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Fire place "cavity"

Discussion in 'Canada' started by go5, Jan 10, 2018.

  1. go5

    go5 Registered User

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    Hi all,

    Behind my natural gas fireplace there is a cavity (that is a build out of the back of the house). It is brick exterior but plywood with SOME insulation on the inside. The op 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. cda

    cda Sawhorse

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    My guess is for clearance to combustibles, from the fireplace

    Sheet rock sounds great to me Type X

    Not a building code expert just makes since
     
  3. Ty J.

    Ty J. Sawhorse

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    Can frame across to create a horizontal barrier. Use of OSB or sheetrock should be fine but not necessary if an air-barrier is installed properly. Make sure to consult the listing for the FP insert for minimum clearance to combustibles. Most inserts will have standoffs that establish the minimum clearances.

    Insulate the walls and lid with fiberglass insulation (rockwool is better for flame resistance) and use a fire-resistant air-barrier (like FSK).
     
  4. Ty J.

    Ty J. Sawhorse

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    As an additional thought, make absolute sure that the space behind the FP is not an air plenum for combustion air. The FP (if a direct vent type, most are) will have an exhaust flue and a fresh air supply. These may be a pipe-in-pipe type. See the listing (installation manual) to verify.
     
  5. go5

    go5 Registered User

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    Thanks for the replies. It is a direct vent FP and has a pipe-in -pipe vent that goes direct outside. So I assume the air is not for combustion air
     
  6. Francis Vineyard

    Francis Vineyard Sawhorse

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    Correct, it may be for ventilation to remove possible moisture. example;
    Vented Roof.jpg

    To eliminate the required ventilation install the minimum R-value of impermeable (rigid or spray foam insulation) below the roof deck and include the additional amount of insulation to meet the total R-value for your zone (see table below for amount of impermeable insulation for your climate zone).
    imerpeable.JPG

    Example of zone 4 below;
    Unvented.jpg

    Ont.JPG

    Additional resource on fireplace air barrier: Info-407: Air Barriers—Tub, Shower and Fireplace Enclosures
     
    my250r11 likes this.
  7. tmurray

    tmurray Sawhorse

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    Natural gas fireplace would not require make up air.

    Just make sure to check the clearance to combustibles for your fireplace.
     
  8. Ty J.

    Ty J. Sawhorse

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    And where did you get this wonderful idea?

    Combustion - Definition - a high-temperature exothermic redox chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke.

    Regardless what type of fuel is burned, it needs oxygen (air), which should be supplied from an approved source.
     
  9. tmurray

    tmurray Sawhorse

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    Sorry, I will correct my previous statement. A gas fireplace (natural gas, propane) does not have a need for passive make up air , which is what was suggested.
     
  10. mark handler

    mark handler Sawhorse

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    Just remember Gypsum wallboard is not a "non-Combustible", It is a "limited combustible" and does need clearances.....
     
  11. go5

    go5 Registered User

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    thanks for all the replies all.
     

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