A Chimney cap helps prevent water, animals and debris from entering your chimney. An open chimney is obviously a hole in the top of your roof. While a fire burns it will push air up and out with no problem. However, 3/4 of the year it is simply a place for rain, bats, birds, squirrels and debris to enter your house. A chimney cap also helps to prevent sparks from your fire landing on dry material that may cause a fire. Chimney cpas come in a variety of shapes, colors and styles.