Chimney breast damp patch?

[Moxi]

Hi

More questions for the forum from my little welsh cottage.

In the main bedroom there is a persistant damp / condensation patch on the outside facing stonewall which always appears after a nights sleep.  Closer inspection has revealed that there is a blocked up fireplace below where the patch forms (there is an air grill in the boarded over fireplace) It dries through the day but always reforms at night presumably because of the warm moist air exhaled from me as I sleep ?  This chimney must link up to the main chimney breast and the use of the 14kW fire downstairs doesn't seem to exacernbate or alleviate the symptoms.  Anyone got any ideas?  I want to dry line the stone walls in the bedroom once I have finished the skielings (see other thread) but don't want to do that until I understand the cause and remedy for this persistant re-ocurring patch).

Moxi

[dhaslam]

Damp patches imply condensation  on a cold surface but they can also be caused by heat.     If a stone wall is heated and it   contains  moisture  the  heat  will evaporate  it  produce  dampness on the  surface.    This happened in my parents house one hot summer in the 70s.   Even though  there had been  no rain the south facing walls were dripping with moisture in the middle of the hot spell.     

[biff]

there is possibly another reason,
                           rain can come down the chimney and settle into the bends or corbles,while the house is active,ie,,people opening doors and closing them and general traffic.the moisture is carried back up the chimney and away,once air circulation dies down the moisture will creep back to the surface,especially if it is a damp night or raining, a roof like vent on top of the chimney will go a way to cure this,
 another possible cause is that there might be a big brute of ashlar stone just under the plaster at that point,sometimes the old masons used these to move the flue over or bend it closer to the one from downstairs to reduce the size going through the roof.if it is a big ashlar then your dry lining will see to that.
                                                                     biff

[Moxi]

Thanks guys, is there any merit in opening up the sealed fire place to look up inside the chimney ?

Its very odd as it goes by lunch time with the bedroom window open but first thing in a morning there is streams of moisture running down the wall but a defined bell shaped patch which i assumed was where the chimney breast was.  Even if I sleep with the room door open and the window ajar I still get a small amount in this one area, the rest of the room is bone dry.

Moxi

[Baz]

Is that flue capped off at the top? It does sound like moisture from you (a person breathes out about 6 pints a day I think) finding the coldest place to condense out. Also the fire downstairs will move moisture from that room up through the floor into the room above.

[Moxi]

Baz,

Not sure if its capped off as the fire place has been sealed up except for a floor level grill, so i may risk the displeasure of the other half by opening it back up to look inside, if its condensate then I can pack out the upper part with mineral wool.

I will invest in a chimney cowl as well as i am sure that rain does get down the chimney and at 93m above sea level on the west coast I do see a reasonable amount of rain 

thanks Moxi

[micko]

Could be something as simple as the chimney flashings, this is a common problem and produces the same kind of marking between the wall and ceiling junction.

[wookey]

It sounds like this part of the wall is cold enough to get below the dewpoint so all the airborne moisture is condensing out there. Does it do it on the windows too? If not then you either have very good windows or there is a freezing gale blowing up that chimney...

As motted it could also be a plain old leak (but the overnight timing suggests not) and it could also be due to salts having oozed out of the chimney into the plaster, making it hygroscopic, but again I'd expect that to keep it damp all day.

You could try measuring the temp of that wall compared to others. (easiest with an infrared thermometer) to try and confirm/deny the condensation theory. If it was me I'd fill that disused chimney with leca and cap it.

[Baz]

You will be able to tell if the top is capped by seeing if there is a draught at the bottom vent on a windy day with the window open. Don't fill an unused chimney though as rain on the exposed bricks at the top gets in and can't escape.

[JohnS]

When I filled my chimneys with Silvapor, I included a drain at the bottom.  50cm of perforated land drain wrapped in geotextile fabric and poking through the bung in the bottom of the chimney.

Theory is that any moisture will decend to the bottom, go through the fabric and drip/evaporate away.

John