If you want to seal off an unused fireplace, you can install wood, metal, insulation, and drywall over the entire wall or opening. If you intend on using it in the future, you should install an insert that is made out of 2 ply foam board. Use a backer rod insulation to seal the insert.
Let's Get It Fixed!
HomeX virtual experts will assess your issue (for free!) and can resolve simple problems.