GC Window Alert Leaf Medley Window Decal Screen Saver Set of 5 Decals Clean glass first with water. Avoid use of chemicals such as ammonia or window cleaners WindowAlert decals are designed to be placed on the OUTSIDE of a window. Decals are best applied when glass is warm (ideally greater than 50 degrees). As a rough guide, we recommend one decal for every 3' x 3' section of glass.