How to tackle damp walls

[kersamson]

Hi,

We're slowly renovating a property in France. Its old - very old, and has extremely thick walls (1m+ thick at the base). They're made from lumps of local rock (granite/shale type stuff) bound together with mud. The end wall in the lounge is giving me concern. Its the longest external wall in this room, and is exposed stone, which requires re-pointing. It contains a huge fireplace, and it going be wonderful when finished! However, this wall is now very very damp ....

The room had a dirt floor, which was dug out, a damp-proof membrane put down, and new concrete floor laid. Outside, there was a grotty lean-to down the entire length of the wall, which was demolished. The condition of the exterior walls gave the builder concern, and it was recommended that we render the whole outside with a lime mortar to bind it all together. It was blasted on and smoothed off. This render has now been painted with Dulux weathershield. (We noticed that the inside of the wall was starting to get damp before the outside was painted by the way). The wall is now very damp from ground level up to about 1.5m. It is uniformly damp along its length (so its not rain water coming down the chimney opening).

During the work on the dirt floor, the builder told me that one day, after a period of heavy rain, a trench that he'd dug across the floor (removing an old soil pipe) filled with water. A french drain was put in along the front of the property, and the water disappeared. We've now got french drains along the front and rear of the place. But not down this end wall that is so damp now.

I am guessing that the installation of the damp proof membrane is forcing water out sideways, and its leaching up the wall. Does this sound plausible? I am hoping that a 3rd french drain, down the length of this end wall will cure it. However, I hear all sorts of opinions regarding whether or not rising damp is a real phenomenon or not! I don't think that injecting a damp course is an option given the higgedly-piggeldy construction of the wall and its immense thickness.

Any advice?

Cheers,

Doug

 

[kersamson]

Come on everyone - someone out here must be able to help.

Thanks!

 

[Joe Malone]

Hi,

We're slowly renovating a property in France. Its old - very old, and has extremely thick walls (1m+ thick at the base). They're made from lumps of local rock (granite/shale type stuff) bound together with mud. The end wall in the lounge is giving me concern. Its the longest external wall in this room, and is exposed stone, which requires re-pointing. It contains a huge fireplace, and it going be wonderful when finished! However, this wall is now very very damp ....

The room had a dirt floor, which was dug out, a damp-proof membrane put down, and new concrete floor laid. Outside, there was a grotty lean-to down the entire length of the wall, which was demolished. The condition of the exterior walls gave the builder concern, and it was recommended that we render the whole outside with a lime mortar to bind it all together. It was blasted on and smoothed off. This render has now been painted with Dulux weathershield. (We noticed that the inside of the wall was starting to get damp before the outside was painted by the way). The wall is now very damp from ground level up to about 1.5m. It is uniformly damp along its length (so its not rain water coming down the chimney opening).

During the work on the dirt floor, the builder told me that one day, after a period of heavy rain, a trench that he'd dug across the floor (removing an old soil pipe) filled with water. A french drain was put in along the front of the property, and the water disappeared. We've now got french drains along the front and rear of the place. But not down this end wall that is so damp now.

I am guessing that the installation of the damp proof membrane is forcing water out sideways, and its leaching up the wall. Does this sound plausible? I am hoping that a 3rd french drain, down the length of this end wall will cure it. However, I hear all sorts of opinions regarding whether or not rising damp is a real phenomenon or not! I don't think that injecting a damp course is an option given the higgedly-piggeldy construction of the wall and its immense thickness.

Any advice?

Cheers,

Doug

OK, here goes...

Firstly, it was a mistake laying a retrofit concrete floor in a property of this type. This will displace water from the floor area and push it up the walls under hydrostatic pressure. The granite, if that's what it is, is highly resistant to moisture so you can guarantee that if you have a damp wall then the moisture pathway is the mortar joints.
As a general rule of thumb, no DPC in the wall so the internal finished floor level must be 200mm above the external finished floor level.

Another mistake was made in your approach to dealing with the wall. These walls are built on the overcoat principle, which means they are meant to breathe and go through seasonal wet dry cycles. It was ok or render with lime mortar as this is breathable but then you render a good material choice useless by coating with an impermeable paint (Weathershield).

The lean to which was demolished was clearly responsible for the damp in this wall so ironically, demolishing this structure would have led to a natural improvement in conditions probably within 6 months. The reason for this is that wall base ventilation is critical to ensuring these structures remain dry. Build a structure against a wall that inhibits natural ventilation and you will almost assuredly experience damp conditions.

