To prevent heat loss here are a few recommendation from Handiman4u

Window casements

1. Apply caulking bitumen all along edges of wooden casing trims.  The type should be Alec paintable latex caulking with silicone
2. Also apply Alec caulking all around edges of casement and wall rub excess off with a wet finger.
3. Do all the windows in your house. 
4. You will notice a difference in your utility bills by 15% for the winter period.

Tip: Light a candle and walk across slowly before applying caulking at each window after application.

       Do the same test and you will notice a difference. A lit candle flame should not flicker.