Feel a draft?
Make sure you've closed your window properly. Sometimes, when you firmly close one window sash, you inadvertently cause another to open — so slightly you don't realize it. This allows a draft.
Here we see examples of how one window sash can open slightly when another is firmly closed
You can accidentally compound this error by thinking you've locked a window when you really haven't.
Because both sashes are not fully closed, this window is not being properly locked the lock isn't positioned to fit into its keeper.
To prevent this, hold one sash closed while you close the other. And make sure the window's lock lines up with its keeper.
With sliding windows, push one sash shut while holding the other shut.
With double hung windows, hold the top sash shut while closing the bottom sash.