Rising damp does exist as a phenomena but its incredibly rare and even you have it then chemical treatments are completely inappropriate for that form of construction. Interestingly rising damp can rise to higher heights in thicker walls. French drains can help if you have a high water table. Dig a small trial pit down to foundation level and see if this fills with water percolating through from the water table, whatever height you get in the pit is the height of the water table at that time. If it remains dry then the water table isn't really a problem and French drains are unlikely to help, even in France.

 

[kersamson]

Hi Joe,

Thanks for getting back! I read some of your other posts yesterday, and was looking for a way to contact you. Looks like my post did the trick!

Thanks for confirming my suspicions. So I have a plan of attack:

1. Get that French drain in place ASAP. (I know for 100% sure that we have a high water table in the area. The bed rock is granite, and water does struggle to drain. The main water stopcock lives in a small concrete enclosure. During the summer this bone dry. After rain it often fills to the brim, so I know water is not far below the surface!
   
2. At the same time generally lower the surface outside (I'll have a digger round to put in the drain, so this is easy).
   
3. Give it a few months - preferably over a wet winter - to see if things start to improve.
   
4. If it doesn't get significantly better, hack off the outside render on the house to (say) 2m above ground level. Re-point the wall with lime mortar and leave it that way - no paint!
   
5. If none of this works cry like a baby!

Hopefully I'll only to need to steps 1 - 3.

While I've got your attention, I have another question:

The builder also used this lime render to plaster over many of the interior walls. Currently these are unpainted, but I can't leave them like that - they are 'dusting' all the time. You only need to brush past them and lots of sandy stuff falls. In a few places there is evidence of (very localized) rising damp. The render is discoloured, feels damp and there is efflorescence present. This is only in one or two spots, the worst one being where an electrical socket has been installed, and the damp seems to be tracking up where the cable comes through from the floor.

I want to sort the damp, and paint the walls. As far as painting goes I am considering some of the breathable clay paints, or possibly lime wash. Have you any experience with these products? So far as the damp patches are concerned, how would you approach that problem? I was considering localised injection, or possibly just undercoating the region with a damp proof paint/treatment first.

Thank you very much for taking the time to read and answer!!!!

Cheers,

Doug

 

[Joe Malone]

Hi Doug, sorry missed this today but I'll post a full response tomorrow

 

[Joe Malone]

Hi Joe,

Thanks for getting back! I read some of your other posts yesterday, and was looking for a way to contact you. Looks like my post did the trick!

Thanks for confirming my suspicions. So I have a plan of attack:

1. Get that French drain in place ASAP. (I know for 100% sure that we have a high water table in the area. The bed rock is granite, and water does struggle to drain. The main water stopcock lives in a small concrete enclosure. During the summer this bone dry. After rain it often fills to the brim, so I know water is not far below the surface!
   
2. At the same time generally lower the surface outside (I'll have a digger round to put in the drain, so this is easy).
   
3. Give it a few months - preferably over a wet winter - to see if things start to improve.
   
4. If it doesn't get significantly better, hack off the outside render on the house to (say) 2m above ground level. Re-point the wall with lime mortar and leave it that way - no paint!
   
5. If none of this works cry like a baby!

Hopefully I'll only to need to steps 1 - 3.

While I've got your attention, I have another question:

The builder also used this lime render to plaster over many of the interior walls. Currently these are unpainted, but I can't leave them like that - they are 'dusting' all the time. You only need to brush past them and lots of sandy stuff falls. In a few places there is evidence of (very localized) rising damp. The render is discoloured, feels damp and there is efflorescence present. This is only in one or two spots, the worst one being where an electrical socket has been installed, and the damp seems to be tracking up where the cable comes through from the floor.

I want to sort the damp, and paint the walls. As far as painting goes I am considering some of the breathable clay paints, or possibly lime wash. Have you any experience with these products? So far as the damp patches are concerned, how would you approach that problem? I was considering localised injection, or possibly just undercoating the region with a damp proof paint/treatment first.

Thank you very much for taking the time to read and answer!!!!

Cheers,

Doug

Doug, I would strongly recommend a mineral silicate paint for the lime plastered walls. They have the benefit of being both breathable and durable. Untreated lime plaster will have quite a dusty surface even if treated with a lime wash. You also have to accept that lime plaster has a downside in that it does stain rather easily. The mineral silicate paint should improve all these issues.

