Filling large gap in brickwork with expanding foam

7 Sep 2009
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United Kingdom
I'm going to box out my fire place with plasterboard to cover the exposed brick work. In the centre there is a very large channel where bricks appear to have been removed to help the original fire draw better. Given that this channel is going to be behind the plasterboard, is there any reason why I shouldn't fill it with expanding foam.
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Expanding foam cost about £4 for a cubic foot! with zero use for fixing to it. That "channel" is it the flue? . If it is, then if its open at the top so rain can get in, then it needs to be vented so it can dry out. Personaly I would brick/block the front up with a hit and miss vent in it at the bottom. Second best would be to batten it up and plasterboard across the lot.
Its not clear to me what you are referring to, can you pic this "channel"? Are you referring perhaps to the flue or a "draft chamber" below the back of the hearth?

As things stand, Dot and Dabbed plasterboard will not hold back the sooty salts from contaminating your decorated surface.

All soot should be removed back to as clean as possible brickwork, and the flue should be swept and ventilated at the fireplace and at the stack terminal.
The photo is a few months old. and the soot has been removed and the chimney swept and a cowl fitted to prevent water ingress. Although the fire place is approx 200 years old, I believe it has been modified at some point. The tiles that are visible are not the actual floor, but have been raised up about 50cm and sit on much more modern brickwork. The neighbouring properties do not have this. The channel does indeed act as a flue, however it was obviously created by removing bricks from an original structure. In fact the back of the channel is only one brick thick through to the adjoining property. Before I did some repointing, you could actually see the back of their plasterwork through the gaps. I had intended to replace the bricks, but I am not confidant in my ability to do it properly, and unfortunately I can't find a tradesman who considers it a job worth getting out of bed for. I agree that expanding foam is not the ideal solution but I don't want to batten and plasterboard it out leaving a large void behind
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This back wall would be an ideal place to learn how to lay bricks! A single thickness of bricks does not meet fire regulations now. Also such a thin wall will have poor sound insulation.
Indeed my original intention was to use this as a brick laying learning experience. However, there are two things that are worrying me. Firstly, the channel was created by removing bricks and I am concerned that if I did not do the work correctly I could end up adding to the stresses on the existing structure rather than supporting it. Also the raised tile floor of the fire place does not extend back into the channnel. There is some brick work about 30cm below the tiles but I don't know how solid this actually is, therefore I cant be 100% confidant in the foundation of any brickwork that I added. Expanding foam does have the advantage of being relatively light.

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