Efflorescence

[baldkev]

Hi all,
I have a customer with an old cottage ( thatched, cob walls etc ) and on one wall which is a party wall with another property and has a chimney, they get problems with efflorescence. This is most likely lime turning to calcium and coming out of the wall. The most likely source of continued moisture is rising damp ( there wont be dpc ). The walls dont appear to be damp on the surface and i havent yet put a moisture meter on it.

They have had it replastered and basically every 2ish years it comes through again.

I am considering putting fireboard ( cement board ) either side if the wiodburner, within the stone faced chimney. Currently its plastered, but gets salts leaching through.

Theres also another part of that wall that is plastered, seperated from the fire by the stonework, which suffers badly. For this i am considering hacking it back and ( using spacers ) overboard with mr mdf or something. Part of me thinks it might work, part of me thinks it needs air circulation.

Does anyone have experience with this? In the past for much lesser affected areas, neutralising the salt with vinegar has worked, but this was on blockwork where the salts were probably from strong portland cement.

Thanks, kev

 

[eribaMotters]

If it comes back every 2 or so years I would be questioning what type of plaster has been used.

Colin

 

[baldkev]

I'm not 100%, but i think she said it was hacked back, rendered and plastered, so i would guess cement render and multifinish. Also a guess, but i would think lime plaster would have made it worse as the lime would tend to draw moisture quicker?

 

[baldkev]

Worth adding they did understand that the paint needs to be breathable and i believe, have used chalk based paint

 

[Woody2Shoes]

I would very strongly suspect that the source of dampness is rain coming down the chimney - either directly (is there a cowl to prevent this) or indirectly through faulty flashings within or near the chimney stack. I would only suspect rising damp after eliminating falling damp and condensation.

 

[Woody2Shoes]

PS I would advise the customer that until the source of the moisture is properly identified and fixed then any kind of plaster will continue to be damaged.

 

[Jacob]

Unused and uncovered chimney masonry can take in masses of water and take years to dry out even if remedied with cowlings etc as suggested above by W2Shoes. They also need air let in at the bottom for through ventilation.

 

[eribaMotters]

Well after what has been said above it's a combination of things. A cowl at the top and an air vent at bottom are a must. I would also check the flaunching around any chimney pot, the stack pointing and any flashing. The render is also a no-no. It should be lime to allow the fabric to breath.

Colin

 

[baldkev]

Thank you all for the replies, some great points. I havent checked the state of the chimney or pot etc, as it was a 'while you are here' request. The reason behind my rising damp thoughts were that the ground level behind the properties is a lot higher than the floor level, but next time i am out there i will take a look at the chimney. Might also be worth having a poke at the plaster to see what it is.
Thank you all!

 

[MikeJhn]

Rising damp is a fallacy, don't get sucked into that one, (sic) if the ground level outside is higher than inside that's penetrating damp and will never be cured without extensive tanking, lowering the water table in the area of penetration or easier dig out the ground, most so called rising damp issues are in fact rain driven or leaking pipes, one so called rising damp issue identified by a Building Surveyor, I investigated was a blocked main drain in the road outside the property, whenever it rained cars passing would splash water onto the walls of the house.

 

[Adam W.]

I'm not 100%, but i think she said it was hacked back, rendered and plastered, so i would guess cement render and multifinish. Also a guess, but i would think lime plaster would have made it worse as the lime would tend to draw moisture quicker?

My bold.

That might be your problem, right there.

Lime plaster and render is best for cob and cob also needs protection from the top with a large eaves.

 

[baldkev]

Ok thanks, I'll have a dig about. Out 9f interest, what would it be about the cement render etc that would cause a problem?

 

[Adam W.]

Cement and gypsum are known to trap moisture in walls and cause salt build up in their surfaces. Exactly why, I can't tell you but it must be due to their chemical composition. Further googling might tell you more, but it's above my paygrade.

Lime renders and plasters readily allow the transmission of moisture from masonry and cob to their in/out surfaces, but they must be painted with lime based paints for the moisture to evaporate, and also be wearing a good hat like thatch with a large eaves to prevent rain from wearing away the surface and protect the base of the wall. (That probably doesn't read very well)

The penetration of rain from the outside to the inside in cob is highly unlikely due to the thickness of the wall and the moisture on the outside is driven off by the wind anyway. The last thing cob wants is to have moisture trapped inside, as it just "melts away" with frost erosion.

 

[Adam W.]

Also, if the inside is plastered with gypsum plaster then it's likely it has been painted with alkalyd emulsion paint, as lime paint doesn't stick to gypsum unless it has casein mixed in with it.

 

[Lard]

Rising damp is a fallacy

….in your opinion….

 

[Woody2Shoes]

….in your opinion….

I wouldn't say that there's no such thing as rising damp, but I think it's fair to say that it's very often mis-diagnosed (for more than one reason) and is a relatively unusual cause of unexpected damp patches (matron...).

 

[Vintage Frames]

One way of dealing with this is to hide it all away with plaster board or mdf, as you've suggested. That isn't going to cure the problem, just a way of keeping it hidden.
If you want to cure it then you must hack away all the cement/plaster that has been incorrectly used on a cob stone wall. These walls require the use of traditional lime plaster and paint. That way any moisture in the walls will naturally evaporate into the atmosphere.
As it's a party wall, there is the problem that the neighbours have cement/plastered their side also, so all the evaporation is forced to one side only. I wonder do the neighbours have the same problem.
Another thing to check is whether the chimney capping is cracked. This will cause rain water to get into the chimney stack and sink downwards.

 

[dannyr]

I wouldn't say that there's no such thing as rising damp, but I think it's fair to say that it's very often mis-diagnosed (for more than one reason) and is a relatively unusual cause of unexpected damp patches (matron...).

I'm not a builder, or surveyor, but re Mike's comment on rising damp: in my 1890 house: cellar, central brick supporting walls (ie far from any outside wall) strong efflorescence, even if brushed off many times, then chemical injection (by me, so not perfect) a couple of years ago now efflorescence almost all just on bottom course (below injection) so if rising damp is a fallacy, what's happening here? (nothing piled against wall, no paint or render.)

 

[Woody2Shoes]

I'm not a builder, or surveyor, but re Mike's comment on rising damp: in my 1890 house: cellar, central brick supporting walls (ie far from any outside wall) strong efflorescence, even if brushed off many times, then chemical injection (by me, so not perfect) a couple of years ago now efflorescence almost all just on bottom course (below injection) so if rising damp is a fallacy, what's happening here? (nothing piled against wall, no paint or render.)

I think you've made my point!

It's fairly unusual for people to do their own chemical injection, and it's also pretty unusual for anyone to bother doing what you've done in a cellar - unless they've converted their cellar into some more habitable kind of space, perhaps - in that case, I might have been tempted to insert a piece of slate/DPM as an alternative.

In most cases, damp walls are caused by faulty rainwater goods or plumbing, or faulty roofing/flashings, or faulty masonry, or condensation, or some combination - perhaps exacerbated/triggered by new work inside/outside.

 

[Woody2Shoes]

I think you've made my point!

It's fairly unusual for people to do their own chemical injection, and it's also pretty unusual for anyone to bother doing what you've done in a cellar - unless they've converted their cellar into some more habitable kind of space, perhaps - in that case, I might have been tempted to insert a piece of slate/DPM as an alternative.

In most cases, damp walls are caused by faulty rainwater goods or plumbing, or faulty roofing/flashings, or faulty masonry, or condensation, or some combination - perhaps exacerbated/triggered by new work inside/outside.

PS They had and used DPCs in even quite modest houses in the 1890's