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Dayton OH for code purposes (climate zone 5).

I have been preparing to blow in fiberglass insulation to my attic (had likely-asbestos-containing vermiculite abated) by installing soffit vents, using expanding foam to seal holes and gaps, making sure the electric is all done so I never have to come up here again: once I have 17+" of fiberglass (to achieve R49), I know that getting around is going to be terrible.

I realized today that I had given no through whatsoever to the giant chimney. Currently unused, but we'd love to eventually get it operational again. I say "unused" but I think the water heater might be venting into it in the basement. Anyway, after some research it seems that combustibles need to be 2" away. Fiberglass isn't a combustible, but I'm trying to go above and beyond code since I live here and far into the realm of "best practices."

Without any guidance, my plan is to get some 3.5" bats of rigid-ish mineral wool meant for stud bays and just... wrap the chimney up above the 17" mark, holding it on there with a bunch of aluminum foil tape, then ignore it while I blow in the fiberglass. Is this acceptable? Is there something I ought to be doing (instead)?Image of chimeny in attic

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    wear a helmet to avoid one of those nails in your skull Commented Jul 27 at 1:52
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    You need to check to see if the cans for your lights are rated to allow for insulation around them. Some are not and that would be more serious than insulation around the chimney. Commented Jul 27 at 13:14
  • @RMDman They are IC-rated; I installed them myself. They’re also holding LED bulbs. Commented Jul 27 at 15:34
  • @DIY75 Good call; no incidents so far but they’re nasty and all over Commented Jul 27 at 15:35

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You are good on your plan. The batts will keep the blowing wool from dropping down in the gap of the chimney to the framing. Not that a lot will, but it will. In actuality, you really only need enough around the chimney to do just that.

The only reason I would add more would be to cover the areas that the blowing wool might not get too, such as the blind side of the chimney, where the roof gets close.

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Before you blow fiberglass insulation into your attic, take a little time to prep your chimney. Start by looking it over carefully—if you see cracks, gaps, or crumbling mortar, seal them up with fire-resistant caulk or mortar. This keeps insulation from slipping through and keeps it away from hot surfaces.

You’ll also want to make sure there’s a proper metal flashing or chimney collar in place. Think of it as a protective barrier that keeps a safe space between the chimney and the insulation. Without that gap, there’s a risk of heat building up where it shouldn’t.

Lastly, double-check that your chimney is clean and in good shape before you add insulation. A quick inspection now can prevent problems later and make sure your home stays both safe and energy efficient.

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