I’ll be interested to see the professional responses on this. In my non-professional opinion, as someone who has lived with the same situation (one zone cooling in a house that should have two) if you live in an area that has 4 (or even 3ish) seasons, the cheapest/easiest option is the portable unit that vents through the window. Roll it out in the summer, roll it back in a closet in the winter. The mini-split (ducted or not ducted) is as you mentioned an expensive proposition and you either have the huge unit on the wall that (IMO) makes it look like a motel room or a huge cartridge in the ceiling (ducted) that makes it look like you live in an office building conference room. Why they haven’t come up with nicer/more aesthetically pleasing options yet I don’t know, but the demand has got to be there given the current choices available. Even with the ductless mini split, the outdoor condenser unit to run/drain the behemoth indoor wall unit is still needed.

I see you're in the same situation lol. I might also look at a unit in the attic for just the second floor, but I didn't want to make it that big of a project.

Here is a similar post in 2013: https://www.houzz.com/discussions/561824/a-c-in-old-house

Yes, I forgot to mention the other alternative of a second unit in the attic. Honestly, with the cost and work involved with the mini-split it seems like it’s about the same level of project as adding a second unit in the attic. That is what we are leaning toward while we use the temporary solution of the portable AC. Every room but the master BR/bath seem to get sufficiently cool (of course!) so we only use one when it gets stuffy and too hot for a fan only. Not ideal, and it makes me angry we have to use it since we (in theory) have central air, but it does the job!

I have a similar issue with my older house. When the house was built, the attic was unfinished space and the precious owner added a supply duct to the upstairs when they finished the space. Without a return and limited supply, it does not get enough airflow to make a significant impact on the space.

I had been thinking of adding a mini split but now wondering if it would be the best option. Our previous house had a whole house fan and I am wondering if I could get sufficient airflow by installing a fan to remove the overheated air from the finished attic area and pull up cool air from the main floor.

I would not mind a window unit, but not on the front side of the house and the window on the side gable is a slider, not a double hung. I love my old house but this is one aspect I just wasn't really prepared for. I don't know if there is any good answer to these issues, but it does give me a new appreciation for the challenges faced by HVAC system designers

As I understand it, a whole house fan will just suck the conditioned air you are paying for right up and out - which doesn’t make sense. I would only use the attic fan if you weren’t going to use the A/C at all. I have seen window units for sliders, and some portable units that will work in sliding windows as well, I think you have to look for them specifically though vs just any “regular” unit. We have an attic fan that turns on automatically when the attic reaches a certain temperature and it has made a big difference on the second floor. There is some debate over whether they are effective, but we have found in our situation it works well and it’s cheap to run so we just let it do its thing until we have no other, more pressing projects, taking up available funds and we can put in a second unit and ducting for the second floor. Until then, the attic fan with a tower fan in the bedroom is working fine and when the heat/humidity really sets in, the portable unit will be rolled out and put into service.

Since ripping up everything and starting again is impractical, try a series of incremental improvements:

Make sure attic insulation is generous

Maximize airflow to balance house. Whole house filter box should be adequately sized, especially if using a paper filter. Replacing a 1" filter box with a 5" deep box can make a significant improvement in airflow if using paper filters.

If possible, add ducts or replace with higher diameter going upstairs-both supply and return. Leave house fan in on not auto. May be some benefit in reducing airflow on first floor to force more air upstairs.

My second floor is just too hot! If you reside during a two-story home, you'll have found yourself saying an equivalent thing. To your dismay, once you return home, the second floor remains much hotter than the primary . Some common reasons that your second floor is just too hot include: Hot Air confine mind, that heat rises. Hot Roof one more reason why your second floor is just too hot within the summer might be due to your roof. Fix the matter Now that you simply know what can cause the matter , it is time to repair it! Redirect airflow to the second floor If you've got a basement, you'll locate your HVAC system's dampers on the ducts resulting in the primary floor and shut them halfway down or more. If you do not have a basement, or can't locate the dampers, you'll close your ground floor register vents down some which can also help push more air to the second floor. An exhaust fan also will help circulate the air, which successively decreases the quantity of hot air that reaches the second floor.