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Chimneys & Fireplaces

What is the correct way to 'seal' a fireplace and chimney to stop damp please?


I have an unused chimney / fireplace that has an electric 'coal effect' fire and wooden fireplace surround. The previous owners had the top part of the chimney stack removed. It now finishes in the loft, at upstairs ceiling level. I believe that the top was originally 'open' to the loft space, although this may have been covered over by the contractors who installed the thick insulation (roll type) in the loft.

Particularly in the colder months, I had been noticing (around the 'joins' between the wood parts of the fire place surround) a 'furry' type of white substance (presumably a mould - does any one know what type this is most likely to be please?)

I have removed the fire and wooden fireplace surround to have a look and found that the fireplace behind had been completely 'faced' with plasterboard (sides, rear and top) fixed to battens. As these looked a bit damp / mouldy, I removed them (I assumed that, in this condition, they would need to be removed anyway!) Behind the plaster boards are bricks. When I removed the top horizontal plasterboard, I found there was also another horizontal plasterboard level above this. When this was removed quite a lot of debris (soot, etc) came down as well.

In case it is relevant, the fireplace opening is 950mm wide x 1420mm high x 480mm deep (approx) and the actual chimney (stack/flue?) is 300mm wide x 230mm deep (approx).

My question is what is the best way to 'seal' the fireplace (to stop possible draughts into the room & loss of heat, from the room, up the chimney) whilst allowing sufficient ventilation to the chimney (to prevent mould / possible related musky smells, etc) please?

I assume that at least some chimney ventilation is necessary to prevent the problems that have been experienced (although please confirm this) but I've seen mention of people 'sealing' the bottom of their chimneys with inflatable 'balloons' (or other similar devices) to prevent heat loss / draughts. Presumably the top of my chimney (at ceiling level in the loft) should also be left uncovered (as mentioned this has probably been covered with loft insulation)?

In case it has a bearing on any advice provided, currently I wish to refit the electric 'coal effect' fire and the wooden fireplace surround, although sometime in the future I may wish to have a gas fire fitted. Related to this, for future reference, would I probably need to have metal flue liner fitted up the inside of the chimney (and also through the loft and the roof tiles above; or, if this is an acceptable alternative, through the adjacent external wall in the loft rather than the roof tiles)? Alternatively, are gas fires available that have rear flues (that could just go out the rear of the chimney to the outside)? If both are acceptable options, are there advantages / disadvantages of either option please?

Thanks in advance for any advice provided.

Best wishes


5 Answers from MyBuilder Chimney & Fireplace Specialists

Best Answer

idea there a small air flow, so rain cap at top with vent or chimney balloon at bottom.


Answered 30th May 2019

good morning

everyone keeps mentioning a bird guard cowl of capping cowl, this is not the case as the stack has been taking down below roof level. two options really . you could just remove the loft insulation but this could lead still to something nesting in it if they can get in to the attic space or you could block the top off it off an get an air vent fitted into the side of the stack , at the bottom you would need to have a vent fitted if you are going to block off the hole again as long as some air can get past the electric fire then you wont have a problem . the main thing is to make sure air can still flow through the complete flue by having it checked out by a sweep to make sure there are no old nests in there

as for the gas side you could need to contact a registered gas installer who could advise you more


Answered 7th Jun 2019

When coal is burned in oxygen it produces a salt called ammonium sulphate. This salt absorbs moisture from the atmosphere, making your unused chimney hygroscopic and prone to dampness. All chimney flues must be ventilated top and bottom. If you have damp patches on the chimney breast , it will have to be re rendered with a waterproof render . This is a very common condition and even removing the chimney doesn’t stop it because the back wall is also contaminated.


Answered 20th Aug 2019

Vent in downstairs chimney breast and vent in upstairs chimney breast


Answered 5th Jun 2019

Ideally a bird guard cap on top of the pot, this would stop birds from nesting, as they notice when a chimney is not in use!!
The guard would allow the chimney to breathe, as long as you have a vent at the bottom of the remaining chimney breast.
Thanks 🙏


Answered 4th Jun 2019

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