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  1. #1

    Should my "doghouse" chimney walls be vented at the top?

    I'm in the process of replacing the fireplace in my home. I removed the fireplace today and stepped into the bump-out space for the very first time. As I expected the builder took shortcuts and I have issues in there. I have this one chance and one chance only to make this space as it should be.

    The bump-out "room" is two stories high with a built-in "ladder" for climbing the wall. I noticed at the top that the builder punched holes in the wall under the siding, presumably for ventilation. I don't understand why this was done. I thought that this space should be completely enclosed within the building envelope. Would I be doing harm by sealing it?

    The builder only insulated half of the exterior wall with fiberglass and he faced the paper face to the outside. I know this isn't right. I'm thinking about removing it completely or turning it around and covering the interior wall with some sort of fire barrier (not sure what yet).

    Should I board up those holes, insulate the remaining wall cavities and air-seal the entire space or should I leave it vented?

    I appreciate any and all advice.

  2. #2
    Join Date
    Aug 2004
    Location
    S.E. Pa
    Posts
    6,141

    Smile

    A 'doghouse' or 'dogshed' is usually just one story. Otherwise, it is officially referred to as a 'chase' up to three stories. After that, the codes treat it as a 'shaft', which requires fire rated assemblies.

    What you are describing sounds like holes punched into OSB siding panels for caebles as a crane swung them into position. This is a very common practice though it is technically against code because it breaches the exterior sheathing. With stucco, it is a disaster as the underlayment paper is virtually guaranteed to be penetrated which leads to rot. I find a lot of that. If you have jagged holes in your exterior sheathing, you might consider shearthing patches over the holes.

    I would seal every seam and penetration or gap you can using foam, caulk or duct mastic. Install new fiberglass batts with the vapor retarder facing inwards then apply a thin structural sheathing over it such as Thermo-Ply with the seams taped and caulked. You will need fireblocking at each ceiling level and every 10ft vertical. If you weatherize a one story dogshed then install a gas direct vent out the rear you should not need to weatherize the entire chase. You would want to replace the chase top with a solid sheet metal cover.

    If you do as I describe here, you are bringing the fireplace into the conditioned space and thus would not ventilate it. Don't forget to weatherize the cantilever as this is usually a major air bypass.

    HTH

  3. #3
    Hearthman I thank you for putting my mind at ease...at least for this project. I'm a little unnerved that the builders of my home violated fire code but if they had followed it then I wouldn't be able to fix some of these problems now. I now have a clear direction on what I need to do and I've already started. Building a fireblock is going to be a bit tricky; my radon exhaust standpipe goes through the length chase to the roof but I will get it done with a little resourcefulness.

  4. #4
    If I may ask one more question. Should I also put a vapor barrier sheathing on the "ceiling"? The ceiling is not the very top of the chase; it's the bottom of the top section of the chase that extends above the roof line (giving the fake chimney look)

    Right now it's just an OSB board floor with some cellulose on top of it; not much. I was thinking about fastening a sheet of foam board to that wood for insulation and covering it with reflectix.

  5. #5
    Join Date
    Apr 2010
    Location
    Brooklyn, CT (NE CT)
    Posts
    6
    I am sorry to hijack this thread, but my issue sounds similar. I do not think they finished my chase correctly from what I have read, but I do not know the easiest way to fix it. I get an extremely cold draft through the bottom of my propane direct vent fireplace which is in a chase.

    Here are some pics I took of the chase opened up. This pink insulation was added by 2 2nd builder who was trying to fix the issue. Originally there was no insulation at all. Can I add in pink rigid foam insulation to seal it up? Also, should it be wide open with no ceiling like the current setup? I am sure a lot of cold air come in through teh siffit into he chase right now.

    http://dl.dropbox.com/u/9620333/Fireplace/IMG_2102.JPG

    http://dl.dropbox.com/u/9620333/Fireplace/IMG_2103.JPG

    http://dl.dropbox.com/u/9620333/Fireplace/IMG_2104.JPG

    http://dl.dropbox.com/u/9620333/Fireplace/IMG_2105.JPG

    http://dl.dropbox.com/u/9620333/Fireplace/IMG_2106.JPG

    http://dl.dropbox.com/u/9620333/Fireplace/IMG_2107.JPG

    http://dl.dropbox.com/u/9620333/Fireplace/IMG_2108.JPG

    http://dl.dropbox.com/u/9620333/Fireplace/IMG_2109.JPG

    http://dl.dropbox.com/u/9620333/Fireplace/IMG_2110.JPG

    http://dl.dropbox.com/u/9620333/Fireplace/IMG_2111.JPG

    http://dl.dropbox.com/u/9620333/Fireplace/IMG_2112.JPG

    http://dl.dropbox.com/u/9620333/Fireplace/IMG_2113.JPG

    http://dl.dropbox.com/u/9620333/Fireplace/IMG_2114.JPG

  6. #6
    Join Date
    Aug 2004
    Location
    S.E. Pa
    Posts
    6,141

    Cool

    With regards to vapor barriers, don't install them unless it is required by code and you understand the reason for them. I highly suggest you read Dr. Joe Lstiburek's book correct for your climatic zone.

