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Spray foam attic as a plenum

Daryl_Losaw | Posted in Mechanicals on

Designing a affordable option for my house plans.  My HVAC contractor and building science consultant mentioned a possible solution.  We are located near Autin so Zone 2

We will be using spray foam open cell for the attic and walls.  Running a short return air plenum to a attic installed air handler for noise reduction.  Then running a short plenum off the supply side to direct the air across the attic.  The plenum would be short to help with fan operation.  That way we have necessary pressure to avoid fan burnout.  Then using ceiling registers(diffusers) to supply the conditioned air to the house.   Basically use the spray foam as a plenum.

We will have a ERV for house ventilation independent of the air conditioning system.  

Just doing conceptual to see if it is possible.

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  1. GBA Editor
    Martin Holladay | | #1

    Are you proposing that you skip the ductwork that normally connects the galvanized supply plenum to the ceiling registers?

    Instead, you would just let the supply air blow loose and free, into the attic, and leave the ceiling registers as holes in the ceiling, hoping that the attic is pressurized sufficiently to deliver the air to the ceiling registers?

    1. Daryl_Losaw | | #3

      Instead, you would just let the supply air blow loose and free, into the attic, and leave the ceiling registers as holes in the ceiling, hoping that the attic is pressurized sufficiently to deliver the air to the ceiling registers?

      Yes. The supply side will have a plenum to direct air and help with positive pressure. There would be no ducts to the register. Plan was to try and use commercial practice in a residential application.

  2. Expert Member
    BILL WICHERS | | #2

    If your plan is to try to pressurize your attic to avoid ductwork, I think you’ll be disappointed with the results. I work with a somewhat similar system commonly at work, which is a raised floor system used to deliver cooling air. The air handlers run higher positive pressures than typical residential air handlers to make this work. You lose a lot of air movement efficiency when you lose the air-velocity component that you get with ductwork.

    The short answer is you’ll be surprised at how little air you get out of vents if you try using your entire attic as a common plenum. You’ll also be pushing air into every little leak in the attic. I’d advise against trying this, and just install ducting in the usual way.


    1. Daryl_Losaw | | #4

      Thanks for the reply. Trying to figure a better way of doing things. This is done commercially all the time successfully I guess residential building just do not work the same way.

      1. GBA Editor
        Martin Holladay | | #5

        Even in a commercial application, there are big disadvantages to this approach, as Bill / Zephyr explained. In addition to much higher electricity use by the blower, you'd soon realize that lots of little leaks in your attic would allow your conditioned air to escape to the outdoors.

      2. Expert Member
        BILL WICHERS | | #9

        This isn’t commonly done commercially. The common arrangement in commercial buildings is to use the space above a drop ceiling as a common air RETURN plenum. Supply air to vents is typically ducted.

        Dana makes an excellent point too, as soon as you use your attic as an air handling space, everything needs to be “plenum rated”, per fire codes. This is a big deal.


        1. Daryl_Losaw | | #12

          TY all for the responses you filled in the rather important part safety from fire and health threats.

  3. Expert Member
    Dana Dorsett | | #6

    If the "plenum" is the attic air, the foam needs to be on the other side of a timed thermal barrier to meet fire safety codes, even if you can get away with less than that with ducts & air handler in the attic, but not exchanging air with that attic. Air handler driven toxic smoke spread is the primary rationale.

    1. RussMill | | #7

      My FIRST thought! Fire ball in the attic!

      1. Expert Member
        Dana Dorsett | | #8

        The smoke-bomb in the attic is the real problem. The smoke is quite toxic, and you don't want to be actively moving it in a rapid fashion to locations in the building where they could easily escape the fire in time, but not necessarily the smoke.

      2. Daryl_Losaw | | #10

        Winner winner no fire for dinner!

    2. Daryl_Losaw | | #11

      Thank you for filling in the rest of the story. This is why I asked.

  4. walta100 | | #13

    I know it is common but ductwork and HVAC equipment in your attic fall in the category of stupid! Just say no to stupid.


    1. Expert Member
      MALCOLM TAYLOR | | #14


      The OP is proposing to build a conditioned attic.

  5. walta100 | | #15

    If you conditioned attic, the conditioned space just got 20% larger.

    I said keep all the ductwork and equipment out of the attic!

    Get the stuff inside the house an cut the operating costs by 20%

    When I have seen it done in commercial building it is always the return air without ducts and they require all power wiring in conduit and all low voltage wires to be plenum rated cable, not cheap stuff.


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