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Discussion Starter · #3 ·
I would use mineral wool. Is this in an attic? If so, build a sheet metal enclosure to accommodate the depth of the new insulation and seal with intumescent caulking.
Yes, the fireplace runs from the crawl space to the attic and out the roof. I plan to sheet metal around it from the crawl space to contain the insulation and fill the groove to the attic. I had some rodent intrusion in the past and they used this space to gain access to the attic and destroy drywall and other stuff which is another reason for filling this space besides the thermals.
 

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Usually, there is a 2" required gap that cannot be filled. It can be bridged with sheet metal and sealed in place using a fire rated caulk. Then the dam 2" away to hold back the insulation. The 2" is often ignored or may be otherwise based upon local codes, but it is a good practice.

Bud
 

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Discussion Starter · #8 ·
Got some pictures? I wouldn't put a fibrous insulation against the chimney given that it isn't really needed. Put the sheet metal on it and seal it with fireproof caulking. You can back the sheet metal with caulking if you really want to, but its not necessary.
I don't have any pictures but essentially if I lay on my back in the crawl space and look up from the base of the brick chimney, I see a 3" gap between the brick and wall studs and can see straight to the attic space. the back of the chimney runs through my master bedroom so insulating it will help since it will stop the cold air from the crawl space and attic from coming through the wall section. Cellulose is non flammable so why would it be an issue having it against the brick? I have a couple bags of the stuff sitting in my garage so I just need to pump it in from the attic
 

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Placing insulation against a chimney can cause heat to build up in those areas. The gap is there to meet fire codes. Air sealing both top and bottom with the flashing should make an improvement. If the wall in the bedroom is still cold then we would need to see/hear a description of what is there, stud wall or just drywall?

Bud
 
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