DIY Doctor

What's the Correct Method for Sealing up an old Chimney and Preventing Damp?

Postby Mike34 » Tue May 21, 2019 11:16 pm

Hi

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.
Mike34
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