The windows in my apartment are simple aluminum ones that were installed years ago and lose a lot of heat. I don't have the budget to replace them right now, so I'm looking for options to improve insulation without taking them out. What can be done?
What works best is sealing the cracks and gaps with good foam tape or quality silicone, because that's where most of the warm air escapes. Check carefully around the frame, especially at the corners and where the aluminum touches the glass. Expandable foam tape is cheap and easy to install yourself; if you can find the double-sided kind with strong adhesive, even better. You can also use caulking silicone for larger gaps, though it's a bit more work.
After that, consider adding a thermal insulating film on the inside of the glass. It adheres to the pane and creates an air chamber that helps quite a bit with heat. There are options that look almost invisible or more opaque-looking, depending on what bothers you less. Another complementary option is hanging thick curtains or thermal blinds in winter; it's not the prettiest if you want natural light, but it works.
If you have a bit more budget, D-shaped weatherstripping or foam sealing that nails into the frame are durable and make a big difference with air drafts. Combining crack sealing + weatherstripping + some glass insulation, you'll notice the change pretty quickly in your heating bill.
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