Lining a damp stone wall


I remember reading a previous discussion about this subject but can't find it now so apologies for rehashing the subject.

We need to line a damp stone wall (you know the kind - 50cm thick)- it's a mix of rising and penetrating damp, we reckon and we need to placo it. We have space to build an inner wall and leave a gap but we were wondering if anyone has actually done this and avoided future damp problems and if so, how?

We were planning to build a wood frame leaving about 10cm gap, insulate between the studs with laine de verre and then put up a vapour barrier before placo. We also thought about airing the gap to the outside by drilling some holes and putting vents over the holes. Or we could drill some holes from the outside but just stop short of going right through the wall so as to help it dry out but not vent the air gap to aid the insulation effect. We could paint on hydrofuge to the wall to stop penetrating damp but then it would stop the rising damp from drying out....

Ideas anyone?

Earlier I did say that the the thing to do is to ask around. The local historical society is a good bet. Builders may want to sell you a modern process that only they can do!

The fleur de chaux mix dries to a glaring white, but soon mellows to a warm honey colour.

Someone else mentioned that the mix ratio needs to be adjusted to suit the sand. As a rule-of-thumb, coarser sand needs less lime.

Also see what they have done with lime based render at the Hassan II mosque ablution facilities in Casablanca - simply outstanding, always hot and humid and no fungal growth.

Careful with the advice Mike. Bear in mind the local stone varies from place to place. Even for us, the limestone is very much different to 20km away. Locally fleur de chaux is not used, everybody is also very careful about going to local builders' suppliers and then say where we live for the sand for the colour particularly, we are very yellow here and not so far away on the other side of the Dordogne river it is a light beige. So, always get the right stuff from the builders and not pot luck at the brico.

The sand/lime ratio does depend on your sand. It is worth doing a series of tests 1:2, 1:3, 1:4, 1:5, etc. Spread on a board or a bit of wall. Watch and see what happens when it dries. Too hard and cracking, or too soft and crumbly. I hosted a Tiez Breizh course last year and they rendered my walls with one part lime to 3 parts washed and 3 parts unwashed sand, i.e. 1:6. The clay content of the sand affects the mix, and sand is cheaper than lime. And sand comes in lots of colours.............Lime wash plus a bit of pigment is a cheap and easy paint finish.![](upload://eWNgDUhvPuYSEtybyUw00Pr71DF.JPG)

Perhaps I should elaborate a bit, because lime mortar pointing is such an easy DIY job - well suited for people with no building skills.

I use "Fleur de Chaux" and coarse sand, with enough water to make the mixture putty-like without being runny. This will keep a long time if wet - unlike cement, it only hardens when it dries out.

Preparation is often the hardest part. The best tool is a small pointed pick. Just chip out the old mortar to a depth of around 2cm.

Then its just a matter of filling all the gaps with the mortar. Wear thick rubber gloves - this stuff will play havoc with your skin. Push the mortar in hard, you can use a trowel and hawk or even push it in by hand. Don't go for any fancy pointing, because you are going to brush it over with a stiff brush before it is completely dry. This will give the authentic rustic look. You don't have to do the whole wall in one go. Do a couple of square metres, then do more the next day. The joins won't show and you won't be breaking your back with unaccustomed work.

You can do the same finish on inside walls. It looks very good in an old building. On the inside, once the mortar is dry, you can seal the surface with dilute (1:4) PVA glue. (The same stuff they use for woodworking and crafts, but you can buy it in large tubs from DIY shops.) Brush or spray it on - it is invisible when dry - but it will stop the wall shedding dust and (maybe) help to prevent water evaporating on the inside.

The materials are cheap and you will be surprised at the improvement it makes to the appearance of your house. Probably the cheapest and easiest way to add value to your property.

Yes let it breathe..........Lots of information on damp stone walls at and There may be a maisons-paysannes group local to you with someone who could advise. Tiez Breiz offers advice in Brittany. (Thought I'd joined this group before but I don't mind joining it again.)

One to three lime and sand mix, not too wet either. We were shown it by local builders who do a lot of work in two UNESCO listed villages near us where it has to look right as much as anything, which modern mortar never quite does. A friend who is doing up her house in the town nearby is rendering her house at present and has promised to come to show us how to mix a proper lime render. So I can't tell you that mix yet. Between the two you are less likely to suffer damp because they breathe.

Mike’s lime based plaster is the most natural and best option. As he says you need to know how the building is designed to work.
I also received details of a new spray applied plaster type finish product this week although I have not seen it in action. Applied to the internal faces of external walls, it claims to make them wind and water tight, thus pushing the moisture to the exterior. Don’t know how it would work in a higher relative ground level situation. In Paris we let the moisture come through and deal with it from the inside of the building.

I think the moral of the story here is that using modern materials is certainly not the way to go. On older buildings all thats going to happen longer term is that the situation will be worse than it is now. Using tried and tested methods used for hundreds of years work well, the important thing is to ensure the building can breath.

