Reinforced polymer composite materials (PCMs), incorporating diverse fiber types, are extensively utilized across multiple industrial sectors. While PCMs possess acknowledged advantages over conventional materials, empirical evidence indicates that prolonged exposure to extreme climatic conditions induces material ageing, leading to a deterioration of elastic and strength properties. A primary design criterion for PCM components intended for cold-climate applications is a low moisture absorption, a property intrinsically linked to material porosity. This study examines pore formation during the environmental ageing of flat basalt-plastic (BP) composites under extreme northern conditions and estimates threshold porosity values. The identification of these limiting values accounts for the layered structure of BP, which exhibits plane-parallel symmetry. Porosity formation during climatic degradation initiates at the external surface of the composite. In order to describe pore formation mechanisms, a statistical model for the formation of open porosity in flat BPmaterials was developed. The porosity distribution across material layers is treated as a two-dimensional cluster structure on a square lattice. Furthermore, the analysis subsequently identifies coincident elements of these two-dimensional porosity clusters across multiple BP layers (two, three, or more). The outcomes of the evaluative and statistical computations were compared against experimental data derived from studies of open porosity in multilayer BP panels. These panels underwent open-air weathering exposure in the natural climatic environment of Yakutsk. Thus, the developed statistical model for porosity formation in multilayer basalt-plastic composite materials can be applied in the design of PCMs for service inArctic and Subarctic climatic zones.