I have a layer on the outside. It’s welded steel. Before I fit it to the room, I cut all the drywall out. Filled it with foam and fire board. There is one layer of fire board inside. The only reason I’m thinking of going with another layer is because my house is basically a barn. It’s sided with barn wood. I live out in the woods. If there were a fire, I’d say 30-45 minute response once someone noticed.