Yeah... that was a major hassle for me up hear in my Holland Landing studio.
I've got air conditioning but there's no way you can have it running while you're doing vocals. So you boil your ass off cause you have to turn it off.
In my cornwall studio, I'm reading up on it so I can run an air conditioner when necessary so that it ventilates the place but doesn't cause any noise.
The weather down there isn't so much a problem cause it generally hovers around 70 deg in summer. If it gets overly hot, you can open the front and rear doors and the ocean breeze totally airs the place out.
In Canada though, I just use a remote control air conditioner and turn it off whenever I'm doing vocals or acoustic guitar. The rest of the time all the instruments are self contained so it isn't a problem.
The vocal booth is the major hassle. All you can do is ventilate it while it's not in use and then stagger your sessions so that you don't overheat.
In our old rehearsal studio it was torture.
During the winter, we had to use a propane heater and it stunk like hell.
The place was freezing.
Then in summer, it was so hot you were literally streaming sweat.