A fire in an enclosed space in an aircraft is, for example, visually detected early since smoke generated by a beginning fire is initially permitted to escape from the enclosed space. The enclosed space is, for example, an overhead luggage bin or a goods container holding food or freight. The smoke can pass through a flow passage either in the form of a gap or a leak between a lid and walls, or between a door and a door frame, or between walls and other walls joined together. Intumescent material is installed along the gaps or leaks and expands into the gaps or leaks when the fire has generated a temperature sufficient for causing the expansion of the intumescent material. Preferably a guide surface assures that the intumescent material expands into the gaps and leaks and cannot expand away from gaps and leaks.