Tom Silva explains how ice builds up on storm doors and how to prevent it
I replaced a entryway storm door that had rusted because ice formed frequently on the inside of the glass panel. The following winter, ice formed on the inside of the new door. In fact, the ice builds up to about ½ inch thick. What can I do to stop this from happening?
You need two things to make ice: cold temperatures and a source of moisture. Moisture vapor leaking into the space between the door and the storm door might cause some icing, so make sure that the main door is weatherstripped around its entire perimeter. If the weatherstripping is in good shape, it might be that the storm door is actually too tight and not allowing trapped moisture a way to get out. The solution in that case is to drill four 3/16-inch-diameter holes, two about a half inch from the top of the door and two about a half inch from the bottom. This will allow air to circulate without reducing the effectiveness of the storm door.
from December 2005