Homes on pier and beam become unlevel and when they do, some windows
no longer close properly enough to latch the lower to the upper half.
That lets north wind create a draft through there.
A simple solution is to cover that ledge. To the left you see just such a cover. It is 3 inches in width and custom tailored to the length of the ledge.
I have 17 windows and got Marilou Moseley to develop techniques to make some "snakes". She got quite a bit of practice.
She is willing to make more for the public at large for a fee that depends on a number of factors. You can email her at email@example.com to talk about it. Texting will be her preferred form communication.
If any one is partly interested, I can loan you one of my "snakes" to see what it is like.George Wyche