With how little time before winter arrives, Ronnie’s suggestions to stop infiltration are your best bet.
For windows that you won’t operate, you can use the window insulation kits like this
If you want to know how much of the cold is from drafts, just get yourself some thin painters plastic and some masking tape. Cover all the windows and see how it goes. I know from experience in my drafty apartment that the majority of heat loss is from infiltration.