Storing a sleeping bag in a garage can be problematic if the garage is prone to extreme temperatures or high humidity. Both dampness and extreme heat can negatively affect the materials and insulation of a sleeping bag. For instance, damp conditions could lead to the growth of mold and mildew, which can damage the bag and cause unpleasant odors. Similarly, extreme heat or direct sunlight can degrade the insulation, reducing its effectiveness. If your garage maintains a stable, cool, and dry environment, it could be a viable storage option. However, for the most part, it's recommended to store your sleeping bag indoors, in a space that is cool, dry, and away from direct sunlight. Always ensure the bag is stored loosely or hung to maintain the integrity of the insulation.