Damp Chimney Breast

16 Jul 2008
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United Kingdom
I have a damp patch that extends from the top left front of the chimney breast, round the side and onto the (cavity) wall to the left of the fireplace. It runs down the wall from the ceiling about 2ft. Many years ago when i moved here i spotted this damp and had the chimney re-rendered and the flashing replaced. This seemed to cure the problem although i still got damp readings from the plaster, the walls did not feel wet.

At the time the building was a bungalow, but recently i had the roof removed and the existing walls extended upwards to form a 2-storey house. I also demolished the upstairs chimney breast that was now part of the bedroom and stuffed a piece of fibreglass insulation in the flue beneath the floorboards.

A few months ago, i noticed that the damp patch had returned and water was actually forming a drip down the wall. Since then i've been more vigilant in venting the room and the patch doesn't get wet but still feels very damp. The weather seems to have no bearing on how damp the patch is, and the outside wall is rendered, apart from a single course corbel between the old and new build.

Can anyone please tell me exactly what the problem and remedy might be? My guess is that salts in the plaster from the original defective flashing might be causing condensation in that area, possibly compounded by the extension work and/or blocking the chimney off.

Also, I'm about to have freebie cavity wall insulation injected into the lower half of the property, but i'm worried that this might complicate the damp problem and/or compromise the 80 year-old wall ties.

Any helpful comments would be appreciated.
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Auto, Hi.

Sounds as if you have condensation occurring within the old remaining flue what happens, if the flashings are in tact, and there is a cap on the chimney pot, condensation WILL occur in an un-used flue, When blocking off an old chimney flue, it is imperative that there is an air flow through the flue, if not condensation will occur.

in your case it appears that the flue has been blocked at its upper end. Question? ios there an air brick inside the room where the fire place was? It is important to provide an air flow from the room, but in your case the top of the flue is blocked, in this instance i suggest that the only way forward is to cut an opening into the flue, on the external wall and fit an air brick on the =external wall to allow the flue to ventilate correctly, also you need a vent, i prefer hit and miss vente that you can control inside the room, so when it get really stormy, you can stop the flow of air , but when calm allow the vent to be open and do its work.

The next thing you will get if you do not vent the flue is a brown, dark mark starting to appear on the internal walls, this is the residue of the "tar" produced over a long number of years "leatching" through the bricks and plaster into the inside decor of the room.

Bottom line, provide ventilation top and bottom of any disused chimney.
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Thanks very much indeed Ken. I'm sure that your advice is good but just to clarify a couple of things.

The original (1930s) chimney was 'internal' i.e. the outside wall of the building is flush, and the flue was blocked off underneath the upstairs floorboards and the chimney above this level was demolished. So obviously no more flashings, and the flue extends upwards just a couple of metres inside the downstairs fireplace that is still there. The flue is still vented (i.e. open) at the botttom. There is also a vent closeby at skirting board level that i put in when i installed a gas fire some years ago.

Given this clarification, i assume that your advice is still the same - to install an airbrick in the outside wall of the house just below the level of the sitting room ceiling? This would presumably require drilling through 9" of brick and judging it to break into the flue (which is obviously nowhere near as wide as the breast and offset to the left)? This might be a less than simple task! Or have i misunderstood you?

And is it the whole story? I've posted a picture below which shows the extent of the damp patch including an area of the (cavity) wall to the right of the window. Is this likely to be caused entirely by the problem you have highlighted, or partly at least a result of ingress from the old defective flashing leaving the plaster in a condensation-attracting state? If so, presumably a hacking off, followed by sand/cement followed by plaster skim (in conjunction with the airbrick) should fix the problem? Or perhaps there are other posibilities?

Again, many thanks for your response.


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