    As for the pics, you must protect the insulation with some sort of sheathing. I strongly endorse thin structural panels such as Thermo-Ply or Thermo-Sheath taped and sealed.

  7. #7
    Join Date
    Apr 2010
    Location
    Brooklyn, CT (NE CT)
    Posts
    6
    I'll check out Thermo-ply. Thanks. Do you think it should have a "ceiling" in it. Right now it is open right up to the roofline like an attic. It seems like cold air will just bellow into to the chase through the soffit vent if there is no ceiling.

    Thanks for the help Hearthman!

    P.S.- Is there any kind of a trick to taping the last part. Obviously I can put up the thermo-ply and tape 85% of it, but how to you put up and tape the last section on the way out when almost complete because at that point I will not have access inside..
    Last edited by Scottrichards; 11-06-2011 at 10:41 PM.

  8. #8
    Join Date
    Aug 2004
    Location
    S.E. Pa
    Posts
    6,141

    Cool

    What you are describing sounds like you are missing your 'fireblocking' as addressed in the codes. The IRC prescribes several material choices such as plywood or fiberglass insulation batts if properly supported. That's the key--support so it doesn't fall down on your fireplace. Above the fireblock goes insulation per your climatic zone. I'm R-38 for attics.

    I leave one stud bay for last. Ensure everything is well sealed and take pics. Cut a piece of T-ply shaped like a box lid so the side tabs will fit inside the stud bay. Attach the piece by nailing along these 4 tabs then goop over the joints. Insert fiberglass insulation batt then sheath over. I goop the joints in this last pc of sheathing.

    Tight as a duck's rear end!

  9. #9
    Join Date
    Apr 2010
    Location
    Brooklyn, CT (NE CT)
    Posts
    6
    I couldn't locate Thermo-ply at my local Home Depot. The closest thing I could find to match it was Dow Tuff-R and Super Tuff-R. Will that work?

  10. #10


    It's okay; I don't mind. In fact, I learned something from the posts.

    After two solid weeks of work I'm nearly finished with the renovation. I'll post some pics when I'm able. I followed Hearthman's advice along with the advice of a local HVAC contractor who's been in our area for a long time. This company has been air sealing, insulating and drywalling chases for 25 years to deal with the problems I had and not once did they get a call back for mold or moisture issues.

    This company also offered one other piece of information that I'd like to share as an aside: do not turn off your pilot light during the summer. The heat from the pilot dries the air inside the fireplace so that condensation and moisture does not rust out critical parts. That's probably why our valve went out; last summer was so humid that we had condensation all over inside the fireplace.

    Now back to the post at hand...

    The logistics of the work were a little complex as the space was so narrow I could not get a ladder inside. I had to use the one the builder built into the wall and tear it down gradually during the final phase. I'll explain in general what I did.

    First I covered all of the holes in the wall with plywood and air sealed every gap I could find with caulking or Great Stuff (R) fireblock foam. It's best to do this work in daylight so you can see the seams of the OSB board on the outside wall. I used the foam in the wall cavities and along the joints where the framing of the chase met the house, but not in places where it would interfere with the later installation of drywall. For that I used caulking.

    Second I examined the fiberglass batts for mold, rot or dead bugs. No mold but on several batts I had dead bug guts which means bacteria--those got tossed. The others were removed and turned around so that the vapor barrier faced the correct direction. I then installed new R-13 kraft faced insulation with the vapor barrier facing the living space and stapled it to the joists. However, don't fully staple the pieces in the side walls right away--at least not on the bottom; you'll need them to be loose to rotate your drywall pieces from vertical to horizontal...more to follow on that.

    Third, I covered the insulation with 3/8" drywall panels that I had to cut into 64 x 24" panels to fit into my car and through the fireplace opening. I inserted them vertically and then turned them horizontally to fasten them to the long walls. This was one of the hardest parts of the job. Do the long walls first and the side walls last or you won't have room to rotate the drywall. Staple the If you damage the paper facing on the side insulation just repair it with tape. Seal every seam and the corners with caulking as you go and don't forget to staple the fiberglass batts on the sides as you go.

    Fourth, I had to run a new electrical cable to provide power to the new fireplace. Our old one didn't have a blower nor did it have special lighting that required power. I finished that part last night.

    My final step which is yet to come is the installation of a fire block between the first and second floor of the chase. For this I'm going to fasten pieces of 2x4 to the wall frames and set a cut piece of fireboard on top of them. I will then seal the gaps with the fireblock foam.

    Once that's done I'm ready for my new fireplace !

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