Contaminated salts in plaster around chimney breast

asxx

On an upstairs bedroom we have condensation problems on a chimney breast which comes and goes depending on the atmospheric relative humidity.

We have had several damp proof contractors look at the problem and they have suggested that the plaster is contaminated with hygroscopic salts from previous fossil fuel burning in the chimney. These have leeched through into the plaster and attract moisture when the atmospheric relative humidity is high.

They both suggest taking the existing plaster off, and then before re-plastering adding a barrier to prevent any further salt contamination into the new plaster. However one suggests using a plastic waterproof membrane as the barrier and another suggests using VANDEX.

Does anyone have any information on the pros and cons of the plastic membrane and VANDEX?

charlie792

Sorry I really can't help you, but your post is of interest as we've been having a strange condensation type damp patch keep appearing on our chimney breast too! It was suggested to us that there was a leak with the capping of the chimney so this was replaced and yet the damp patches keep flaring up and no idea why - I think you may have solved it for us!

DVardysShadow

Salts only leach in the presence of water [ie dampness]. I am assuming the chimney breasts have been blocked off. If this is the case, bearing in mind that average rainfall is about 1 metre per year, the chimney breasts will be very damp. Chimney breasts should not be blocked off - they should have small vents so they can breathe.

Moocow5

DVardysShadow wrote: »

Salts only leach in the presence of water [ie dampness]. I am assuming the chimney breasts have been blocked off. If this is the case, bearing in mind that average rainfall is about 1 metre per year, the chimney breasts will be very damp. Chimney breasts should not be blocked off - they should have small vents so they can breathe.

Very true. It is important that the chimney is not totally blocked off but that it has a ventilated cap. There should also be a vent in the old fireplace to allow air to flow through.
Dampness may be caused by lack of ventilation but may possibly be caused by problems with the flashing.

I'm afraid I cannot comment regarding the membrane as opposed to the Vandex as I do not have experience with those.

I hope you get the problem sorted:)

madrhino

My mum has the same problem. We went to B & Q today and spoke to a plumber there who said to paint it with a mixture of 5 parts water one part PVA. It seals it in apparently. Haven't tried it yet.

owls

some good advice above here,
as said,it is crucual to allow air to circulate in the chimney to avoid the chimney sweating plus an additonal air brick in the room to allow for air cirulation.
as regards the salt coming through-
is crucual that you form at least some kind of barrier between any new plasterwork and any signs of salt on the stone/brickwork.
vandex is a poor solution to the problem as this will act as a deterant for any damp in the wall if used correctly, but the salts will eventually eat through over time.you need a physical barrier so the chap that suggested a plastic membrane is spot on for me,it is quite similiar to visquine but of slightly more rigid plastic and perferated so you can plaster directly over it.

pva? is that a wind up:rotfl:
i can tell you for free, that pva will not work to keep salt residues on an old chimney at bay.
pva does have its uses eg as a primer to kill suction but not for this application.so speaks a damp proofing contractor.

25rts

Whilst Vandex products are very good this seems an over specification to me and they would normally only be used below ground, in old barns and on rare occasions heavily contaminated chimneys. Replacement of the affected plaster using a 3:1 fine washed sand and cement render incorporating a waterproofing additive skimmed with a multi-finish plaster is usually all that is necessary. Chimneys should ideally be swept before they are capped and vented.

Should you require further advice a list of UK wide independent damp surveyors can be found here

http://www.independentdampsurveyors.co.uk/index.html

WhiteHorse

Chimney breasts may suffer from cold spotting rather than damp. Other walls can suffer from this too. This is your first option after ordinary damp has been excluded.

It is also true that residues from coal fires can in time percolate through plaster. There are a variety of options, some of which have been given. You can also hack off the old plaster and replace it with a sulphur resistant render.

asxx

Thanks for all of the replies. Some further info. The problem has been apparent ever since we bought the house (about 4 years ago). At least from ground level the flashing looks sound and there is no sign of water penetration on the ceiling of the room near to the chimney breast which I might have expected if there was a leak. I think the chimney itself is capped (not sure about ventilation at the top) but it certainly has ventilation at the bottom from the existing open fireplace.

As the existing plaster has degraded from the salt contamination it definitely needs to be hacked off and replastered. It sounds like

1. Some form of waterproof additive to the new plaster (as suggested by 25rts) or a waterproof barrier should be used when replastering. Any more comments on what is best would be appreciated. I am tempted to go down the route of a physical barrier (Vandex or membrane) as this sounds like it will be a more long-term solution. Is this likely to be much more expensive than just using a waterproof additive?

2. I need to check that the chimney is ventilated at the top.

25rts

5 litres of waterproofing additive will cost you £10 from a builders merchant / Wickes. One bag of Vandex BB75 with VAT will be around £60 and you probably need to pay to have it delivered as most builder's merchants will not have even heard of it. Membrane with fixings £200 again plus delivery. And you still need to re-plaster anyway.