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  1. #1
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    Nov 2013
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    Question How to Insulate a Fireplace Chase

    I've read mixed things on doing this but need some advise anyway.

    I'm a new homeowner and while I was in my attic replacing some insulation from when I moved my attic door from an inaccessible location (they had it in a closet at the lowest point of the roof, maybe 8 inches of room to actually get into the attic) I noticed something odd. While up there I noticed my attic chase opened up into the attic (the chase is about 4' Wide by 2 to 3' deep from ground floor up to the second floor roof). I looked down inside and noticed the entire chase from floor to ceiling has no insulation what so ever. Looking at it, I don't even think there's a vapor barrier on it but I can't be certain. The Chimney flu is only a 12" (estimated) metal tube so there's tons of room on the sides. I don't know if it is supposed to be insulated at all but the fact that the "crawlspace" between the first and second floor opens up into the fireplace chase and the attic is open to the chase makes me believe I am loosing a ton of heat from this. I was thinking about climbing down into the chase from the attic (not seeing any other way into it) and putting up some stone wool insulation into the chase walls (all the way up).

    Is there any reason that I shouldn't do this? Is there a better way of doing this? Do I need to use Stone wool or is there a better alternative that won't cost me an arm and a leg?

  2. #2
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    Aug 2004
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    First of all, you should not be able to see down the chase because it should have fireblocking and a listed firestop at the attic level and each ceiling below. If attic insulation can come in contact then an "attic insulation shield" should also be installed to keep insulation off the chimney to maintain the stated clearance to combustibles. On an exterior chase, the outer three walls typically should be weatherized. This includes air sealing such as foam or caulk then insulation then rigid sheathing on the interior of the chase. This sheathing serves to restrain the insulation in place so it can work but also so it cannot fall against the hot chimney or fireplace and cause a fire. The code requires interior walls to be sheathed and technically these would be interior walls. If only the rear wall faces the outside then only that wall would need to be weatherized.

    To shinny down into a chase would constitute a "confined space" according to OSHA. This would require a written plan in the event a worker became incapacitated down there and include a plan how to get him out. You would also technically need to pull a permit-if hiring someone to do it. You can put yourself at risk all you want.

    It can be quite difficult to do under the best circumstances. Ideally, you would open the rear wall of the chase and work from there, sealing that last.

    Whatever this 'crawlspace' is you call it must be fireblocked off per the code.

    Check the archives on this site for more discussion

  3. #3
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    Nov 2013
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    Thread Starter
    Yeah.... its an exterior chase and I can see light coming in from multiple cracks in all three walls of the chase, no insulation, foam, or anything of the sort can be seen (Basically I see the framing from attic to ground level) and possible boards before the siding. Its most definitely open to the attic with nothing stopping any insulation from falling down into it, I can't tell if its directly open to the crawl space. I'll check later tonight when I get home if there are any visible firestops at all in the chase. So... Problem 1-5 identified.

    What I call a crawl space is a 12-18" gap between the first and second floor where the ducting is located. This space was open to the sofit over my front porch which I have since put insulation and backing to separate the inside from the out. This soffit extends to the fireplace chase and is actually open to it as well. I don't know when either the chase or this soffit was built and can only assume it was original to the house until I find records stating it wasn't. So if I were to add insulation to the chase, note that pulling walls (inside or out) isn't a possibility at this point in time, I would need to first caulk/foam the cracks to the out side, then put the insulation up, then the restrictor board (I'm guessing sheet rock?). I would then close off the soffit and the floors with a fire stop and then I would close it off to the attic with a fire stop?

    Any information on how one wold potentially build a firestop into an existing fireplace chase? I know I haven't confirmed whether or not there are any between the floors but information is power at the momemnt

  4. #4
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    You need the fireplace make and model first. Then get the listed instructions which will include the part# for the listed firestop and pick that up if possible-one per ceiling. In the attic, it goes flush with the tops of the joists while lower levels it goes flush with the ceiling. This component is only about 16" square. To block off the rest of that big opening requires "fireblocking" which is described in your building code. It can be a lot of materials but 3/4" plywood is very common as is sheetmetal if properly supported and secured. This same stuff must be used to block off the chase from that "soffit/ crawlspace".

