This is similar to this Q: Best way to seal gaps in sliding window?.
But the big difference is that I regularly open and close the sliding windows (at least once a day). (Whereas that other Q is about a once-a-year aircon installation & removal.)
Also, my main goal is to block out/reduce noise from a busy road nearby. (Whereas that other Q is about keeping out insects.)
So, the suggestions in the other Q probably don't work well:
- Foam rubber: Falls out as I open and close the windows. Even if I manage to tape/glue the foam pieces in place (which seems hard for a DIY newbie like me), over time (with opening and closing), the pieces will be torn.
- Spray foam: I haven't tried this yet, but I think similarly, over time, this will be torn.
- Tape: I haven't tried this yet, but I don't think this will do much in reducing noise.
The sliding windows come in three. When they are closed, there are altogether four big gaps--two big gaps between each pair of sliding windows, one upper and one lower.
One such upper big gap:
One such lower big gap: