Bubbling paint above chimney

[Magrah]

Hi All

I wonder if anyone has an answer.

We have a bedroom which has an old fireplace that has been blocked and the whole wall re-plastered before we moved in 8 years ago. All was fine and I decorated the room when we moved in. I have been meaning to put a cowl on the chimney for a while but only managed to do this a couple of months ago. All was fine but a year ago the paint above where the old chimney was started bubbling a little, it got bigger and bigger and is now an area of about 2 square feet in total. Having taken the paint off it looks like areas of small white crystals which I can easily scrape of with a scrapper. My guess is this is caused by water getting in and no ventilation. The cowl will help stop water getting in but it obviously has done before it was fitted.

Question is do I need to go to the hassle of making a whole in the wall and fitting a vent (I don't think this will solve the problem short term) or can I buy a sealer (perhaps unibond, the paint comes of easily back to plaster in the effected areas) to paint on and stop the salt / sulpher? coming through while the cowl dries the chimney out? Hopefully the latter as don't want to butcher the nice plaster and think a vent will look unsightly in my 9 year olds room!

Thanks

 

[KenGMac]

Magrah, Good evening.

There are a couple of things.

In general an un-vented flue, that is a chimney flue that does not have an opening inside the room is well known to cause condensation inside the flue and problems will occur to the plaster finishes, which comes in many and various forms. You are correct in your diagnosis about salt / and or / Sulphate attack

The latter is caused when any fossil fuel is burnt, which release.s Sulphur that then combined in soot adheres to the linings of the flue, if exposed to moisture, rain water from the chimney pot, or condensation in the flue the moisture combines with the Sulphur to produce a very, very weak Sulphuric acid, the acid even in a weak form attacks the lime mortar in the flue.

Question? when the flue was capped, was there any damage or missing flashing's at the roof?

Most obvious fix would be to install a vent inside the room??

Ken

 

[bobasd]

hack off face of c/breast and open it up.the nice plaster will prob be a gypsum skim.sweep it. open and sweep the c/breast below if its blocked off .
dont use anygypsum to make good use render. and leave with a vent.
do you have any photos of the stack flaunching and flashing?

 

[Magrah]

Thanks both for the replies.

In answer.

Yes when the chimney was capped there was a little render missing, this has been made good.

Here is a photo.

A few points.

1) Can you seal against this i.e. with unibond or similar?

2) I don't want to reopen the chimney and vent it, any other ways? The chimney is now water tight with a cowel.

3) It's an external wall, worst case could I vent from the outside?

4) The damage is only in a small area above what was the fireplace in the bedroom, the chimney was blocked before we moved in, that was in 2010 and its been fine until recently. Can it get much worse?