Reducing the rate of fire spread in buildings through the intersections of enclosing building structures with polymer pipelines is achieved by using fire clutches equipped with liners made of expandable materials. In case of a fire, it is ensured that the spread of flame through polymer pipelines is hindered due to formation of a heat-insulating foamed layer. This layer does not allow the polymer low-melting pipelines to heat up to 120°C. Comparative assessment of heat-insulating and mechanical properties of two intumescent composites and their charred products were carried out to clarify the reasons for their heat-insulating efficiency. Composites included the same intumescent system (ammonium polyphosphate / pentaerythritol / dolomite / thermally expanded graphite) and different polymer binders. The research was conducted using the following methods: complex thermal and X-ray analyses, scanning electron microscopy, a number of standard and original techniques. It was established that heat-insulating ability of a charred layer is determined by temperature intervals in which interactions of initial components occur. The best mechanical, heat-insulating and morphological properties of investigated intumescent composites and their charred layers are achieved when temperature ranges referred to formation of organo-mineral framework and volatile thermolysis products are coinciding. For the composites studied in this paper, this temperature range was 350–400 °C. Thanks to this knowledge, it becomes possible to develop new fire-retardant composites with improved properties instead of selecting the components in empirical way. As a result, the general quality of fire-retardant materials may grow and their properties will be sufficient not only to meet the construction requirements, but to properly operate in case of fire as well, practically justifying the predicted effect.