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Least-bad way to run return in unconditioned attic (4A)

CrisPA | Posted in General Questions on

Long, complicated question…  
I’m working on an attic that was converted to a bedroom (lower side of 1950s split-level).  I’m insulating the kneewalls with RockWool and polyiso to about R-28. The cathedral ceiling has foil-backed fiberglass. No air-sealing and thin, badly-installed insulation on the kneewalls has always made the room uncomfortable.

Another problem is that the room has no air return. This is probably OK in the winter as the cold air can run down the stairs, but is really bad in summer for the same reason. (We use a window AC unit there to try to keep it comfortable). The supply vent comes up through the floor  via panned studs.

It would seem that the time to add a return would be before I finish all the insulation and foamboard installation (and before summer hits again). The available returns are
– Next to the supply vent
– About 15′ from the N edge of the room

Cutting new holes in the floor for a return that’s right next to the supply vent seems like a non-starter, so it looks like I should aim for the other one. If you look at the plan, you can see that the supply is near the N (bottom) end of the room. The easiest place to put in a return would be the N end, but that seems to create about a 5′ circuit between the supply and return. I think I’d want the return pretty close to the other end of the room, so I could either try to run a duct along the triangular space at the peak of the ceiling or else put one through the top of the  kneewall on the W side close to the S end (maximizes distance between return and supply).

Clear so far?  Running along the ridge and trying to ensure a good seal seems like a challenge, so going through the kneewall seems like the least bad option here.

On the other end of the insulated duct… the return now 25′ feet away is another 2-wide panned stud job. The options for connecting the duct seem to be:
– going straight down from the attic into the return, which would mean cutting through 3″ of stacked 2x4s and trying to fit a 3.5×14  register
– going through the ceiling in a closet that backs upto the panned return space and putting in a 90 degree register into the return

I hope this is all somewhat clear and that the diagrams are useful.  Any thoughts on all of this?

Thanks,
Cris

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Ideally the return register grille would be at the far end of the space from the supply register.

    What directions do the floor joists run?

    How much space do you have behind the kneewalls? Building another kneewall and insulating IT (rather than the existing kneewall) to create a service cavity could work, as could installing an appropriately sized R8 insulated flex duct run in the kneewall cavity for the return (with well sealed boots at the wall & /or floor penetrations.)

  2. CrisPA | | #2

    Dana,

    The floor joists run E-W (horizontally on the plan view).

    If I'd thought of your service cavity idea earlier I'd have gone with that - brilliant! I'll probably go with the R8 flex.

    Thanks!

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