Air pollution is one of the important environmental problems in Poland. The main cause of its occurrence is emissions associated with individual heating of buildings. The reduction of the primary energy demand in a building is influenced by its proper thermal modernization, including in particular insulating. In view of the above, this article presents the results of studies on the possible environmental benefits of thermal modernization of single-family houses for the area of Southern Poland. The analysis was limited to determining the impact of measures to reduce air pollution emissions by insulating the building envelope of single-family houses. The research was conducted for two voivodeships: Śląskie and Małopolskie. Its aim is to identify the financial costs and achievable ecological effects of the thermal modernization of single-family buildings. The geographical selection of the research area was based on the fact that it covers the most polluted region in Poland. This region is characterized by many features that influence poor air quality. Among these features, the most important are: diversified building structure, a diverse topography, and very high population density. To limit multiple variables, we have selected a scenario method that has already been used in similar research. Four scenarios were established to show the relationship between the value of emission reductions and the level of funding for thermal modernization. The analysis allows a comparison of the effectiveness of individual variants and the transposition of their results into the possibilities of action in the region. This research will help to supplement the knowledge of the impact of insulating building envelopes on reducing pollutant emissions by reducing the energy demand of a building. They also identify a possible link between the level of this reduction and the grant amount for thermal modernization. As a result, it was found that a high share of external funding—stimulating the tendency of the inhabitants of the analyzed voivodeships to effective thermal modernization, and thus reducing the buildings’ energy consumption—has a significant impact on the improvement of air quality.