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Worried about air sealing fireplace attic vent cavity

I'm an insulation contractor and I also try to close up the "big" air sealing holes before we blow in additional loosefill insulation. I want to close off the enormous cavity the builder left for the fireplace vent stack. My concern is how does a fireplace draw fresh air from combustion? Do I need to leave this cavity open to prevent back drafting?
Thank you for your responses.



Asked by Randy Ransom
Posted Dec 3, 2012 9:10 PM ET
Edited Dec 4, 2012 9:13 AM ET


4 Answers

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What kind of fireplace? The ordinary type just draws huge amounts of air from the living space.

If you can find the brand of chimney you can look up the installation detail, but I would guess that square flange is for floor penetration and can be framed around.

Answered by K Willets
Posted Dec 3, 2012 11:44 PM ET


I tentatively agree with Kendall--the square flange is a firestop and should be part of a solid "lid" above the fireplace, at the level of the attic floor. You should not be able to look down and see the top of the fireplace, there should be a plywood panel over that cavity with the firestop as part of it. You may have to do some framing to support the plywood correctly and air-seal the whole thing.

You MUST get the installation instructions for the fireplace and the vent pipe system to verify the clearances to combustibles and other requirements, but the goal is to close off the cavity, install an insulation baffle around the pipe, then install the insulation. The stuff that has fallen down on top of the fireplace should be vacuumed out first.

It wouldn't hurt to get down inside the chase and improve the wall insulation, too... at least do something to secure it in place, or better yet enclose it in an air barrier.

We fixed one that looked a lot like that recently. There were large holes in the sheet metal fireplace box--it was a MASSIVE air leak. about 200 CFM50 if I remember right.

Answered by David Meiland
Posted Dec 4, 2012 12:42 AM ET
Edited Dec 4, 2012 12:47 AM ET.


The question of how to seal the penetration where a metal chimney goes through a ceiling has nothing to do with the source of combustion air or makeup air for the fireplace.

First of all, it is essential that you seal the air leak around the chimney. Once that is done, you can determine whether the fireplace has combustion air needs. (I'm not belittling the issue -- of course every fuel-burning appliance needs combustion air, and you need to be sure that the appliance is safe and is burning cleanly. But one problem at a time...)

As others have mentioned, contact the fireplace manufacturer and find out whether there is a chimney kit for this location.

Otherwise, the standard solution to air-sealing around a chimney is to use two or more sheets of galvanized steel, cut to fit closely around the chimney. Then seal all seams, as well as the perimeter of the metal, with high-temperature caulk.

Answered by Martin Holladay
Posted Dec 4, 2012 8:35 AM ET


Also note- all that exposed pink fluff in the walls of the flue chase are radically impinging on the performance of the insulation due to free convective exchange with the air in the chase.

I'm not sure how easy it would be to add an air barrier to that side, but this is one of the rare instances where perforated aluminized polyester radiant barrier stapled to the studs may be a "right" solution. While not ideal (the perforations would still be subject to infiltration pressure), it would impede the convective currents and dramatically reduce the radiated heat penetration of the insulation.

You'll probably be able to legally & functionally block the big hole at the top with sheet metal, but fixing the sagging batt situation would still be critical (especially with the paper facers.) It's not clear if you'd be able to seal and loose-fill the part that appears to be exterior wall, but you'd probably be OK with R15 rock wool batts, with or without an interior side air barrier.

Answered by Dana Dorsett
Posted Dec 4, 2012 1:14 PM ET

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