Two small upstairs (children's) rooms in my house are a nightmare to get the temp right. Set temp to 75 in summer - and they'll be 87 while the rest of the upstairs is correct temp. Set temp to 75 in winter - and they'll be 63. The vents do blow properly. I know there are other factors in play - one is above a garage and immediately under the black roof, and the other takes afternoon sun on brick.
I have easy access to the attic above both rooms. Both have just a single supply to them in the ceiling and no returns. And both are the closest rooms to the HVAC unit in the attic, so there's plenty of pressure.
1. Can I just tap into their existing ducts and install secondary supply vents?
2. Or should I install returns, probably using a cell of the wall until I reach the attic where I'd continue a return to the plenum?
3. Since the doors are usually closed, should I put vents in the doors?
3. Or should I do all? Or can I squeak by with some improvement without doing all?