Good luck with everything that you are doing guys, will be worth it in the end.


I think the crystals you are talking about are what is called efflorescence in English. I don't know if the French have a different word.

Sounds like you are taking on a big job. Hopefully, Kate's problem is rather less complicated.

We are repointing the worst area on the east side of our house. The other thing we have to on inside where patches of cement have been used to treat cracks (lezardes I think) is to treat it with a special product that allows the hemp lime to stick or else it would drop off ive been told. Luckily our walls are mostly pure stonework but where the concrete cuve was removed we need to treat it before we apply the insulation.

Hi Mike, its not full excavation as the house is on a hill so most of it is already above ground level. It was built part underground on the east side and then filled in with remblais and mud. Part of the basement was used as a stable. We have excavated most of the earth and internal (non supporting) wall and gone down a metre and a bit to sort out the foundations which were as you have seen early concrete and very friable. I will try to pop down and get a photo with doors open to get light in as I think you can clearly see the early concrete foundation, then the stone work beginning and in one area on the north wall I think mud. The other day the north wall looked wet but I wasnt sure if that was from the demolition man hosing it down to keeo the dust down when he was digging out. I havent been in this week as hes off but will be back in soon.

Have a look at my blog for pictures so far.

In the stable area on the stone wall above ground and foundations were little white crystals which I cant remember term for but builder told me this is a sign of the water coming up.

These rooms will be our bedrooms for some years so I am really hoping the investment to allow the wall to breathe works!



Excavating a basement is not quite the same thing, because you are working below ground level. Above floor level, a simpler solution should be possible.

Hi Kate, having the same problem and excavating our basement I sought several professional views on this and did lots of reading. In the end we have decided to go for a thermal correction so 8cm hemp and lime mix sprayed on and then finished with a lime plaster and lime breathable paint. Its not cheap and is being done at the sacrifice of finishing the rest of the house but in the long run I hope will be worth it.

I read old house eco handbook, les materiaux naturels decorer, restaurer et construire and English Heritage plus Batiments de France advice. It was actually our thermal technician Frederic Loyau from Fiabitat who persuaded us to go hemp lime but since we have researched it we are hoping this is the best solution for us.

It allows the walls to breathe and dry out naturally without causing vapour to be trapped in the insulation layer causing long term damage to the stone wall and the insulation. I know many will say its nkt a problem as long as it doesnt come through but the thought of mould growing behind it was enough to give me cause for concern.

If you want more info on lime hemp let me know and I will dig out the info I received.

You can also use hemp blocks to build a wall instead of spraying it on.Look up chanvribloc.

If u havent seen it yet la maison ecologique and natura maison magazines are really useful with lots kf examples and costs, suppliers, products on these type of buildings every month.

Things that go wrong with houses are rarely the result of faulty construction. Mostly, problems occur because owners do not understand how the building was intended to work.

The first thing is to check if the outside soil is higher than the interior floor level. But the most likely cause of problems with old stone walls is cement pointing. Originally, these stone walls were built with lime mortar (or even mud). When cement became widely available, many owners had their walls re-pointed with cement mortar. This makes he outside of the wall waterproof, so water inside the wall cannot evaporate into the outside air, but migrates to the inside of the wall.

The usual answer is to hack out the cement and re-point with a 1:3 lime and coarse sand mortar mix. But ask around locally first. The houses in your areas will have been constructed with local materials that suit your climate. In the days before RSJs and reinforced concrete there were well established building techniques, based on centuries of experience, that worked very well.

Hi Kate, we are renovating a property and have a similar problem, there is some damp in the walls. Until recently I thought it would simply be throw up some studding, bit of insulation, vapour barrier, some plasterboard and skim, throw some paint on and all is done.

However I am beginning to realise that this is exactly the wrong way to do things. I am no expert here so please if anyone wishes to correct this then feel free. Stone walls breathe, air flows in and out as does moisture so whilst there is no covering things are great, start blocking this and there lies the problem, moisture gets trapped which actually results in more problems.

From what I am reading (some really sad technical stuff) you need to ensure that the stone walls can still breathe, this means breathable insulation such as wool, wood fibre, covered with something like wood fibre board and then skimmed/finished with a lime coating/skimming.

As I say, I am no expert but do a bit of investigating on the net such as breathable insulation, insulating an internal stone wall etc etc, you will soon realise that its not a simple job of shutting it all away behind another wall or studding.

One final thing is to use the right type of paint, use cheap nasty stuff and you will be creating a seal, again you need to use a paint that breathes although in France I don't know the brands. So, whatever you use throughout make sure the building can breathe, forget the vapour barrier... apparently.

Damp is a specialist subject, using coatings and coverings to try and keep it out can make things worse. See if you can find a book called The Damp House by Jonathan Hatrred, most interesting. Also look at insulation of historic houses by English Heritage, a completely different view of dealing with damp.