Where in the mid-Atlantic region do you live? As a Virginia native and resident, I know all too well about the extreme temps!
I store most of mine in a temperature controlled storage unit. However, Halloween is stored in my attic and they do just fine up there. No problems or issues caused because of the extreme heat of the DC summers. They'll be just fine. I have a friend in Florida who stores his entire collection in the attic, and they are fine too. I mean, if you are able to keep them in a climate controlled place... that would be the best, but it's nearly impossible as the size of the collection gets bigger.
I do recommend putting the molds in clear plastic bags for storage. They keep the paint protected, clean, and nothing can crawl up into them and you can carry a lot more at one time .