Push them into the opening inside your existing windows to create an insulating pocket of air.
Building interior window insulation panels.
Windowtherm insulation panels formerly known as advanced energy panels are interior panels not storm windows and not replacement windows.
Insulating panels solar shades or interior storms can significantly reduce heat loss through window openings.
Energysavr window inserts are effective interior window insulation panels.
Keep the cold air out in the winter.
Topher belknap is a fellow member of the midcoast green collaborative here in maine and he came up.
But attention to air leaks and where foam panels are used attention to fire safety are important as well.