Fibrous insulating materials represent a relatively specific group of materials whose structure is often very difficult to define. From the point of view of reaction to fire and combustion, these materials are mostly described from a macroscopic point of view and evaluated on the basis of standard tests carried out to determine reaction to fire in accordance with EN 13501–1 and related test standards. However, the research work carried out at Brno University of Technology seeks to describe the combustion of these materials from a different perspective at the microscopic level and to use some principles from the field of building physics to describe the heat process in more depth and to parameterize it by key properties of the fibrous material, so that the fire response of these types of composites can be estimated at the design stage and better optimized when developing them. This paper describes the initial phase of the research work, the formulation of the basic dependances and the initial results of the experimental tests. It was found that it is necessary to combine chemical and physical approaches to combustion propagation. The basic dependences of flame height when loading samples according to EN ISO 11925-2 on fiber type, thickness and bulk density were determined.