The diploma thesis is an analysis of the problems of the current condition of fire closures, focusing on the requirements with respect to the basic properties, which have a major effect on spread of fire.The reason is a better understanding of fire resistance as a basic property of a fire closure through a suggested experiment. Specifically, by the test of a source of radiant heat to explore what happen inside of the fire closure and also to record the development of temperatures, including temperatures inside of the door wing construction. The experiment was carried out on two samples - corner fragments of a fire door with different construction and different uses of insulating materials in the door leaf using a methodology for testing fire closures adapted for the use of radiant heat source and thermocouples placed directly in the insulating material of the door leaf. The first result was the finding that when one type of insulating material is used, the temperature inside the door leaf has a similar process from a certain moment. The second result was that, we were successfully identified critical details on both samples of fire closures and then proposed measures for their removal, by observing ongoing events.