We own a cob and stone property, which suffers from internal damp and mildew. Why is this and what can be done about it?

The most common causes of internal damp are usually two fold- cement render on the external walls, (usually painted with plastic masonry paint) and pink (gypsum) plaster on the internal walls. Cement mortars and gypsum plasters by their very nature do not breathe and have a hard set, unlike lime mortars.
 The solution is to remove the inappropriate render and plaster and re-do with lime mortar, which will let the walls breathe. Don’t forget to paint with a breathable paint such as lime wash and the damp problems will dissipate.
It’s worth noting that modern conveniences such as central heating and double glazing can also add to the problem as this hinders the internal air flow. If there is a fireplace in the room that has been blocked up, unblock it and install a wood burner which will also help to circulate the air around the property.

The render on our house has cracks in it and in places it seems to be coming away from the wall. I have tried patching it with cement but that also ends up cracking and falling off. Is there a better solution?

This sounds like a typical problem that is encountered when external walls on older properties are rendered with cement. Because cement, (unlike lime mortar) does not flex with the building, over time cracks will start to appear. The problem is then exacerbated when rain water gets into through the cracks and freezes during a hard frost, only to melt again as the weather warms and in doing so, the water molecules expand and push off the cement mortar.
Lime mortar on the other hand, will flex with the building. If the movement is small, the micro cracks recrystallise through the action of 'free lime' effectively self-healing the affected area.

I am thinking of buying a cob property. What sort of issues should I be aware of?

Cob properties were generally  built on a stone plinth, so don’t be dismayed that there is no damp proof course.  Ideally, the property will have been maintained in the correct way, and has a sound roof that has not allowed water to leak into the cob structure.
Cob needs to maintain its own micro climate, so that it’s neither to wet or too dry.  Lime mortar has a key role to play here as it allows the cob to breathe and ‘wick’ away any excessive damp.
If the cob has been incorrectly rendered with cement, then check the soundness of the structure by tapping the walls- if they sound hollow or the render has come away from the cob underneath, then you need to be aware that remedial works will have to be undertaken as soon as possible.
We always try and look at the top of the cob walls by getting up into the attic roof space, where more often than not, will give you an indication as to whether the walls are sound. If you have any doubts about the property, then it’s always best to get expert advice from someone like ourselves who specialises in working on cob buildings.

Why should you not use modern paints on lime mortar?

There are several reasons why you should not use a modern paint on lime mortar.
In the trade, we often refer to this as ‘plastic’ paint as it forms a solid coating over a surface, which peels and flakes off when applied to lime.
Cob and stone & brick properties constructed before 1930’s are built with lime & need to breathe.  By using modern paint, the lime mortar is sealed and cannot breathe or let moisture escape from the substrate and so gives rise to damp issues inside the property.
The second reason is that the very nature of lime will react chemically with a paint which is not designed to be used on lime. This combined with the entrapment of damp within the structure will make the paint peel away from the lime mortar, and leave the mortar underneath exposed to the elements.

What is the best type of paint to use on lime mortar?

Interior: Lime wash is the perfect paint for lime mortar as it is also made from lime putty, one of the key ingredients that go into making lime mortar. It allows the substrate to breathe and damp to escape.  It’s soft, warm appearance gives shades and depths of colour that are not obtained with any other paint and because it allows the lime mortar underneath to breathe, will help wick away any damp. Easy to apply, it comes in a range of colours that will enhance any home.
 If you want a more modern look, or perhaps have one wall that has lime plaster abutting one with gypsum plaster, then we recommend using Milk Paint, which we stock. These paints are also breathable and give a uniform finish. They come in a range of colours and again are easily applied; however they cannot be used on external finishes.
 Exterior: Lime wash is commonly used on many properties that are not prone to the prevailing weather. However, for properties that are in exposed areas,  or even on a busy road where fumes from traffic and splash back from tyres means that the property is constantly subjected to pollution, we would recommend using a mineral silicate paint.
 This is a breathable paint, made from lime and potassium water glass (sodium silicate), which unlike lime wash, does not have various depths of colour and so to all appearances is very similar to modern paint but still retains its breathability combined with durability against the elements.
 Another scenario where mineral silicate paint comes into its own is when one part of a property has had a new addition with a cement render. Using lime wash, which darkens when damp, next to a cement render painted with modern paint, will accentuate the differences between the two. Mineral silicate paint on the other hand will cover both surfaces and give a uniform finish, yet allowing the walls with lime mortar on to breathe.

Back To Top