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Thread: Cavity/Doghouse

  1. #1
    Join Date
    Jan 2011
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    Hi all,

    Behind my natural gas fireplace there is a cavity/doghouse that is on an outside wall. It is brick exterior but plywood with SOME insulation on the inside. The top of the cavity is a slopped roof. I am getting a bad draft from the top due to a soffit on the outside that is allowing the air in. After I removed the fireplace there was insulation above the fire place with a vapour barrier. I have removed both just to get up into the cavity.

    My question is can I make a more solid separation from the top of the cavity? run some 2 x 4's and add insulation and then maybe drywall? Is there a need for this cavity to have direct access to behind the fireplace?

    Thanks for any help

  2. #2
    Join Date
    Aug 2004
    S.E. Pa
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    Wow, you're really into DIY weatherization. The fireplace must be installed in accordance with the building code and the mfr's listed instructions. Typically this means fireblocking above the fireplace in a chase. The chase must be weatherized per the code. In construction under the residential code, you can line the chase with ordinary construction just like in the rooms in your house. Most important is air sealing. Regarding sheathing the chase interior, the walls should be sheathed just as the rooms are. I recommend thin structural sheathing such as Thermo-Ply or Thermo-Sheath. These materials typically do not support mold growth, are excellent air barriers, hold insulation batts off the fireplace and being thin typically don't encroach into the stated clearances to combustibles off the back and sides of the fireplace.

    I strongly recommend you have a qualified professional re-install the fireplace and test fire it.

    To answer your question of materials your building code lists alternatives such as 3/4" plywood but you can use alternative materials if approved by your Authority Having Jurisdiction (code official).

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