Outside in the garage for a few months, even a year or two, is probably no big deal. But humidity can be hard on kits because it allows mold and mildew to colonize ruining the wood and discoloring the outside of the box. Also, many kits use plans and instructions that contain high acid levels (like Bud Nosen's blueprints!) which will turn brown and crumble over time and the moisture accelerates this. Steel hardware rusts too. Folks who live near the gulf coast (salt air + extreme humidity) should definitely keep their kits inside air conditioned houses for longer term storage. Here in Central Texas an attic can rise to 140 degrees in August so thats also out of the question if you don't want canopies and wood to warp. I keep my kits stacked in a closet in my spare bedroom. Fortunately stacking isn't that big a deal with R/C kits because they tend to be made from heavy corrugated cardboard but there are limits.

I bought a Royal Pitts S-2 kit from a guy in Houston that had left it out in his garage for 20 years. The outside of the box which was originally white had turned dark gray. Although the wood looked new, the plans and instructions were orange/brown and crumbly, especially the top layers exposed most to the air within the box.

I bought a Proctor Nieuport 28 kit from an estate liquidator in Alabama somewhere. The box and contents were fine but the 1/32 plywood pieces had mildew/mold and had to be replaced. The blueprints had also yellowed but weren't crumbly. A receipt in the box showed it was purchased new in 1992.

I've never had problems with bugs or other vermin but I hear infestations can silently destroy a cherished kit over time.

This is an interesting topic. I'd be interested in hearing what the guys up North with basements and real winters do.