I live in the damp pacific north west.
I have found that if you remove the decals from the kit and place them in a three ring binder with plasic pouch pages you will be O.K.(keep the binder in the house)
I have stored my kits in boxs in my garage for many years with out problem.
It does get hot here but the kits are holding out well.
HINT: store your kits on their side in the boxs.If you store them flat the weight of the top boxs "will" warp the kits on the bottom of the stack.
Another HINT:Keep your best kits in the house .This way you will be sure that happens to your favorites.