Replies

1. 
   Joe_Bob | May 22, 2013 09:44pm | #1

   I would put wrapping the chimney with mineral wool in the category of: definitely couldn't hurt. Of things to try, it would certainly be one of the less expensive and obtrusive measures. If you were to do that I would use a foil-faced insulation or metal jacket so you have something to stop air movement through the insulation.

   That aside, I would take a second look at your attic insulation and air-sealing. You don't say how much insulation there is, but you may still not have enough. Likewise, when the air-sealing was done were there efforts made to accurately locate leaks and thermal bypasses, e.g.: with a smoke pencil or thermal camera? If not, it's likely you could still find a lot of opportunities to improve air sealing and insulation - and you would probably be losing a lot more heat via that route than what is being radiated from the chimney.

   Alternately, you could address the ice dam issue by increasing the ventilation in the attic. The closer the attic is to the outdoor temperature, the less melting you will get. The risk with this approach is that increasing air flow in the attic could exacerbate air leaking issues from the heated part of the house.

2. GBA Editor
   Martin Holladay | May 23, 2013 05:13am | #2

   Adrienne,
   Joseph's answer is a good one. If you want to do a super-duper job, encase the chimney with mineral wool, and then surround the insulation with studs and drywall.

   After that, follow the usual guidelines to reduce attic temperatures: track down air leaks (ideally using a blower door), add more insulation to the attic floor, and increase the amount of ventilation -- in that order. More information here: Prevent Ice Dams With Air Sealing and Insulation.

3. Expert Member
   Dana Dorsett | May 24, 2013 03:35pm | #3

   Hanging anything on a chimney is a code violation, even insulation. To meet code you can build an insulated chase around the chimney with the requisite 2" of clearance, without attaching the insulation on the chimney itself. This is definitely a place where rock wool is right material, since the binders on fiberglass batts have a very low melting temp.

   You're explicitly BARRED by code to put the insulation or any other material directly between combustible framing and the masonry, except as fireblocking:

   http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_10_par061.htm

   "R1003.18 Chimney clearances.
   Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum air space clearance to combustibles of 2 inches (51 mm). Chimneys located entirely outside the exterior walls of the building, including chimneys that pass through the soffit or cornice, shall have a minimum air space clearance of 1 inch (25 mm). The air space shall not be filled, except to provide fire blocking in accordance with Section R1003.19."

   I suppose if you used steel studs it's OK to butt the insulation up against the masonry, but not if you used wood, and you wouldn't be allowed to use a paper-faced gypsum board or any other combustible material to box it in. Fiber cement board would probably pass muster though- even the 1/4" tile-backer stuff, since it isn't being used as an ignition barrier, but only needs to not be combustible itself.

4. 
   user-945175 | May 24, 2013 08:58pm | #4

   I'm not sure I understand Dana. Would wrapping the chimney in mineral wool batting be a problem? I thought it allow airflow through or maybe I am wrong about that. Sounds like from the answers I could do some insulating, but that I'd be better off sealing any other airleaks.