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Insulated chimney

Hello Martin,

I'm considering wrapping this chimney in 2"- 3" of Roxul or Thermax or spray foam insulation before we cover it w/ sheetrock. Code is likely to require Roxul I think.

The picture shows I'm standing on the 2nd floor and shows the chimney as it comes up from the 1st floor and then goes through the 2nd floor ceiling, into the attic. This house will be getting an unvented attic, w/ 6-7" of CC spray foam on the roof decking / rafters, so the attic will be regarded as conditioned.

The 1st floor portion of the chimney will be covered w/ fieldstone veneer. (typical) I look at this chimney as a huge wick that steals heat from the 2nd floor and conditioned attic when the fireplace isn't burning. Do you see any problem w/ doing this insulation strategy. I recognize fireplaces like this do not promote building performance, but the homeowner was going w/ the fireplace no matter what.

Thanks for your help.

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Asked by Kevin Hanlon
Posted Jan 16, 2013 10:25 AM ET


9 Answers

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From a thermal performance standpoint, the best thing to do with this chimney is to eliminate it.

How many flues does the chimney have? What types of appliances does it serve? Are these appliances gas-fired, oil-fired, or wood-fired?

If there are wood stoves or wood-burning appliances attached to some of the flues, then it makes sense to leave the chimney uninusulated within the home's thermal envelope. When the flues are hot, the chimney can radiate some of the heat upstairs.

If the house had a vented unconditioned attic, it would make sense to insulate the attic portion of the chimney. But it sounds as if your attic is conditioned.

Answered by Martin Holladay
Posted Jan 16, 2013 11:42 AM ET


Codes are a bit squishy and somewhat self-inconsistent about chimneys, but code demands a 2" clearance from the chimney to combustibles, EVEN IF there is 12" of masonry between the flue liner and combustible (which is considered OK if the chimney is part of a structural wall). You're also not allowed to use the chimney as the structural support for anything else, not even rigid foam, Roxul, or spray foam- something ELSE has to be the mechanical support for the insulation.

That said, with 3.5" mid-density R15 rock wool between the masonry and combustibles at zero clearance (no gap), there is extremely low ignition potential in most real-world scenarios.



I concur with Martin that for a chimney passing through conditioned space it's better to leave it uninsulated, but DO take care to use sheet metal air-barriers at each floor to keep the chimney chase from becoming a huge air-sucking flue in it's own right, driving air infiltration.

Fireplaces suck. I mean REALLY suck! An open hearth wood-burner runs at best 7-10% efficiency while burning, and that's only improved to the ~15% range by the addition of tight sealing glass doors and better damper controls. The sooty particulate air pollution generated is also quite large. By contrast, non-catalytic EPA rated woodstoves (and some wood burning inserts) start at about 75% efficiency these days, and some hit the mid to high 80s, a 10x improvement in efficiency but on the order of a 98% reduction in particulate air pollution. With an EPA-rated wood burning appliance the vast majority of the air pollution occurs on cold-starts. If they insist on the fireplace ambience, there are some relatively attractive ~75% efficiency wood burning inserts out there that don't look totally out of place in new construction, eg:




(You don't have to buy or use the blowers that are often sold with them to boost the max BTU/hr output.)

By using only 10% of the fuel they may even get to use it more often. The higher the efficiency (and the hotter you run them) the cleaner the viewing windows stay too. Anything under 70% is a dog to be avoided these days (if only for the viewing glass cleaning aspects), but they do exist.

If it's a straight-ahead open hearth fireplace, consider going with a gasketed top-sealing damper to limit the "cold hearth" convective loop within the non-operating flue, eg:


Answered by Dana Dorsett
Posted Jan 16, 2013 5:03 PM ET


When warm gases meet with a cold chimney liner, condensation will form. To guard against this condensation issue, insulating will keep the liner warmer throughout the burn cycles.

Answered by Jenny Belman
Posted Jan 17, 2013 11:01 AM ET


Your advice applies to exterior chimneys, not interior chimneys. If the chimney is surrounded on all sides by interior conditioned air, then the portion of the chimney under discussion is at room temperature.

Answered by Martin Holladay
Posted Jan 17, 2013 11:10 AM ET


I have a chimney with an insert that is used to vent a natural gas stove in the old fireplace. We use the stove periodically, but when it's off I suspect cold air from outside penetrates our living space by coming down the chimney. Would it make sense for this reason to insulate the chimney?

Answered by Hari Kamboji
Posted Jan 21, 2014 3:18 PM ET


You are describing an air leakage issue, not an insulation issue. The air may be leaking in between the liner and the old clay flue tile, or the air may be leaking through the new flue itself.

You need to consult a chimney expert to examine your chimney, to safely seal any air leaks that can be sealed, and to make sure that the flue has a safe damper than can be closed when the gas fireplace is not in use.

Answered by Martin Holladay
Posted Jan 21, 2014 3:34 PM ET


Thanks Martin. I never noticed a problem with cold air leaking in. I really should have framed my question differently, in terms of slowing down the transfer of warmth from the living space into the cold interior of the chimney and from there right out of your house. It seems that a layer of Roxul around the chimney would slow down this process. Maybe I'm incorrect in thinking that the temp on the interior of the chimney is likely to be the outside temp?

Answered by Hari Kamboji
Posted Jan 21, 2014 4:06 PM ET


Most of the time, the components of an interior masonry chimney are at or near room temperature. The usual direction for the air flow (air leakage) through an unused flue is from the heated home up the flue to the exterior.

Of course, the direction of this air leakage can reverse -- most notably when the house is depressurized by a powerful range hood fan. Under those circumstances, exterior air can be drawn down the flue and into the house. But that is not typical.

Any masonry chimney is a huge thermal bridge and an energy nose-bleed. If any GBA readers are designing a new house, they shouldn't include a masonry chimney.

Once you have one, though, you are kind of stuck with it, unless you want to demolish it and remove it (an expensive proposition). I don't advise adding insulation around an interior masonry chimney.

Answered by Martin Holladay
Posted Jan 22, 2014 8:00 AM ET
Edited Jan 22, 2014 1:44 PM ET.


Thanks for the reply. Less work to do = good news!

Answered by Hari Kamboji
Posted Jan 22, 2014 1:29 PM ET

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