My house use to have old windows with storms I had to install outside each winter. Through a grant, I had new windows installed. They are double hung, single pane, with screens to the outside. Do I leave the screens on during the winter? How do I keep the cold out with single panes?
Well, that wasn't much of an upgrade. I'm afraid you're stuck with having to use that plastic stuff or installing extra storm windows.
Install insulating shutters.
Solid shutters will decrease both heat loss and summer heat gain. Insulating shutters consist of wood panels, a vapor barrier, and sometimes a decorative covering. If you fit them tightly against a window frame, they'll provide an insulating air space between the shutter and the window.
Too bad... I would have thought any grant would specify modern efficient windows.