Condensation On Internal Chimney Breast - Stack Removed

EViS · 6 Nov 2015

Since last year, the inside of our unused chimney breast becomes soaked in high humidity. Brown stains and salt streaks are present.

Thinking this was a leak around the flashing, I decided to simply take down the stack to loft level and extend the roof. This did not cure the issue.

I've since researched the cause on the forums and am now aware that the cause is condensation. However, I cannot find the best solution to solve this issue.

We have an internal vent downstairs and internal vent upstairs where fireplaces used to be sited. I would presume this is sufficient ventilation? What else needs to be done to prevent this condensation from taking place, now that the stack has been removed and the chimney is open in the loft?

dilalio · 7 Nov 2015

I would suggest that you take further steps to increase ventilation in the room(s) concerned...

Do you have windows in these rooms? Do they have trickle vents fitted? If not, try leaving windows open on vent lock if mechanism supports this (slightly open but still locked). Trickle vents can be retro fitted on upvc.

It appears that the condensation is finding a "dew point" on the chimney breast - as the coldest surface in the room. Houses of a certain age, in this country were not designed to be hermetically sealed and need constant air circulation. Do you dry clothes on rads or clothes horse in this room? As that is another cause of such problems?

Try a dehumidifier and see if that helps.

If it persists, then you can go the route of battening out the chimney, in filling with polystyrene insulation and over boarding with new moisture resistant plasterboard and skim coat.

ree · 7 Nov 2015

Have your flue swept - have all flues swept - and the stack flaunching and lead flashings inspected.

What do you mean by: "the inside" of the chimney?

theprinceofdarkness · 8 Nov 2015

If the back wall of your flue is an external wall then it will be at outside temperature. So any warm moist house air getting into the flue will cool and condensation will form. Like wise the hole of the flue in the loft, will let hot air out and cold air in. Its best to make the whole stack part of the inside of your house. Just insulate the whole of the top of the stack and flue as the rest of the loft is. Leave a small bleed hole if its solid foam, of say 1" diameter, just to bleed of a little hot air.
Frank

EViS · 9 Nov 2015

dilalio said:
I would suggest that you take further steps to increase ventilation in the room(s) concerned...
If it persists, then you can go the route of battening out the chimney, in filling with polystyrene insulation and over boarding with new moisture resistant plasterboard and skim coat.

We don't have trickle vents, but also don't dry any clothes in the house. Nothing about the house or our lifestyle has been changed in a very long time. What else could cause this to suddenly become a problem after so many years?

ree said:
Have your flue swept - have all flues swept

What do you mean by: "the inside" of the chimney?

The chimney is not used, there are no fireplaces or any access other than through two internal vents (one on each floor).

By "inside" I am referring to the internal wall of the breast. This is flush with the external wall of the house. The chimney breast protrudes externally out past the side of the house.

theprinceofdarkness said:
If the back wall of your flue is an external wall then it will be at outside temperature. So any warm moist house air getting into the flue will cool and condensation will form. Like wise the hole of the flue in the loft, will let hot air out and cold air in. Its best to make the whole stack part of the inside of your house. Just insulate the whole of the top of the stack and flue as the rest of the loft is. Leave a small bleed hole if its solid foam, of say 1" diameter, just to bleed of a little hot air.
Frank

I must admit that after all these years we still haven't got around to insulating the loft (!).

So I'm still stumped on the solution to this...

ree · 9 Nov 2015

If i understand you - you have a chimney breast containing two redundant flues. Each previous fireplace now has a vent installed.

Its irrelevant whether the flues are active or redundant - they should have been swept, and cleaned of soot before blocking up.

You will have to open any and all blocked flues, and have them swept.
(And maybe a camera dropped down to confirm the effectiveness of the sweeping, and the condition of the flue linings?)

The : "brown stains and salt streaks" are chemically liquidised soot from the unswept flues.

After opening up, sweeping and making good with replaced vents - you must knock off all affected plaster, and render up with a sand & lime render.

