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Community and Q&A

Exterior insulation behind stone chimney

TimL_in_ND | Posted in Green Building Techniques on

We are planning to add exterior insulation to an old house. House has 2 layers of siding. There is a large external field stone chimney on the east side and the siding runs up to this. I don’t know if this is the same for the inner layer of siding; could be that the siding was installed and then the chimney built. This inner layer has lead paint and may be left in place when adding the exterior insulation (likely spray foam). If removing all siding to access the wall sheathing gives us options for insulating the wall cavity behind the chimney, seems like we should consider this.

House had formaldehyde insulation blown in (70s). Not sure if they did the wall cavities behind the chimney – don’t think so since I don’t see drill holes on the inside of the house where the chimney goes up the wall.

What options should I consider for insulating the wall behind the chimney? Tear it down as much as possible and add a new metal chimney seems extreme.



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  1. GBA Editor
    Martin Holladay | | #1

    The easiest approach would be to open up the stud bays from the interior.

    It's possible (but challenging) to insulate the entire exterior side of the chimney with mineral wool. (Note that this approach raises code compliance issues; not all code authorities will approve doing this.)

    Here's a link to an article by Chris West describing this approach: Providing Outdoor Combustion Air for a Wood Stove.


  2. Expert Member
    Dana Dorsett | | #2

    You may need to do some investigational probing from the interior side. The masonry needs a 1" clearance to combustibles to meet code:

    If then necessary clearance to the sheathing exists it's fine to insulate the wall cavities (assuming they are currently uninsulated.)

    It's also possible to insulate on the exterior, but that would also have aesthetic issues to be addressed.

    Is the chimney currently in service? (If so, by what?)

  3. charliexu | | #3

    Hi Tim,

    Curious what did you do with your chimney in the end. I have the same situation here and I am exploring ideas. One idea right now is to frame it out with metal stud and put sheathing , air /water barrier then Rockwool insulation and siding. But that will have to terminate before touching grade which will leave out some space for leakage.

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