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Converting building “cavity” into a plenum?

user-1137156 | Posted in Energy Efficiency and Durability on

I know that the use of building cavities, things like spaces between joists, rafters and studs shouldn’t be used as part of HVAC distribution. However if one had a “plenum” that covered the entire floor area, located in the ceiling, air distribution would be easy regardless of the partition layout below and the friction losses would be about as low as possible. So my question is how can I construct/build such a plenum and not have it be a building cavity?

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  1. Expert Member
    Dana Dorsett | | #1

    There are rectangular ducts designed/sized to fit reasonably into standard framing bay sizes. eg: a 7' x 14" comfortably squeezes into a 2x8 joist bay, a 3.25" x 14 fits in to a 2x4 stud bay.

    Panned framing with sheet metal installed on the framing was once commonly used for both supply and return ducts, but it's notoriously leaky, often banned by building codes at this point.

  2. user-1137156 | | #2

    I don't think you've understood what I want. I want the WHOLE area covered by a HUGE plenum. A cavity between two layers say 6" apart. If the building were say 25'x40' the plenum would be 25'x40'x6", all well sealed and slightly pressurized. Some where in that area would be a vertical inlet, from a fan below and anywhere else there could be "outlets" through the bottom. A possible construction is all fiberglass "duct board" with a "latitce" of metal studs to separate/reinforce the top and bottom. But even with all duct board is it still a "building cavity? If it's ok with duct board, what other materials can make up the walls? I'll give it a name, a "distributed plenum" , kind of a 2 dimensional extended plenum.

  3. azgreg | | #3

    Here's a plenum truss

  4. Expert Member


    Aren't you trying to kill a fly with a shotgun? It reduces friction losses, but introduces all sorts of complications. Beyond the code difficulties, you have the practical problems of constructing something that large but thin, with a smooth, sealed surface on the inside, have to then address how to achieve acoustic separations between the spaces it serves. and also need to find alternative locations for the rest of the services, the electrical, plumbing, ventilation and return-air, that can no longer be located where this large plenum is situated. All just to eliminate a few ducts. Unless I'm missing something, it seems like lot of work for little return.

  5. Expert Member


    You know immensely more about mini-splits and their installation that I do. My concerns were just about buildability.

  6. user-1137156 | | #7

    I Thank you for helping me get that, not so good idea out of consideration. I do have to build it and get it through the inspections!

  7. user-1137156 | | #8

    I do believe you are right! I'll probably end up with ducts in a "dropped" ceiling over a central hallway with a "slim ducted head on each end.with the return air coming up a vertical duct hidden in a wall (return air grill either low on the wall or in the floor). I'm thinking of connecting the output ducts through a motorized damper and adding 2 motorized dampers, one between each unit and it's output duct.while also adding a selector switch to control the three dampers. The switch will select zone (normal) or all on east or all on west. The rewards for all that complexity is the ability too run as two zones (normal) or one zone using either unit alone, for redundancy, in the event either unit craps out, or to reduce capacity and stop cyclic operation.
    I keep forgetting the K.I.S.S. principle! The dampers aren't cheap, certainly not worth $600+!!! Two systems! Which also dramatically shortens and simplifies an ERV inlets from my kitchen and 1/2 bath! If one craps out leave the doors open ,must remember this when we vacation! On another thread I described the units and predicted the performance and showed that for ODT above 32f only one should be operating, big deal! On that thread I discus the fail soft capability, basically either will carry the load if the ODT is above 17f, below that the indoor temperature will fall but will stay above freezing for any ODT above -3f (which is 3f below design temperature). Also if both are operating 70f can be maintained at-22f, well below any recorded temperature here.

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