I like the idea of setting up a coop using an existing barn. You may want to look at all access points to the barn to see if predators have other ways to access your chickens. Every time I look at my coop and run, I try to think of ways predators could get in. I drive my husband a little crazy with it but for all the time, effort, and money we have in our chickens; I want to be sure predators aren’t picking them off.