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.
KH
 | 988.52 KB |
Asked by Kevin Hanlon
Posted Wed, 01/16/2013 - 10:25
Posted Wed, 01/16/2013 - 10:25
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Kevin,
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.
Posted Wed, 01/16/2013 - 11:42
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.
http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_10_par020.htm
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_10_par061.htm
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:
http://www.hearthstonestoves.com/assets/images/product_photos/product_ma...
http://bowdensfireside.com/product_images/vermont_castings_wood_burning_...
http://www.fairviewhardware.com/wp-content/uploads/quadrafire7100fp.jpg
(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:
http://www.woodlanddirect.com/Chimney/Top-Sealing-Chimney-Dampers/Lock-T...
Posted Wed, 01/16/2013 - 17:03
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.
Posted Thu, 01/17/2013 - 11:01
Jenny,
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.
Posted Thu, 01/17/2013 - 11:10