Partitioning L-shaped room. Should I move a register?

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Too Ambitious

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Currently I have 2 bedrooms on my 2nd floor that open straight off the stairway. One of them is L-shaped. I am partitioning that bedroom into a smaller square bedroom. I will cut the original wall against the stairs to a half wall to create a semi-open flex space. (It will be used as a playroom at first, and eventually I may add laundry.)

The current HVAC setup: there are registers fed by 3.5x12 wall stacks under the windows. Someone came later and added 10x4 wall registers to the opposite wall against the knee wall attic. As it is, all 3 registers will be in the new (smaller) bedroom.

My question is whether I should move the 3rd register from the bedroom to the new flex space.

Floorplan is below. The red wall is the wall I added. The green wall is the original bedroom wall that will be cut to a half wall.

Screen Shot 2022-03-30 at 1.45.56 PM.png



I will also be adding some sort of return air to the bedrooms (probably jump ducts / transfer ducts), too, since we're putting new doors on and don't want huge undercuts.
 

Too Ambitious

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Yes id move it
Thanks. Should be pretty easy considering I have the attic access, and it's run with flex duct. Might not even need to replace the duct run.

Agree with fitter30, move it. Doors don't need much undercut to be effective.
Right now the upstairs bedrooms get extremely hot if you keep the doors closed in summer. That's probably a combination of problems, return air being one of them.

The biggest is probably poor insulation on the knee wall. It's deteriorating R-15 mineral wool. That's an expensive problem to fix, though. I've gotten quotes for spray foam @ around $5k (which would bring the knee wall attic into the building envelope). I don't want to do that until I finish all the MEP I need to run in that attic space though.
 

Too Ambitious

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Here's my plan for the returns...

Screen Shot 2022-03-30 at 1.45.56 PM.png


In the left bedroom, I will put a return where the original register I moved is. I can easily drop this through a 1st floor closet down to the return trunk in the crawl space. I think I should use 8" flex for this given the 2 registers are 6".

The right bedroom, it's difficult to find a good place for a return. The only open wall space is where that yellow register is, which obviously is not a good place for a return. I can't easily get a duct to the attic above because it would have to squeeze through a 2x6 rafter bay. I think there's a way to do it, but the jump/transfer duct would be a heck of a lot easier.

Hopefully this will work.


Also working on a plan to better insulate the knee wall without spray foam.
 

ArayT

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That's a common problem with this type layout (assuming there's no or very limited attic space). Normally the problem is the limited space to install ceiling insulation between the 2x6 rafters vs the knee wall.
8" return and the transfer duct should be sufficient. Make sure the flex is pulled tight, flex has about twice the pressure drop as metal duct.
 

Too Ambitious

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That's a common problem with this type layout (assuming there's no or very limited attic space). Normally the problem is the limited space to install ceiling insulation between the 2x6 rafters vs the knee wall.
8" return and the transfer duct should be sufficient. Make sure the flex is pulled tight, flex has about twice the pressure drop as metal duct.
I think I am going to run rigid for the drop through the closet; flex only in the attic and crawl space. The main reason is to conserve as much closet space as possible. The closet is conditioned space, so I don't need insulation. I'm also going to throw a return in the that room while I'm at it since the door usually stays closed. I may run 10" from that point to the return trunk in the crawl space.

I'm starting to realize the only realistic solution for the attic is spray foam. It's not my first choice, but I can't seem to come up with anything else that wouldn't be a ton of labor. The best DIY solution I came up with is to do foam board + rock wool in some form (R-15 rock wool in existing stud wall + 4.5" XPS gets me R-38; would need a 1/2" drywall thermal barrier though)
 

Reach4

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I'm starting to realize the only realistic solution for the attic is spray foam. It's not my first choice, but I can't seem to come up with anything else that wouldn't be a ton of labor. The best DIY solution I came up with is to do foam board + rock wool in some form (R-15 rock wool in existing stud wall + 4.5" XPS gets me R-38; would need a 1/2" drywall thermal barrier though)
You may want to read up on turning the attic into conditioned space. You get rid of the attic vents. You get rid of the current insulation. You insulate the roof either under or even over the wood. You use foam to seal at least. This could be to supplement the XPS or totally foam only.

One advantage is that the attic stays clean, because dust is no longer coming in the vents.

The conditioned attic space is generally more expensive. Read up on it.
 

Too Ambitious

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You may want to read up on turning the attic into conditioned space. You get rid of the attic vents. You get rid of the current insulation. You insulate the roof either under or even over the wood. You use foam to seal at least. This could be to supplement the XPS or totally foam only.

One advantage is that the attic stays clean, because dust is no longer coming in the vents.

The conditioned attic space is generally more expensive. Read up on it.
I've read up on it. I'd go that route if I went with spray foam. I have a quote to remove all current insulation & spray foam the rafters & gable walls.

If I DIY I'd keep the attic vented. It's a lot less complicated that way. Problem is I can't get to R-38 on the attic floor above the 1st floor ceiling without raising the attic decking. (re-reading, I think that's what @ArayT was saying)

Either way I'm probably keeping the upper attic (above 2nd floor bedrooms & not accessible for storage) vented w/ blown cellulose. Even the spray foam contractor told me I'd get more bang for my buck just doing the lower knee wall attic. Eventually I need to top off the cellulose since it's been disturbed by contractors over the years.
 

Reach4

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Going with closed cell foam, or the cheaper open cell foam?
 

Too Ambitious

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The quote was for open cell. Will probably get a few more quotes.

With closed cell, I could get R-38 without overfilling the rafter bays. That would let me use drywall as a thermal barrier instead of paying almost $1k for intumescent paint.

I've also considered doing 2" of closed cell on the underside of the roof deck, then the rest open cell.

I don't love the idea of overfilling the rafter bays w/ open cell because I'm sure I'll hit my head on it and dent it many times.

Seems like there's no real reason to use closed cell in my climate, though.
 

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Now, as is typical, I'm second guessing the jumper duct. The problem is that my attic is very tight. I don't think I could get 10" R-8 flex in there without completely blocking access to one side of the attic. R-6 is possible.

Maybe 8" R-8 would be enough. It's not enough to cover the CFM of the supply registers, but maybe combined with a normal door undercut, it would be okay.

Though I guess a 6' run of R-6 for a jump duct isn't that awful in the big scheme of things.

I'm also considering a transfer grille. That seems like the better solution in terms of efficiency (no duct in hot attic), cost, and installation, but I'm not convinced that the honeycomb cardboard will do enough to stop sound transfer.

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