Hey thanks for your response. The window sill probably doesn’t need replacing now that I think of it, it’s just really loose. I’m thinking I should be able to nail it back in. For below the window sill, Im gonna just fill in the gaps, tape and spackle it. You like using durabond? I was reccommended USG 20min joint compound. Any preference?

Was the apron board missing below that window? If it was, it should be replaced. That will cover all the gaps etc.

I would probably use the durabond . It is stronger for a stress area. Though it is a pain to work with since it drys so fast and hard.

Cat_KY makes a good point regarding the apron. Just seal between the apron and stool.

There is no apron below the sill on any of the windows lol. So maybe I should seal it up and then put on the apron.

Jim,

Depending on the age of your home, that is a great idea to seal it before you cover over the crack to avoid any potential infiltration at that point. Here are a couple of different methods. Sorry I didn't have any close ups but you can zoom in.

Since apron is trim board, usually, it might be a good idea to put one under all your windows. Its pretty simple to do. Pretty obvious looking at the one in your picture, that it once had one. Someone removed them, and never replaced them.