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Due to a leak from a corroded fireplace cap and other maintenance issues, the interior wall and flooring along the South side of the upper bedroom was damaged and needs to be replaced. The exterior was repaired and I am now addressing the interior repairs.

(note that this picture is a collage of 2 photos and so the angles of the wall studs may appear off center - but they are vertical)

Interior view: Interior View

So far I have cut out the bad 3/8" plywood and the 3/4" MDF along the wall by cutting along the edge of the joist 16" away from the wall. I have also attached a new 2x8 to the joist so that the new flooring will have a nailing surface.

I planned to nail in a 16" strip of plywood and OSB over the top of the new 2x8 over across the joist and to butt the new plywood and OSB up against the wall bottom plate.

However, now that I'm looking it over, I'm concerned that there will not be enough support with just the 2 floor joists to support anything heavy on top of it.

So my question is: Would adding cross supports be enough? (added 2 so far on the left side).

But there is an issue because I will only be able to add 2x4's sideways as supports on the right hand side due to the HVAC line

Or, after adding the 3/8" plywood strip, should I pull up the rest of the 4x8 MDF sheets leaving the existing plywood in place and replacing the MDF with full sheets of OSB so that there is a full 4x8 sheet of OSB to the wall? (Note that getting full 4x8 sheets into the room would be difficult and I may have to cut the OSB into 2x8 sections)

Or some other solution?

Exterior views during Sheathing / Siding renovation:

Sheathing in process: Sheathing in Process

Sheathing complete: Sheathing Complete

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More support is almost always better. But yes, you are in a bind with the duct. Personally, I would rip the subfloor at least to the next joist bay and replace at least that far. The additional subfloor span will give you better support over the section where the duct is, and you should be able to put blocking in the adjacent joist bay (unless there's another duct). If you can still put some flat 2x4 blocking above the duct, that's great.

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