Damp on chimney wall - hydroscopic salts
Discussion
I've done a lot of reading on this but not sure if on right path. We have a bricked up chimney in our bedroom. Spots on the wall keep getting damp.
There has been historic water ingress but fairly sure all fixed now. We added a vent where there was none too and have had chimney and pointing all checked and re-checked. Even though wall was skimmed nothing was done at the time to seal it - so new plaster has been damaged I think.
From what I can work out "hydroscopic salts" can be the problem. i.e. condensation in the room sticks to old salts in new and old plaster. We used just standard plasters back when this work was done.
I've read about various tanking methods and membranes that can be used to fix what seems to be a common issue. The idea is we should hack off all the plaster, fit plastic membrane and board/plaster over it.
My problem is that we have no depth to install all this (because of plaster cornice). The plaster itself is actually totally sound - really rock solid.
I'm thinking I could strip any paint off. Treat the exposed plaster with salt neutraliser. Use a cement slurry (looked at Sika waterproofer) over the whole lot - which should seal the wall. Then leave that for a few months to see if it stays dry before finishing and painting etc.
The wall itself seems dry except where old water damage was - the plasterer used bonding to patch areas too which I think was an error as that seems to act like a sponge for water. We do get a lot of condensation from shower etc. so it seems to add up. Sand and cement with SBR seems to be best.
Anyone any experience? Seems a swine of a problem!
There has been historic water ingress but fairly sure all fixed now. We added a vent where there was none too and have had chimney and pointing all checked and re-checked. Even though wall was skimmed nothing was done at the time to seal it - so new plaster has been damaged I think.
From what I can work out "hydroscopic salts" can be the problem. i.e. condensation in the room sticks to old salts in new and old plaster. We used just standard plasters back when this work was done.
I've read about various tanking methods and membranes that can be used to fix what seems to be a common issue. The idea is we should hack off all the plaster, fit plastic membrane and board/plaster over it.
My problem is that we have no depth to install all this (because of plaster cornice). The plaster itself is actually totally sound - really rock solid.
I'm thinking I could strip any paint off. Treat the exposed plaster with salt neutraliser. Use a cement slurry (looked at Sika waterproofer) over the whole lot - which should seal the wall. Then leave that for a few months to see if it stays dry before finishing and painting etc.
The wall itself seems dry except where old water damage was - the plasterer used bonding to patch areas too which I think was an error as that seems to act like a sponge for water. We do get a lot of condensation from shower etc. so it seems to add up. Sand and cement with SBR seems to be best.
Anyone any experience? Seems a swine of a problem!
Very common problem when the flue has historically vented gases from the burning of wood or bituminous coal.
Hygroscopic salts get lodged in the plaster, via the masonry, and airborne moisture forms on the surface (often brown staining also forms).
Strip the plaster and render the masonry with a barrier coat - this holds the salts in the masonry and prevents them from re migrating into the new plaster.
The Sika suggestion should work but check with Sika (or Sovereign) first and spec what they recommend.
Hygroscopic salts get lodged in the plaster, via the masonry, and airborne moisture forms on the surface (often brown staining also forms).
Strip the plaster and render the masonry with a barrier coat - this holds the salts in the masonry and prevents them from re migrating into the new plaster.
The Sika suggestion should work but check with Sika (or Sovereign) first and spec what they recommend.
Salt laden plaster will attract moisture you're right.
If you just want to cover up the problem rather than fix it (I'm expecting a bashing from the lime loving wall breathers for this
) a shower tanking membrane paint would do it.
https://www.screwfix.com/p/mapei-shower-waterproof...
Couple of coats of that with a roller. Not sure how well emulsion adheres to it so it might need a coat of BIN prior to final finish.
An SBR/Sika 1 slurry might do the trick but I'm not sure how good a finish you'd get.
If you just want to cover up the problem rather than fix it (I'm expecting a bashing from the lime loving wall breathers for this
) a shower tanking membrane paint would do it.https://www.screwfix.com/p/mapei-shower-waterproof...
Couple of coats of that with a roller. Not sure how well emulsion adheres to it so it might need a coat of BIN prior to final finish.
An SBR/Sika 1 slurry might do the trick but I'm not sure how good a finish you'd get.
Brilliant, thanks.
One part I'm struggling to understand (and I have read elsewhere so assume it must have a lot of merit): If the problem is not really water getting in from outside (rain etc.) then why does painting or coating it with some kind of tanking paint/slurry seem less of a solution than a plastic membrane and new render?
I'd actually agree that painting with some "special" paint seems too good to be true but I don't really understand why it won't work? Or is it more than it will fail a few years down the line from chips to the coating etc.?
It feels less like I'm covering anything up if the water is actually coming from inside the room?
I'd normally just hack it all off but in this case I think it will mean removing the bricked up area as it sits too far proud. It's a lath and plaster wall otherwise too - so will be one heck of a mess stripping it right back. Took us about 5 years to get the dust off everything last time we stripped one!
One part I'm struggling to understand (and I have read elsewhere so assume it must have a lot of merit): If the problem is not really water getting in from outside (rain etc.) then why does painting or coating it with some kind of tanking paint/slurry seem less of a solution than a plastic membrane and new render?
