How can I prevent air leaks in my single-hung windows?
On the windows I have installed they have a clear plastic or white wiper that seals to the frame, when these windows get older that plastic seal fails then the outside air can make it through the gap. You may need to remove the slider and look on some windows I have seen several of these, wiper type weather seals. The seals pull out to a side if they don’t slide out with players look for a staple keeping them from moving.
You may be able to find a generic weather strip that will replace yours. It’s not horribly difficult pulling the window can be the hardest part.
The entire sliding part it probably will need to come out so you can remove and replace.