Ironic this thread for bumped because I was reading multiple threads and Reddit posts about storing drums int he garage with heat, humidity, etc.
I moved in to my own place after getting married last May (2018). Prior to moving in to my new place, I had my drums in a temperature controlled living room at my parent's house. I never had t worry about weather extremes, especially since I live in the South bay Area, where there is no humidity and the heat is dry.
That being said, after moving in to my new place, it is an older place with no central air. In addition to that, I don't have room indoors to keep my drums set up, so I have them out in the standalone garage out back. The garage is basically a big garage structure, with 3 separate one-car garages (I live in a duplex). It gets really warm in there and I was concerned about my Mapex Velvetones, as well as my hotter drums and snares.
I ended up installing a window AC unit to help keep the air moderate during the hot summer months (it gets up to 98 degrees F in that room on a warm day, even hotter during the heatwaves). The garage isn't insulated and there is no ceiling as it goes straight to the roof, however, I've found the AC unit does a decent job keeping temperatures from getting out of hand. I haven't had nay problems with my drums thus far.