I'd actually agree that painting with some "special" paint seems too good to be true but I don't really understand why it won't work? Or is it more than it will fail a few years down the line from chips to the coating etc.?
It feels less like I'm covering anything up if the water is actually coming from inside the room?
I'd normally just hack it all off but in this case I think it will mean removing the bricked up area as it sits too far proud. It's a lath and plaster wall otherwise too - so will be one heck of a mess stripping it right back. Took us about 5 years to get the dust off everything last time we stripped one!
You're quite right. You haven't got a damp problem. You've got a salt problem. Which is causing what appears to be a damp problem.
This makes interesting viewing on the subject.
https://www.youtube.com/watch?v=KuoKhF31hL8
This makes interesting viewing on the subject.
https://www.youtube.com/watch?v=KuoKhF31hL8
This might help OP if you don't want to start stripping plaster Zinsser Watertite Probably falls under the heading 'botch job' but worth a go. It's hyGroscopic btw.
Sounds like an old house soooooo................Lime render.
Moisture has to evaporate. If you use sika render, etc, you will never ever ever in a month of sundays get rid of the damp problem. it will manifest at another point.
also, why is there dampness there in the first place? solve the source and damp goes away. it makes its prescense known when people try to apply modern techniques to old buildings. the building tends to turn around and say f*&k off
Moisture has to evaporate. If you use sika render, etc, you will never ever ever in a month of sundays get rid of the damp problem. it will manifest at another point.
also, why is there dampness there in the first place? solve the source and damp goes away. it makes its prescense known when people try to apply modern techniques to old buildings. the building tends to turn around and say f*&k off
Edited by kurt535 on Monday 25th September 20:52
Op, I think you have a fair understanding.
Consider also that salts only travel in solution, therefore if there is no ongoing dampness replastering, including the basecoat will do the trick.
I've never know it to be an ongoing issue once replastered 3 parts sharp sand to 1 part cement, integral salt neutraliser and waterproofer.
Plasterers will often using bonding plaster to hold the beads, as you say they may as well hang a wet sponge.
Or use lime and replaster every few years.
Consider also that salts only travel in solution, therefore if there is no ongoing dampness replastering, including the basecoat will do the trick.
I've never know it to be an ongoing issue once replastered 3 parts sharp sand to 1 part cement, integral salt neutraliser and waterproofer.
Plasterers will often using bonding plaster to hold the beads, as you say they may as well hang a wet sponge.
Or use lime and replaster every few years.
My girlfriend has the same problem, check all damp sources such as chimney flashing pointing for and water ingress,
In my opinion having a fire would always dry the stack out and with it being a vent to outside it will always be colder than your room, so always be a condensation point as soon as you remove the heat source.
Salt and water are really strong and will push through new render in time!
In my opinion having a fire would always dry the stack out and with it being a vent to outside it will always be colder than your room, so always be a condensation point as soon as you remove the heat source.
Salt and water are really strong and will push through new render in time!
Cheers again. I'm not 100% sure of everything outside but I've taken a brick out inside and it seems drier the further into the wall I go through mortar etc. Chimney has been re-pointed and has a vented cap on top. Inside the chimney is cold and damp but then it's outside air and been raining a lot.
I can also see a few holes in the lath and plaster further up the wall we filled years back with bonding - the water just sticks to that. That is bonding hanging on laths and not touching the stone wall. We've used modern vinyl emulsion which will be another problem. As someone said its a really old house.
A bit worried about covering anything up - we've so far managed to avoid bodging but this one just had me a bit stumped. Will see what I can hack off and if I can get it deep enough for a good skim of sand and cement (even if I can't do the air gap membrane). That won't take too long and it still comes back will need to go for something more drastic.
I can also see a few holes in the lath and plaster further up the wall we filled years back with bonding - the water just sticks to that. That is bonding hanging on laths and not touching the stone wall. We've used modern vinyl emulsion which will be another problem. As someone said its a really old house.
A bit worried about covering anything up - we've so far managed to avoid bodging but this one just had me a bit stumped. Will see what I can hack off and if I can get it deep enough for a good skim of sand and cement (even if I can't do the air gap membrane). That won't take too long and it still comes back will need to go for something more drastic.
happy to: pm me and i can send through pictures of one I've just done.
one mistake i've constantly seen done is people trying to eradicate damp in old houses (like a new build); candidly you won't. liberal use of blackjack, sika render, sand cement render, dpc injection systems...all will make the situation worse/move the damp problem onto another area where you may not even be able to sort out when it becomes a problem.
the old houses have to breathe. its really vital to understand why the damp is there in the first place and then find a way to reduce/manage it.
one mistake i've constantly seen done is people trying to eradicate damp in old houses (like a new build); candidly you won't. liberal use of blackjack, sika render, sand cement render, dpc injection systems...all will make the situation worse/move the damp problem onto another area where you may not even be able to sort out when it becomes a problem.
the old houses have to breathe. its really vital to understand why the damp is there in the first place and then find a way to reduce/manage it.
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