Temperature fluctuations and extreme heat/cold will cause damage in the shellac/finish on some machines. It may cause belts to crack or crumble, bobbin winder tires to disintegrate and other parts like nylon gears to suffer. One summer may not give you problems but climate controlled areas are preferred for long term storage. I have a few machines in the garage now, but they won't be there longer than a few months for the above reasons. I don't have any humidity issues where I live...