You don't say where you live - that can make a difference. I've been married to an A/C guy for 42 years. We live in south Florida and our house is zoned - meaning we have one A/C for our bedroom, one for the guest side of the house, one for the center main part of the house and I have a mini-split in my sewing room which is the top floor of the garage. Energy wise it's cheaper to only cool the part of the house we're using but I was able to get the units cheap!! Look into mini-splits - that's the ones that have an outdoor condensing unit and an inside part that hangs on the wall - no ducting needed. You can then only cool the areas you're using. They work very well and have come a long way since their inception. Good luck!!