I have a new house in NC. I took one upstairs bedroom and made it into a computer room (his & hers). Because of all of the equipment, that room is always hot. I put reflective window coatings on, but that is not enough. I need to add an A/C duct for the summer and be able to turn both off for the winter. If I manually close the vent, it whistles. Do I just add another duct and vent and put (2) dampers in the attic? Will this stop the whistling? Is there a better way to solve the problem.
MOST of the time (95%) your house A/C will not have enough extra capacity to handle the additional heat load. More ducts won't help. You need a window shaker or a minisplit:Fujitsu Ductless Mini-Splits: Wall Mounted Systems
We get your scenario dozens of times a summer. Along with excess heat from home entertainment rooms.
"Cut it twice and it is still too short".