How to Replace a Return Duct in an External Stud Wall?
sgrepp
| Posted in Energy, Heating & Insulation on
We recently purchased a 1966-built, Cape style home here in northern coastal Massachusetts.
The house has insulation and air-sealing details typical to this age home — meaning they’re woefully inadequate in today’s thinking.
I started a project to repaint a room on the first floor, and decided that it made sense to replace the drywall on the two external walls, both because they were in rough shape cosmetically, and as a way to get a sense of the level of insulation that was done in the original build. I found 1” (maybe 1.5…) of fiberglass, surprisingly foil faced on both sides — something I’d never seen before.
The puzzling thing I found is shown in the photos. I came across this stud bay with the insulation methodically face-stapled to keep it compressed against the external sheathing. After scratching my head for a bit, I came to the realization that I was looking at a cold air return duct for the furnace. There is a duct-in-joist return air handler in the basement ceiling, and this bay was apparently used to extend the return air duct system to the 2nd floor, up through the front stud wall and into the Cape’s eave space. Not sure if this was part of the original build, as you’ll be able to see the hacked wall plates in the top and bottom of the photos, which suggest that maybe they were done after the build.
I’d obviously like to eliminate this gaping hole in the insulation/air seal layer, but how? There’s no easy way to move the duct to the perpendicular inside wall, as there is a doubled floor joist directly beneath it supporting the stairway opening. So my choices, as I see it, are:
1 – Fill the bay with insulation like the rest of the wall, but build a bump-out encroachment into the corner of the room to house a conventional metal air duct. This is likely the correct solution, but I hate the aesthetics of having that bump out intruding into an already-small room.
2 – Leave the return air duct in the wall, but modernize it by lining it with 1” sheet foam on the external sheathing and stud sides, and sealing the corners with spray foam. This would be “better” than the existing duct, and provide a better air seal, but less insulation than the rest of the stud bays. (And it feels like a hack solution…)
So, my questions:
– Have you ever come across this type of thing? I’m an advanced DIYer, and in my 45 years or so of rehabbing homes I’d never seen anything like this before.
– Would you choose one of my solutions above, or do you have some sage advice on how to address this that I haven’t thought of?
Thanks so much for any direction you can offer.
-Steve
-Steve
Replies
I would not want foam board in an air return. Fiberglas duct board would be a better choice.
Is there some other way to get return air from the upstairs?
You said there was not a lot of room, but you could also bring the whole wall in a few inches if you don't want a bump in the wall.
Thanks for the thoughts.
Why duct board? Because of potential out-gassing from the foam? Or because of better insulation values? If I leave the duct-in-bay setup, I want to have the most insulation value I can in such a small space, which is why I thought rigid foam -- I'd think I can get at least an in, so around R-6?
I've looked at this for a few weeks, and really can't spot an obvious place to move the duct to. If possible, I'd move it to the perpendicular internal partition wall to the left in the photos, but like I wrote before, there's a doubled 2x8 floor joist under that to support the stairway opening, so that's a no-go.
Thanks for taking the time to reply -- every question helps me think my way through this!
fire. if there was a fire in the home close to this foam, the hvac would spread toxic smoke fast.
foam board has to be covered with drywall or other barrier just sitting in a basement wall.
in a plenum, with air blowing past, it is really a no-no.
Ahh, hadn't thought about that angle. Thank you,.
Look at the whole house. in my house, all the upstairs rooms return air via cracks around the door (and below), through a hall, and to a central return.
There is an open stair from the hall way, and I bet most of the return air heads down the stairs to the first floor return.
You might get good results with a vent in a door, or some other path for the return air.
Interesting! The top of this return exits into a handmade distribution (I guess, really "collection") box in the eaves space (again, it's a cape, so there's a roughly 48" triangle of eaves storage area...) and as I sleuthed around there, I was surprised that the return vent (in one of the upstairs bedroom closets) is essentially blocked.
Reading what you wrote, I wonder if that was their plan -- to have the two outside bedrooms return air via the other return -- a centrally-located collector in the bathroom that sits between those two rooms. And that bathroom was obviously added on later, as it's in a shed dormer that looks much newer than the rest of the house, and is framed much differently.
If so, maybe they just didn't bother to remove the (now unneeded) old return duct and fill in that cavity in the exterior wall when they added on that bath & new return air run?
If that's the case, I can likely remove that old stud-duct and just insulate it like the rest of the wall. But how do I verify that I have enough return air before I make that leap?
You could run an ACCA Manual D – Residential Duct Systems analysis and re-design the air distribution system.
but if the duct is blocked off, and there is a return path elsewhere, I would not sweat it, and just insulate and seal it up, happy to have made an improvement.
Learn how building cavities should not be used as supply or return ducts.
I just did this myself, built a soffit so the stud bay can stay insulated.
How to Install a Return Air Duct
A return air duct is what carries air back to the furnace after it is dispensed throughout the home through the supply duct. Without a return air duct, the air would not properly circulate and your energy costs would rise.
Return air ducts usually use ceiling joist spaces as the actual "duct," which then runs back toward the furnace. These joist spaces empty into an actual duct that runs perpendicular to how the joists run, and then down into the side of the furnace. With the right tools, you can install a return air duct successfully.