven walls with rounded corners, usually on a stone plinth. Common right across mid and east Devon.
Old walls are rarely flat or square. If nothing is straight, you are almost certainly on solid construction.
Persistent damp in an old house is very often a modern plaster or cement render trapping moisture, not the wall itself failing.
If it is listed or in a conservation area, lime is usually a requirement, not a preference. Check before anyone starts.
It is worth being specific about this, because it is the single most common and most expensive mistake we get called out to.
Moisture enters a solid wall, from driving rain, from the ground, from ordinary humidity. In a breathable wall it evaporates back out and nothing happens. Seal the inside with gypsum and the outside with cement render, and the moisture is trapped in the middle.
It then finds the weakest route out: through the timber joists and lintels, which rot. Through the floor. And it pushes the plaster off the wall. You get damp in a house that was perfectly dry for two hundred years, and the cause is the plaster that was supposed to fix it.
Stripping cement render off a cob wall and re-doing it in lime is a large, expensive job. Not doing it in the first place is free.
It costs more. It takes longer, several coats, with weeks of curing between them. It cannot be rushed, it does not like frost, and there are far fewer plasterers who can work it properly.
Be wary of a lime quote that looks suspiciously cheap. It is often cement with a bit of lime added, which behaves like cement and causes all the same problems. You will not find out for a couple of years.
If you have an old Devon property, see our lime plastering work, or send us a photo and we will tell you what you are dealing with.
The ones we get asked most.
If your walls are cob, stone or solid brick, you need lime. These walls were built to breathe, moisture moves in and evaporates back out. Lime plaster is breathable and allows this. Modern gypsum plaster is far less permeable and traps moisture inside the wall, causing damp, rot in timbers, and plaster that blows off. Modern cavity-wall properties can use standard gypsum plaster.
Measure the wall thickness at a window reveal. A solid wall is typically 400mm or more, and often much thicker if it is cob. A modern cavity wall is around 250 to 300mm. As a rough guide, properties built before about 1920 are likely to be solid wall construction.
The gypsum seals the inside face of the wall and traps moisture inside it. That moisture then finds another route out, through timber joists and lintels, which rot, and through the plaster itself, which blows off the wall. It commonly creates damp problems in a house that was dry for centuries.
Lime is applied in several coats with weeks of curing between them, so the work takes far longer. It is also a more specialised skill and behaves very differently to gypsum. A suspiciously cheap lime quote is often cement with a small amount of lime added, which behaves like cement and causes the same trapped-moisture problems.
On listed buildings and in conservation areas, lime is usually a requirement rather than a preference. It is worth checking with the local authority before any work begins, as using inappropriate materials can breach listed building consent.
Send a photo on WhatsApp and we'll tell you straight, including if the answer is "leave it alone".