I have significantly improved the area behind the walls of my mancave.
You have gone through the door on the right. Note all the new OSB board
And the new duct work in the next two pics below.
You have gone through the door on the left. This side had exposed fiber glass insulation and tremendous dust. It was becoming a health hazard.
This is the inside wall. Note the return. No filter there anymore.
Inside wall on other side. This was long overdue but I didn't have the time or energy.
Finally after 30 years, I have a room and a hall going down stairs that is "right". I'm looking forward to a summer with much better temperature control on the east side of the house. All the old duct work in the downstairs is going and a complete revision of the HVAC system. A massive revision.