A great way to insulate windows in the winter and summer, is to use Bubblewrap. It not only insulates, but it still lets light in. It won't grow mold, and can be washed. I put it inside my windows. You can hang it up with just a few tacks or push-pins.
I have never tried this, but have read that if you spray the windows with water, the plastic bubble wrap will stick and you won't have to use push pins to hold it up.
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