Been storing boats out side for 30 plus years with out a problem.The main thing is to have proper ventilation.
What I do is,I build a rib system over the boat and a minimum of 2 feet past the engine. the extra length is
for air. The back of my tarps are open.To stop any humidity from forming in the boat.
I had a balsa core boat store for 10 years this way with out any dry rote.
Shrink wrap is one of the worse ways you can store a boat. It locks the humidity inside.Do to the air
inside can't change to the condition outside.
When you build your garage. Insulate it very well.So once it gets cold out it stays cold in the garage.