Louvers are allowed in fire doors, but only if they are labeled by a certified agency. The louvers must have fusible links, which helps them close to prevent smoke from spreading. Glass windows are allowed, but keep in mind they cannot be used in combination with louvers. Doors with a 45-minute rating can have up to 1,296 square inches of glass; doors with 90-minute rating can have up to 100 square inches of glass. Three-hour fire doors may not have glass windows.