insulate window, but still be able to open it

ap wrote: ...

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That would be a storm window also w/ movable pane...

oooooold fashioned storm windows were 'hinged' at the top for this reason. Perhaps a sheet of plexi with foam around the edge would work ok. cut to the right size it would snug in there and could be popped out on a nice day or for fart duty....

This is a partial solution but it may prove worthwhile nonetheless; and although it won't eliminate air leakage around the sash, it will help reduce some of the heat loss through the glass itself.

First of all, go for the 3M brand of window kits. Nothing else, in my experience, comes remotely close to the quality of this product and for the few extra dollars, it's well worth it.

Instead of applying the sticky tape around the outer edge of the window trim, apply it directly to the front of the lower sash. In this case, the window film will cover just the lower half of the glass but you can still open and close the window as you wish. You might fully cover all of the windows in your home with the exception of the one or two you wish to use for ventilation.

Three years ago, I installed 3M window kits on both the inside AND outside of six large fixed windows in my home (a combined glass area of about 135 sq. ft.). Applying the film directly to the window itself and not the outer trim means these films virtually disappear from view. [The outer film is protected by a wooden storm.]

See:

Cheers, Paul

What I did was build removable wooden frames that fit inside the window opening, and which were held in place by wooden blocks screwed to the casing.

In retrospect, it would have made more sense to make the panels fit OVER the window-opening, because it's easier to get a good seal that way.

Look at how a pre-stretched artists canvas is put together, and built one of those, only bigger, and out of plastic sheet.