Merits of removing masonry chimney?
The sixty year old chimney at my 50s ranch is severely damaged from moisture kicked off by our hot-water heater (now replaced with an indirect WH off our boiler). I’ve received several bids to have it rebuilt but I have a different idea. I am thinking of tearing down the chimney to below the roofline, insert a round metal chimney vent in its place, and finally roof & CC sprayfoam the cavity left behind. Rather than a 6′ by 3′ chimney I’d then only have two 6″ metal pipes (basement & 1st floor fireplaces). The chimney is in the back of the house and adds nothing aesthetic.
I assume this would fairly massively reduce thermal bridging associated with all that masonry. Both fireplaces have inserts and flex-metal piping within the flue. My thought would be to slide the flex-metal pipe inside the two new rigid chimney pipes that would protrude the roof-line.
Worth the effort?
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The short answer is that you can do as you propose. Consult with an experienced mason concerning the flue transitions and the metal chimney penetrations.
Of course this raises a lot of questions.
1. Is it worth it to demolish the chimney to the attic floor?
2. If so, why not get rid of the chimney entirely? Who needs fireplaces? Maybe you can get rid of your fireplaces, too.
3. How expensive is the work? Who will be performing the work?
Whatever you do, safety is paramount. If this is a DIY job, make sure that you understand chimneys and relevant building code requirements.
That may be cheaper than rebuilding the masonry chimney(?)
You'll probably need to install safety certified insulated chimneys designed for wood stoves, rather than randomly sliding flue liners designed for masonry chimneys inside a steel pipe.
Why closed cell foam insulation the cavity left behind instead of (completely fireproof) rock wool, or something?
Is this an exterior chimney, that would require building out a 6' wide section of framed insulated wall?