I've lived in my current duplex for about 5 years now and every summer, my upstairs portion of the duplex gets noticeably hotter than downstairs. I couldn't give an exact measurement but 10 degrees Fahrenheit hotter would not surprise me.
Over the years, I've added several things to the house to deal with this issue. It's made the heat manageable but by no means is it solved. I had to replace the AC (it was 20 yrs old), but even that didn't fix the cooling issue. I've added ceiling fans in all three bedrooms. I've added an attic fan as well to get the hot air out of the attic.
What I'm really doing is just guessing what the problem is. I've looked into companies doing energy audits and insulation, but I'm not sure if this is the most effective thing I can do.
How do I go about solving the hot air in my second floor during the summertime? What is the most effective approach to solving this problem? Is this something I can solve or would it be best to hire a contractor?