I am renovating a 250 sq foot building that was previously used in summers only as a party/gathering space, and more recently as a storage space. I'm converting it into a study for my returning-to-school spouse, and a guest bedroom for overflow and pet-allergic guests.

Currently there are places where there is open space between the top of the wall and the roof. Clearly, I need to close these areas in an appropriate way. But what's the best way?

Suggestions??