Damp in the chimney breast

[Wanderwolf]

Hi,
I have an offset link detached property built about 1900. It has three chimney stacks, one where the property abutts up against an adjoining end terraced house ( so the chimney breast itself is not exposed to the weather), one on the outside where the chimney breast wall is an outside wall, and one that goes up through the middle of the house. I had all three chimneys professionally capped with cowels.
The middle of the house chimney starts at ground floor ceiling level, ie lower part was removed to facilitate kitchen modernisation. This chimney breast ( in the upstairs bedroom) has damp patches on it. The damp is not coming up and the damp is not coming in with rainfall so it MUST be condensation. Correct ?
It doesn’t have an air brick but nor is it tightly sealed at bedroom fireplace. I understand the condensation arises because of warm air from the room rising to meet the cold air in the attic and then dripping down.
If I fill the chimney with insulation or somehow seal it at attic floor height would this stop the problem or would the resultant lack of air circulation in the chimney cause other problems. This is what I would like to do if it will work!
I am not keen on massive air bricks to allow expensive hot air to escape !
I don’t have any problems with the chimney against the terraced house which I don’t understand but have similar problems with the chimney on the outside wall, again at bedroom height.
It’s very depressing !

 

[JohnD]

Disused chimneys must be ventilated top and bottom or they will get internal condensation. Each chimney typically contains an upstairs and a downstairs flue, both must be ventilated.

No need to use a massive airbrick, just use an ordinary one.

 

[mrrusty]

so it MUST be condensation. Correct ?

Maybe not. Chimneys are very prone to accumulating hygroscopic salts caused by the reactions between combustion gases and water to form acids, and the acids react with lime and other building products to form salts which accumulate in the bricks. The salts pull water vapour out of the room air and show up as damp patches.

Disused chimneys must be ventilated top and bottom or they will get internal condensation

Not necessarily. A chimney ventilated at the bottom exposed to warm humid room air will show condensation at the top if the top is colder than the dew point. Warm air rises, and will rise up in a top and bottom ventilated chimney . A chimney ventilated like that could just be a condenser! Condensation happens when air containing water vapour cools to the dew point. If the supply of humid air is replenished through unrestricted ventilation, the condensation is continuous if the air is allowed to cool to the dew point. If you want to ventilate the bottom of a chimney, then vent it outside.