Zone 4/5, Central Indiana
I have one of those infamous above garage bonus rooms. I have spent the last 6 months blocking open floor trusses (Foam boards sealed with expanding foam), removing and correctly installing fiberglass batts in the kneewalls, installing rigid backing on kneewalls, dense packing the floor trusses and air sealing everywhere (on interior and attic side of all kneewalls and the end wall open to the attic) I can using a combination of foams, caulks, EcoSeal, tapes etc. Furthermore, I have sealed around all of the IC rated can lights (8) and installed air-tite baffles within them. All bulbs are now CFL. The same treatment has been done to the stairwell leading to the bonus room from the hallway below (within the conditioned space of the home). As part of this process, I have elected to remove the supply registers (4) and the large return duct and install a dedicated single head mini split system that will both heat and cool the room when in use. The room is not used that frequently, but in its previous state, it was pouring seasonally hot/cold air down the open staircase into the hallway/kitchen area below.

I have not yet installed the mini split but will be doing so shortly. Incredibly, there is already a hugely noticable difference in the bonus room temperature as well as the temperature in the living area open to the staircase, even though there is no conditioned air being routed to the bonus room currently.

The supply and return ducting came up from the basement, up through an unconditioned closet in the garage, then into the main house attic. At that point, the return duct terminated in the end wall of the bonus room accessable from the attic. The supply duct was split in the main attic area and branches were run through the kneewall attics feeding the 4 registers. All of this naturally was at the far end of both the supply and return systems. As of now, I have disconnected and plugged both the supply and return ducts where they first entered unconditioned space ( where they entered the floor of the unconditioned closet in the garage). Therefore, there is still approximately 10 feet of rigid metal supply duct running from the main trunk to where I blocked it and approximately the same 10 feet of panned (what else would one expect?) return from the main return to the point where I blocked it.

The main living area of my house is noticably cooler so far this summer, so the system appears to be “working effectively”. Additionally, last winter’s heating bills were reduced very noticably, so I think I am on the right path, even though I am not finished. I should say that I have found and corrected many difficiencies in my system, including disconnected ducts and a completely open section of the main return trunk (where floor joists were used for panned returns, the contractor did not block the back side, meaning that the return was open to the unfinished mechanical room in my basement). I have taped and sealed all supply and return ducts that I can get access to, sealed all register boots to floors, walls, ceilings etc.

So, having said all of the above, I have a couple of specific questions:
1). It seems to me that I would be better off terminating both the supply and returns from where they exit the main trunks as opposed to where I currently have them blocked. Would that be a correct assumption?
2). Since both return and supply were terminated, and since both were at the end of the main trunk runs, I am assuming that I have not terribly imbalanced my system (no, I have not had it checked). Should I be concerned that I could have ultimately “crewed it up”?

Thanks in advance for your input and direction.