French drains sound good given what you have described relating to the water table but do if at all possible reduce external ground levels to ensure they are a minimum 200mm below internal finished floor level. I'm unsure what you have decided to do about the retrofit concrete floor slab but assume you are keeping it. If that is the case then I'd be looking to increase wall base ventilation even more than 200mm. I am concerned that you have built in a problem here that will cause long term damp problems with the walls and the only defence you now have is external wall base ventilation, the more the better.
If the render is very old I'd hack it off for planned replacement anyway. Give it maybe 3 months over the summer to ventilate then plan to replace the render around August time before the bad weather starts to set in.
If all else fails don't cry just invite me to France!

 

[kersamson]

Hi Joe,

Thanks for the top advice! Message received and understood. I'll talk to my builder (the new one, not the one who gave me all these headaches!) and let him know what the plan is. Dropping the ground level should be possible, and we'll see about hacking off some of the outside render at the base of the walls. Its all new (few years old now), but taking away a couple of foot at the base to expose the stone work won't look too bad.

I'll look into the paint you recommended too! And I'll certainly invite you over to France if it comes to it! In the meantime, here are some photos....

The original dirt floor (before digging out and putting in DP membrane and concrete floor). The trench is where a soil pipe was taken out. Then after a few days of rain this happened.....

Untitled

• kersamson
• 9 May 2013
• 1

Here's the outside of the place before we started work...

Untitled

• kersamson
• 9 May 2013
• 1

And here it is today (the end wall is the one with the damp):

Untitled

• kersamson
• 9 May 2013
• 1

Finally here's a good shot of the damp wall:

Untitled

• kersamson
• 9 May 2013
• 1

Many thanks!

Doug

 

[Joe Malone]

What part of France is it in Doug?

 

[kersamson]

Hi Joe,

Its in Brittany. Near Ploermel.

Cheers,

Doug

 

[Graeme1983]

Im in exactly the same boat, four granite stones walls, earth brick partition walls, concrete floors, cement render and modern paint etc. Place was as wet as an otters pocket until the render, floor and carpets came up.

Havent got much to add that Joe hasnt said, hack off modern render, muller concrete floors, replace with lime renders, washes and maybe limecrete floors are worth looking into, french drains all round the house. Im having a log extension built next to the house to contain the bathroom and some bedrooms to take some of the moisture away so I have the advantage of living there and being able to leave the whole old house to have a year or so drying period.

How much difference do you think it would make between letting the stone walls dry and lime rendering them almost right away Joe?

Ive found that my wood fire is much better for drying the house out then the windows being open alone, if you can spare the wood its good to run the fire often.

Couple of decent books:

http://www.amazon.co.uk/Lime-Buildi...d=1368182825&sr=8-1&keywords=lime+in+building

http://www.amazon.co.uk/Using-Natur...d=1368182839&sr=1-1&keywords=natural+finishes

 

[Joe Malone]

How much difference do you think it would make between letting the stone walls dry and lime rendering them almost right away Joe?

A highly subjective question to answer Graeme but accepted principle is that bare masonry dries out at a rate of 25mm per month. If you are heating internally then you could hypothesize that you're getting 50mm of the wall dried out every month. This of course is provided that the moisture source has been managed or eliminated.
You wouldn't fully dry out the wall over a 3 month spell but pragmatically the wall doesn't have to be bone dry, its about getting the wall down to an acceptable level of dampness that doesn't cause decorative spoiling and then to make sure you achieve moisture equilibrium, meaning the water vapour is evaporating from the wall as fast as its rising in the wall. I find it sometimes helps to achieve equilibrium by giving the wall a head start in the drying process.

'As wet as an otters pocket'... brilliant, never heard that one before.

 

[kersamson]

Hi again.

Just been looking at the cost of mineral silicate paints. Blimey! Clay paints seem to tick the same boxes but cost considerably less. How much better are the silicates? Would Earthborn clay paint be good enough?

Cheers
Doug

 

[Joe Malone]

Hi again.

Just been looking at the cost of mineral silicate paints. Blimey! Clay paints seem to tick the same boxes but cost considerably less. How much better are the silicates? Would Earthborn clay paint be good enough?

Cheers
Doug

Judgement call this one Doug, the clay paints will do a job but what is the cost differential? Mineral silicates are far more durable so it's about whole life cost rather than initial upfront cost. Personally I'd go with the mineral option but your choice.

A quick search and I found 10 ltrs of white primer and 10 Ltrs of paint for £142. Compare that to the clay paints which aren't cheap and even a good quality standard masonry paint. This is a highly technical product and I don't think the price is bad.