2nd floor of home too hot, best solution
I know this question is asked a lot and each situation is different so I'll do my best to lay out the specifics.
We've live in our house a year, DC Metro area, and learned once we moved in that the when the previous owners remodeled the first floor they cut off 1 of the 2 returns to the second floor. Our house is about 1200sft (600 sft up & 600sft down), brick circa 1940, duct work is original setup for heat only but with gas furnace in the basement. The 2nd floor has a return in the master bedroom about 8"x 5". Duct work runs through our unfinished basement. The attic seems under insulated (nothing recent) and has no eaves so basically no ventilation except a partially louvered window. Also the windows are original single pane with no storms.
The master bedroom is the main problem. It is twice the size of the other 2 bedrooms, but each has 1 supply and they are all the same size. There is a drop down attic stair in the hallway with no weatherstripping and it just bleeds airs. In the winter our room is cold, and in the summer hot. Really it's the summer though that's unbearable.
We've had multiple HVAC people come out, an insulation/energy efficiency expert, and are getting a free energy audit completed next week. The HVAC people never agree on what to do. Some say run a return into the attic, others say put in a huge attic fan because the return might not do enough, another person said a whole 2nd system in the attic for the 2nd floor and possible far future attic renovation, and a friend suggested a ductless AC unit that feeds into our bedroom.
The duct work was quoted from $3-5K, the 2nd system at $15K, I'm waiting on insulation quotes, and just heard about ductless A/C.