Both storm doors and screen doors are considered external doors. A single device can be both a storm door and a screen door. Storm doors are designed to protect the inner door from water and condensation. They also help seal the doorway from air flow which makes the building easier to heat or cool. Storm doors very often have transparent glazing which allows for light (but not air) to enter the structure when the main external door is opened. Screen doors are designed to allow both air flow and light to enter the building. The screen is usually metal or plastic fiber with a weave tight enough to allow ventilation but exclude insects. Some doors have interchangeable windows and screens that can be swapped. These require a storage place for the window or screen that is not being used. Storm/screen door combinations also come with self-storage features which allow either the window or screen to be raised or lowered as needed.