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Community and Q&A

Insulating Outside of Masonry Chimney

Hgck2JF5Wd | Posted in Energy Efficiency and Durability on

I’m wondering about the efficacy of insulating the outside of a masonry chimney. The attic/loft will be used as semi-conditioned living space, and I am concerned about 1. keeping the chimney from sucking all of the heat out of this area and 2. potentially improving the efficiency of the house overall. I’ve been thinking that I might be able to insulate around the chimney in the loft space– roughly 9′ of chimney– and that at that point, the cinder block will actually represent a small but appreciable r-value, perhaps even as high as r-10. (This page– –lists 8″ block as having an r-value of more than 1) It seems that the chimney mass might function better as well, leading to some synergistic gains. I’m not sure what one would use, though rigid fiberglass and rigid mineral wool seem like possibilities. Fire concerns might dictate building a chase 2″ away from the chimney and insulating that, which would allow for a wider array of insulation materials.

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  1. Hgck2JF5Wd | | #1

    I should clarify that I am talking about insulating the chimney on the INSIDE of the house where it passes through the loft/attic. I've never heard of anyone doing this. I suppose sealing the loft of seasonally would accomplish the same thing.

  2. Hgck2JF5Wd | | #2

    I'm in climate zone 6 and have r-40 walls and r-50 roof. It would seem the masonry chimney would be a significant thermal deficit, though heating it up a couple of times daily would minimize that a bit. Are people building efficient houses in cold climates using masonry chimneys, and if yes, isn't it logical to try to improve their thermal performance?

  3. GBA Editor
    Martin Holladay | | #3

    First of all, superinsulated houses rarely include masonry chimneys, except for the homes of die-hard fans of masonry stoves. What appliance will your masonry chimney serve? The best solution to your dilemma is to eliminate the masonry chimney.

    Assuming you don't want to change your plans -- or if the chimney is already built -- then it is perfectly possible to insulate the chimney as it passes through your attic. Surround the chimney with four stud walls that you pre-assemble on the floor of your attic. Include drywall facing the chimney. Space the drywall an inch away from the chimney. Then insulate the stud bays with the insulation product of your choice. You can add more rigid foam over the studs if you want, on the side of the wall facing the attic.

  4. Hgck2JF5Wd | | #4

    thanks for the reply. The chimney is already built and will serve a small soapstone masonry stove and a wood cook-stove. The flue is rigid stainless wrapped with ceramic blanket. Obviously, the heat loss calculations of the chimney are complicated and depend upon how and when the stoves are used. It seems to make sense to insulate the outside of the chimney to the same r-value as height of insulated chimney-- which seems to be in the neighborhood of r-10. From your response, I'm surmising that you consider masonry chimneys to be a significant energy sink. Do you have any other ideas for increasing the thermal performance of a masonry chimney?

  5. GBA Editor
    Martin Holladay | | #5

    1. Install a stovepipe damper, and be sure the damper is closed when the stove is not in use.

    2. Close the air intake openings of your wood stove when the stove is not in use.

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