Keep the hot air out in the summer.
Building interior window insulation panels.
Energysavr window inserts are effective interior window insulation panels.
Topher belknap is a fellow member of the midcoast green collaborative here in maine and he came up.
Push them into the opening inside your existing windows to create an insulating pocket of air.
But attention to air leaks and where foam panels are used attention to fire safety are important as well.
Insulating panels solar shades or interior storms can significantly reduce heat loss through window openings.
Keep the cold air out in the winter.