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Conditioned attic with ventilated roof deck

edbiagini | Posted in General Questions on

 My sister and her husband recently built their own home and did closed cell spray foam in their attic to make it conditioned space. I like the idea of having the air handler and duct work within the envelope, however the price is steep and the inability to inspect the roof deck worries me. 

My wife and I will be building our own home soon and I was wondering if it is possible to make the attic conditioned space while still having a ventilated roof deck, treating it like a cathedral ceiling. Is this possible to do with rockwool insulation to allow any vapor to get out? Would the attic have to be drywalled? I am in climate zone 5.

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  1. Expert Member


    Yes, your cathedral ceiling doesn't know what you call the space below it. It can be living space or an attic. As long as it is conditioned like the rest of the house you are fine.

  2. Expert Member
    BILL WICHERS | | #2

    Yep, you can do that no problem, and if it's vented, you really don't even need spray foam. With new construction, the easiest/cheapest way to go is likely to be vent baffles in the rafter bays (you need at least 1" vent space per code, but 1.5" is better), then batts, then furr out the space with perpindicular framing, a second layer of batts, then a finished drywall surface.

    You don't really need need drywall, since the mineral wool batts don't need any kind of fire protection, but drywall will make it look better, and will prevent any issues over time with the batts sagging anywhere. You could use plywood too. I would probably go with 1/2" drywall, taped mudded and primed, if I were building this. You don't need anything fancy.


    1. Expert Member
      MALCOLM TAYLOR | | #4


      While drywall isn't necessary, some interior air-barrier and vapour-retarder is.

      1. edbiagini | | #6

        What would you recommend?

        1. Expert Member
          MALCOLM TAYLOR | | #7


          I would install a variable-perm membrane detailed as an air-barrier. If you decide to finish the space later you can add drywall.

          I don't know where you are, but it would be a good idea to see what your building code says about that space, and whether it has any requirements that the ceiling be finished.

  3. walta100 | | #3

    It seems to me every thing on your wish list is a design flaw.
    Spray foam is a poor choice.
    Conditioned attic is poor choice
    HVAC equipment in the attic is a poor choice.
    Rockwool is not the lowest cost option.

    Consider making room inside the conditioned space for your ductwork and HVAC equipment make your ceiling flat and cover them with R60 of cheap fluffy insulation.

    Spray foam is expensive. Spray foam is risky in that sometime it fails to cure leaving a smelly mess.

    Conditioned attic increases the number of cubic feet of conditioned by 20-40% and increases energy usage by at least that much and often more because the cost of R60 on the rafters is unaffordable. Yes, a conditioned attic is marginally than the truly dumb idea of HVAC in a vented attic.

    Putting the HVAC in the attic if a great idea for someone selling a house but a bad deal for the buyer. The seller gets to sell every square inch of floor space as living area leaving the buyer stuck with the high electric bills.


    1. Expert Member
      MALCOLM TAYLOR | | #5


      Heat loss is dictated by the surface area of a building, not the cubic feet of the conditioned space.

  4. walta100 | | #8

    "Heat loss is dictated by the surface area of a building, not the cubic feet of the conditioned space."

    I agree and my point remains the same. Moving the thermal boundary from the attic floor to the roof line increases the surface area 20-40% increasing the heat loss by at least that much.

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