This is the exception...
I had a dozen or so long boxes stored up in my mom's garage from the 80s. I recently pulled a few books out and sent them to CGC. I sent 3 New Mutants #1 & 2 Wolverine Limited Series #1. I got back 2 9.8s and a 9.6 NM 1 & a 9.8 and 9.6 Wolvie 1. My mom's garage was located in lower Michigan (White Cloud). Again, I know this is an exception ... but they were up their for decades w/o any kind of AC/Heat or humidity control. (with all the bugs, cobwebs, and mice - but they were off the floor, maybe that saved them.)