    Inside the chase, I recommend thin structural panels such as Thermo-Ply or Thermo-Sheath. This stuff can be cut and rolled a bit to squeeze it into tight places. I use roofing nails and secure seams with either foil tape listed to UL 181 a/b-FX or duct sealing mastic. If your ext. sheathing is vinyl siding, it can be removed to allow a large access cut into the chase, seal things up, then seal the access hatch and work your way out to the yard then replace the siding.
    hth

  5. #5
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    I run into this all the time, Building inspectors and contractors need to pick up the slack. so many chases are build with nothing but framing. Hearthman hit the nail on the head, it's a sucky job, but well worth it.

  6. #6
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    Thread Starter
    This is what I'm dealing with
    https://www.dropbox.com/sc/m31t63n5l03416d/yX0GpVzie8

    As you can see the chase is open to the attic with nothing more than a 2x4 holding back the insulation from falling down into it. I looked down into it again and there is definitely no firestops in it. In fact I can actually see pink insulation at the bottom of the chase on the flue itself. Also, I noticed there is some insulation on the main wall of the house but it doesn't appear to be secured very well. I did notice that there appears to be a "ladder" (the word ladder being loosely used) going down so I may be able to reinforce it and get down into it and back up and out. I'm not understanding your "get the model number of the fireplace" comment as I wasn't aware a fireplace like this had a model number. Can I still insulate the outer 3 walls of the chase? I will still be building/installing firestops now that I'm aware they don't exist. I wil apparently need to either seal off the chase from the attic or at least build up a ledge to keep from having more insulation fall down and in.

  7. #7
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    You can sheath over the attic framing to keep out insulation as the last thing. Inside the fireplace will be a metal tag or rating plate noting the mfr., model, serial #, mfr date, etc. You'll have to search around but its there. Any pics from outside, esp. chimney top and cap? Also pics inside fireplace? Those stickers on the chimney pipe will tell you the mfr. at least along with the clearance to combustibles (usually 2"). Yes, those walls can be insulated but air sealing more important and tougher to do. Call a pro

  8. #8
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    I'll look up the model number when I get home later tonight. I won't be able to get good pictures of the outside as I won't be getting home until after dark. Tomorrow will be the best time I can add those for reference. I'm hoping I can do all this with relative ease. My biggest thing is going to be climbing down the chase (I'm not a heights person). No matter what I need to get the insulation off the fireplace flue and the pieces that are beginning to fall down I need re-secured to the main wall.

  9. #9
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    Thread Starter
    Image of the outside of the fireplace.

    https://www.dropbox.com/s/lpeusces6f3ad0a/IMG_1745.JPG

  10. #10
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    Quote Originally Posted by hearthman View Post
    You can sheath over the attic framing to keep out insulation as the last thing. Inside the fireplace will be a metal tag or rating plate noting the mfr., model, serial #, mfr date, etc. You'll have to search around but its there. Any pics from outside, esp. chimney top and cap? Also pics inside fireplace? Those stickers on the chimney pipe will tell you the mfr. at least along with the clearance to combustibles (usually 2"). Yes, those walls can be insulated but air sealing more important and tougher to do. Call a pro
    What "pro" would you recommend? I have been calling around but it doesnt seem like anyone around here has done this before or is familiar with the code requirements of the chase. I purchased the home 1 month ago and have found that the fireplace chase is open to the attic and it is very hot in my living room and pantry that share that wall.

  11. #11
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    If your city has a building inspector call him or if not maybe a fire inspector city or state might help.

  12. #12
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    Not sure where you are located but a fireplace specialist who does more than just installing and servicing the fireplace should be able to advise or quote you on properly finishing the fireplace chase.

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