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Community and Q&A

Ventilate dead space behind kneewalls

user-1052093 | Posted in General Questions on

I have recently completed my attic renovation. We moved the insulation layer to the roof deck and brough our hvac inside the thermal envelope. We finished the space for an extra room.I now have keewalls on both sides of the room with a triangular space behind them with all the ducting/hvac etc. behind them. I did not provide any registers or returns to this dead space. Should I be concerned that there is no air movement or turnover in this space? I was thinking it might be wise to provide a louver through the kneewalls to allow some air movement.

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  1. user-1026531 | | #1

    I don't see any issues with this installation as long the furnace is sealed combustion. If it is not sealed combustion, make sure the space is large enough to supply combustion make-up air.


  2. GBA Editor
    Martin Holladay | | #2

    Q. "Should I be concerned that there is no air movement or turnover in this space?"

    A. No.

    [Note to Wes Harding: According to my reading of Rob McKenzie's question, I don't think that he has a furnace in these triangular attics behind the kneewalls.]

  3. user-1026531 | | #3

    My mistake. I am so you used to seeing homeowners cram a furnace into a tiny triangular attic once they have made the old attic conditioned space that I jumped to conclusion. Thanks for the clarification.

  4. user-1052093 | | #4

    Thanks for the respones guys. You are both correct. I did not state the furnace is behind the kneewall(sorry). The furnace is not sealed combustion, however I did have a "Fan in a can" installed to bring in outside make up air for combustion (all inspected and approved). The furnace and makeup air is contained in its own portion/room behind the kneewall. One of my concerns is some portions of triangular dead spaces is over bathrooms on the floor below. The ceiling of these bathrooms had/has no vapor retarder or barrier. They do have exhaust fans and are always used. I am thinking there is the possibility of humid still air being trapped behind the knewalls from any vapor transmission from the bathroom ceiling below. Sorry I was not more explicit with my first post. Thanks again

  5. GBA Editor
    Martin Holladay | | #5

    If your sloped ceilings are carefully air sealed and are insulated to at least minimum code requirements, then your triangular attic areas are inside of the conditioned space of your home, and you don't have to worry about humidity.

    However, I don't like the idea of an atmospherically-vented furnace in such an area. Although fan-in-a-can systems work, they are a band-aid measure. It's easy for a fan-in-a-can to introduce too much outdoor air -- and that incurs an energy penalty. If the fan ever conks out, you'll have a combustion safety problem (although I imagine that their are safety interlocks that probably shut off the furnace in that situation).

    As soon as you can afford to do so, you should replace your older furnace with a sealed-combustion unit.

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