Tracing/curing damp

We've got a damp patch on an upstairs wall that refuses to go away. It feels cold and clammy while the surrounding area is warm and dry.Pictures here:
http://i18.photobucket.com/albums/b104/mkpb/pic1.jpg
It's the inverted L-shaped patch. There's a hairline crack just visible round the edge of the patch and the dampness doesn't spread beyond the crack.The adjoining wall to the left is an internal wall and the dampness doesn't spread to that either.It doesn't get better or worse in the summer/winter or in dry/wet weather. The room is ventilated and warm.
Outside there's a chimney which was the flue for an old boiler that was removed years ago.
http://i18.photobucket.com/albums/b104/mkpb/pic2.jpg
The internal opening was plastered over (no airbrick). There was a steel inspection plate on the outside base of the flue but that's been sealed as well now, as a ground-floor extension has been built round the lower part of the flue.
There are no obvious problems on the roof. The flashing and tiles look fine. The pointing looks sounds (looks like it might have been redone at some point). The guttering is clear and the cowling is solidly set in mortar (but at a slightly wonky angle).
I've just hacked the damp plaster off along the line of the hairline crack, and found that the damp area coincides exactly with brickwork while the dry area is breeze block.
http://i18.photobucket.com/albums/b104/mkpb/pic3.jpg
I'm guessing that the chimney is the most likely cause of the damp, but I don't understand why it's not spreading and never varies.I've Googled the archive and opinion seems to be split on the cure for dampness due to disused chimneys. Some say ventilate at top and bottom (I could add an air brick above the flat roof level --- if there a metal liner in the flue that's connected to the cowling, would there be any through-flow of air or would I just create one opening into an otherwise sealed void?); some say that if the bottom is sealed the top should also be capped with a slate; some say that a cowling on the top is all that's needed.
Anyone got any ideas? Is the chimney the problem or is there something else I haven't considered? Advice much appreciated.
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It's probably got at least a 3 month time constant, which is going to average out most dry/wet weather periods anyway. When you fix the problem, expect it to take at least this long to dry out.
I would guess it's the chimney based on what you've said. Flues need ventilating at the top and bottom, or the inside eventually ends up soaking wet, and seeps through the brickwork. It's also a good idea to prevent rain water going in the top, although that's not essential -- if it's correctly ventilated, this will dry out. I don't know what the effect of the liner will be, but I could imagine that leaving this in a disused flue might well prevent it drying out even if it was ventilated.
Looking at the angle of the cowl or pot (can't tell which it is in the pictures), I would guess the flaunching has failed on the top of the chinmey, which might be funnelling more water into the flue than would otherwise be the case.
If you will never want to use the flue again, you might consider taking the chimney down to the level of the upstairs ceiling, leaving it open (vented) into the loft, and tiling across the hole in the roof. This would minimise any future maintenance required. You will also need to vent the bottom. I would do this to the outside rather than the inside (a vented flue loses you something like 1kW of heat if vented into the room). You might also have to pull out the flue liner.
--
Andrew Gabriel

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Mike wrote:

If your up to it redo the flashing and take of the guttering and inspect the wall behind the guttering whilst its off. I'd suspect the guttering as the culprit because from the pic the rendering looks sound but hairline cracks might be evident behind the guttering.
Tip a bucket of water in the guttering and see how fast it runs off in the area where the internal damp is.
-- Sir Benjamin Middlethwaite
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It is likely that the cause is condensation within the flue. This is why it is damp almost permanently. In addition, salts activated by the dampness could be 'hydroscopic' ie they attract more moisture from the air. The different densities of block and brick is why the block area seems drier.
Venting the flue should cure reoccurance - an air bick at the top and one as low as possible - preferably inside the lounge to draw warmer air up the flue. This will create a through draft to dry off internal moisture.
dg
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