"taking things down to loft level and extending the roof" can you post pics of this? What i'm querying is how come you are dropping the stack into the loft but the chimney breast is external?

Blagard · 9 Nov 2015

I would guess, one of the flues is giving you a problem more than the other. Most likely would be the flue serving the ground floor fireplace.

With external stacks like you have they will be far more susceptible to condensation as another poster has pointed out. Even if the flues were swept before bricking them up there remains a risk of soot deposits becoming damp and giving the type of problems you describe. Certainly sweeping the flue before it is bricked up is and excellent idea, but not a total elimination of some risk. I would think the best solution would be to dryline the chimney breast possibly with an insulated back plasterboard.

I actually have a breast/flue that is partially projecting externally and now used by a gas fire (did have coal fires in its early life), It was swept before the gas fire was fitted but on the floor above the stain patch on the breast displayed a similar problem to yours. While it is not damp, I used a stain block over the affected area before decoration. If it was damp and other issues such as flashings were not a problem I would have dry lined it.

Edit noting the post above, you could replaster it avoiding the use of gypsum based undercoats, a sand cement undercoat would also be OK. However it will probably not stop the damp if that is also a problem.

EViS · 9 Nov 2015

ree said:
"taking things down to loft level and extending the roof" can you post pics of this? What i'm querying is how come you are dropping the stack into the loft but the chimney breast is external?

The chimney breast is partially in the wall and partially past the external wall. The stack was also built slightly inwards of the breast externally. So, by the time the stack was removed, the roof could be extended to the existing fascia either side and the roof covers the entire breast.

I presume a render with waterproofer is necessary. And can this then be floated and skimmed with gypsum plaster?

What's the cure for the condensation? Bearing in mind it has never been a problem in all the year's we've lived here...

Blagard · 9 Nov 2015

A waterproofer in the undercoat may help shield against dampness from within the flue but not condensation on the chimney breast are within the room itself. The cure for condensation is ventilation and heating, or a change of lifestyle so the relative humidity in the room is lowered. Or a combination of all.

A standard plaster skim on a non-gypsum based undercoat is fine.

dilalio · 10 Nov 2015

You could spend a lot of time and money finding and attempting to cure the cause(s)... There are numerous potential suspects and without a site visit, it's impossible to diagnose!

As I mentioned above, if it were me, I would fit trickle vents to the windows and blanket the offending wall with 25mm insulation and reboard with plasterboard. You could stain block the patches first and use foilbacked board.

It is the way my colleagues and I have proceeded to deal with such problems and know that it is common practice by other local contractors.

EViS · 10 Nov 2015

So are we saying that ventilating the chimney breast internally or externally is NOT the solution? But ventilating the affected room itself.

The room is always heated and used but there are no trickle vents. We'll leave one of the windows open on the first lock for the time being and see if that cures the condensation. I presume this would provide more ventilation than a pair of trickle vents would?

joe-90 · 10 Nov 2015

The easiest way to sort it is to stop moist warm air from the house getting into the chimney void. If it can't get in then it can't condense can it? Simple science will tell you that much. Putting vents in allows moisture laden air in that then condenses and gives you grief. Simple science tells you that much.

ree · 10 Nov 2015

OP,

FWIW: i mentioned in my above posts what the matter was and how to remedy it.
Its all very straightforward, and what i suggested is the considered best practice in the Remedial Industry. It works.

FWIW: if you are still concerned about hygroscopic liquidity in your flue(s) then perhaps post some pics of your difficulties?

dilalio · 10 Nov 2015

EViS said:
So are we saying that ventilating the chimney breast internally or externally is NOT the solution? But ventilating the affected room itself.

The room is always heated and used but there are no trickle vents. We'll leave one of the windows open on the first lock for the time being and see if that cures the condensation. I presume this would provide more ventilation than a pair of trickle vents would?

You now have enough information/opinions to bring in a specialist and be confident in discerning bulls**t from beef!

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Condensation On Internal Chimney Breast - Stack Removed

Similar threads