We have a 1954 brick chimney that has a metal liner inside and is only used by the natural gas hot water tank. It's had fiberglass right up against if for guessing 20+ years.

I'm air sealing the attic, it comes right up and through the attic with a couple inches gap between the framing and the brick.

I've read online about using sheet metal and high temperature caulk to close this air gap, and to keep all insulation away except (some say) rock wool.

Is this really necessary and/or required in my situation? This isn't a wood burning chimney, it only exhausts 1 50 gallon hot water tank through a metal flue inside the brick.

I'd like to just put rigid foam and stick it in place with Great Stuff and then insulate right up to the chimney. Most of my google searches have led me to info on insulating between the metal flue and the chimney, that's not what I'm looking for.

  • Was this chimney ever for a wood burning purpose? Is there still a firebox that some future person might use for burning wood? If so, you should assume it will be used that way and insulate accordingly. Apr 3, 2015 at 13:52

2 Answers 2


I would use that Flex Seal, unless you have high heat.

Check it out, can buy it in stores now 14 colors.



I have the same question. This is the best I've seen for the floor seal:


I've read online about people putting a 2" gap around the chimney, surrounding that with an aluminum sheath, fire-proof-caulking the gaps, and then putting insulation on the other side.

I have extra foil-faced poly-ISO, which has a high temperature rating, so I'm wondering if I can do that in one step...

EDIT: the question is: is it safe to use foil-faced poly-iso up against an exterior chimney or at a certain distance from the chimney, without an aluminum sheath?

I think I'll either: 1. go with a full aluminum sheath right against the chimney to serve as the air barrier, and then fill in behind it with Roxul, OR 2. seal between the chimney and the framing with fireblock foam or caulk (to prevent exterior air from coming in), and then put 3" of Roxul directly against the chimney brick.

  • Are you asking a question, or providing an answer?
    – Tester101
    Sep 24, 2015 at 10:12
  • It's a partial answer - I've found information on sealing the floor around a chimney, but not on sealing around the chimney. Sep 25, 2015 at 15:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.