Devon has more cob buildings than any other county in England. Cob is earth, and earth has to breathe. Wrap it in cement render or skim it with gypsum and you will trap water inside the wall, rot the timbers, and destroy a building that has stood for four hundred years.
Subsoil, straw and water, mixed on the ground and built up in courses, each one left to dry before the next went on. No cement, no bricks, no frame. The wall is the structure.
Devon cob is usually built off a stone plinth and it is thick, often two feet or more. That thickness is what holds the heat and what holds the building up.
There are somewhere around twenty thousand cob buildings still standing in Devon, more than anywhere else in the country. Most of them are two or three hundred years old. Some are considerably older. They have lasted because, until relatively recently, people knew how to look after them.
That is the old Devon saying about cob, and it is the whole of the theory in seven words. A good hat means a roof with a decent overhang, keeping rain off the top of the wall. A good pair of boots means a stone plinth keeping the earth up out of the ground.
Cob does not fail because it is old. Cob fails because it gets wet and cannot dry out again.
This is the single most common way people wreck a cob building, and it is almost always done with good intentions.
Cement render is hard, brittle and effectively waterproof. Cob moves slightly with the seasons, so the render cracks. Water gets in behind the crack. And now it cannot get out, because the render it is sitting behind is waterproof.
So the water sits inside the cob. The wall softens. The timbers embedded in it start to rot. And from the outside, the render looks fine, so nobody knows anything is wrong until something structural gives.
Cement render on cob does not cause damp gradually. It causes it invisibly, and by the time it is obvious the damage is serious.
The same logic applies indoors. A gypsum skim over a cob wall seals the inside face. Moisture that would have evaporated harmlessly into the room now has nowhere to go.
You will usually see it first as the plaster blowing, bubbling or coming away in patches, often near the bottom of the wall. People call that rising damp. It is usually just a wall that has been sealed up and cannot breathe.
Hack it off, skim it again with gypsum, and you will be doing the same job again in two years.
Lime. Lime plaster inside, lime render outside, limewash or a breathable mineral paint on top.
Lime is soft, it is flexible enough to move with the wall, and crucially it is vapour-open. Water that gets into the wall can evaporate back out through the lime rather than being trapped behind it.
It is slower to work with, it needs the right weather, and it costs more. That is the honest trade-off. What you get is a wall that behaves the way it was designed to behave.
Some modern breathable systems work too. Sprayed cork render is genuinely vapour-open and it will go over cob without sealing it, which makes it a legitimate option where lime is not practical.
It should come off. That is a serious job and it is not one to rush, because taking hard render off a soft wall can pull the face of the cob off with it.
But leaving cement render on cob is not a stable situation. It is a slow failure, and the longer it stays the worse the wall underneath gets.
If you are not sure what you have got, send a photo. It is usually obvious from the surface, and it costs nothing to ask.
Send a photo of the wall on WhatsApp and we'll tell you straight what it is and what it needs. No charge, no hard sell.