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Community and Q&A

Insulating Masonry Chimney in Unconditioned Attic

MikeHeart | Posted in General Questions on

Hi folks, I have read as many threads as possible and seen different recommendations on how to do this.

My natural gas furnace flue runs inside a concrete block chimney from the basement through the main floor and up through the attic. The attic is uninsulated and therefore the chimney tends to release a lof of heat into the attic space.

I have about 12″ of cellulose insulation underlain by 2″ vermiculite which makes air sealing the penetrations very challenging and dangerous, but I’m doig what I can up there to reduce likelihood of ice dams. I reckon the chimney is a good place to start.

Some folks seem to be okay with wrapping the masonry chimney with Rockwool from the attic floor to the roof, the logic being that the concrete chimney gives the required 3″ of space, and the Rockwool of course being non-flammable.

Others insist on building a stud wall set back 3″ from the chimney and insulating with Rockwool, facing with drywall, and air sealing top and bottom.

From a code and practicality perspective, what do you folks recommend?

I don’t know if the base of the chimney has been air-sealed (I suspect not) given shortcuts I’ve seen the past homeowner make with air sealing.

Photo shows the chimney and the cellulose insulation currently blown against it. The steel flue pipe is just out of sight inside the chimney. The black pipe is a wrapped stove fan vent. The white cable is just a random electrical cable unrelated to the issue at hand.

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  1. Jon_R | | #1

    Rockwool sounds even safer than what you already have - cellulose against it.

  2. MikeHeart | | #2

    Agreed; the cellulose against it is not ideal. Just tryign to figure out just how much I need to modify this.

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