The main level is very comfortable, but when it's sunny the upstairs takes hours to cool down after I get homeI've added adequate insulation to the attic, new windows, but one downfall I can see is not enough ventsIt's not a lot of sqft., maybe 350 - 400, but only 2 small ventsThe home has central airHow hard would it be to run another vent to an upstairs room?
It sounds as though you might have a 2 story home with a single system ACThese aren't as efficient or effective as dual systems (one up and one down)Also I'm guessing you have an attic mounted condenser and air handlerIf so, it should be fairly simple to pull a side duct off the main outlet near the air handler, and put it into a vent in the hottest area upstairsIf you do this, be sure to use the vent which can be closed off in winter, or it will likely get too hot up thereCheck that there's reasonable return air from upstairs to the handler - adding more cool air will work better if the air can easily get back to the systemBefore doing any AC work, please be sure you have adequate attic insulation (I use R-38 blown) and it's ideal to have decent or better attic ventilationRadiant barrier is excellent if installed same time as roof decking, but doesn't seem to work worth a darn as a retrofitAlso, if those upstairs windows face South or West, you might want to consider solar screensThey help a lot.
If you are near your parents, move back and then figure things outIf that's not possible, then look up mobile homes for sale.
