Unit gets 70-80% humidity in Chicago (70 degree weather)

[Byron W.]

Hello, I'm house hacking a 3 unit in Chicago. The first floor unit gets really humid. There's no AC. It gets humid with the windows closed. With the windows open, it helps circulate air more of course but doesn't do much of anything for the humidity levels. The unit is 1350sqft. I'm planning to live here another 1-1 1/2 years before moving into another place and renting out this unit. I'm trying to figure out the best course of action here.

Some things to note:

The basement just below this unit is large, dark and gets about 70%-80% humidity (same as the first floor unit). It has some very small windows that I have open to help circulate air down there. I'm not sure if attacking the humidity down there will solve the problem with the humidty in the 1st floor unit or not.

My other units humidity levels seem fine. Though I haven't measured, being in them, I didn't notice it being as humid when walking through.

The humidty was above 80% when it was 80+ degrees outside. But now that it's cooled down a bit (70-75 degrees) the humidity is between 70-80% daily.

What advice do you all have for me? Should I get an AC unit? Should I install a dehumidifier in the basement? Should I do something else? Thanks!