Large temperature variations aren't good for anything, but I don't think plastic would be effected too much. I could be wrong. UV light will make them brittle, so keep them out of the light. Also, keep them dry, as you don't want to have to wash mold off of them or have the boxes decay. The most important thing is to take your decals out and store them in clear pastic sheets, in a binder, indoors. The decals will decay faster than the plastic kit.