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I found a way to use a large chimney balloon and pretty much block all downdraft. Sadly, the fireplace and hearth are still very cold, and it sucks heat out of the surrounding floor too.

Perhaps the firebox has tiny cracks letting in air, or the whole thing (brick outside, stone facade inside, no idea what's underneath) is just too heat-conductive. I'm already planning to hire/rent a thermal camera (for the whole house) which should help figure it out. But after that, what can I do to insulate it?

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If this is a "Through the wall" fireplace (exposed brick on the exterior wall, which I think is what you are describing) then yes, it conducts heat very efficiently out of your house. Insulation and stucco over the exterior brick would probably be the most effective solution other than removing it if you don't use it.

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If you are committed to not using the fireplace for the forseeable future, why not build insulated false walls on the sides, back and top?

You could use 2" rigid foam insulation, which has an R value of between 13 and 15 (depending on whether you have an air gap behind it).

rigid foam

All the edges need to be sealed with metallic tape to eliminate drafts. Then face it with 1/2 inch drywall (necessary for fire safety), using construction adhesive. The structure would probably be self supporting, but you could make a small frame of 1x2s behind the foam, if necessary, to make it more rigid.

Paint the whole thing matte black and it should look like a fireplace, just shallower. Put a decorative screen in front, or a basket of seasonally appropriate stuff.

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