It is reccomended that powder be stored in a cool, dry place by the manufacturers and all reloading books. Personally, I lived in Tucson for many years and my reloading room was in the garage where it would get over a 100 degrees in the summer. If there was any deteriation of the powders it was nothing I could ever see or notice. I had a can of Bullseyethat I had for over 30 years and it as stored in everything from damp basements,cool rooms and also in my garage. I finally made some shotgun shells out of it, just to get rid of it as I don't use it for pistols at all. The shells fired fine without a hitch. I would certainly follow the mfgr.s suggestions as to how to store it, but I have not seen any problem when it wasn't followed either. It is certainly your call on this..
