You have to have no-spawn items spaced out to prevent spawning.
Basically all interactive objects (aside from doors and the like) make a 20m spawn prevention bubble. So you need work benches, torches, furnaces, beds, fires, fermenters, portals, and the like preventing spawning inside your walls. (there is a full list floating around but I don't have the link handy.)
Roofs, walls, pure decoration, and such don't prevent spawning.