The COVID-19 pandemic has made the world realize how vital indoor air quality is. For healthy and sustainable indoor environments, the “breathable building skin” approach deserves the attention of the building science community. In contrast to the common approach of airtight buildings, the “breathable skin” approach changes what is expected from the solid parts of a building envelope. Here, a new approach, new measurable parameters, and a new practical testing method are presented. Benefitting from the pollutant reduction and self-ventilation potentials of building materials is a new approach introduced here for enhancing indoor air quality. The effectiveness assessment of that approach requires developing testing methods for measuring the pollutant reduction (diffusion and retaining) performance of building materials. Among the occupant-related indoor air pollutants, CO2 is well-known and one of the widely-used indicators for assessing indoor air quality. The testing method proposed in this study assesses CO2 reduction performance of building materials in terms of "CO2 concentration decay rate," "effective CO2 diffusion coefficient," and "CO2 retaining ratio" as the related measurable parameters. Sample use of the testing method conducted on adobe and autoclaved aerated concrete was presented to explain the proposed testing procedure. This procedure involved the combined use of single-chamber and double-chamber diffusion tests. The single-chamber setup is a system that permits CO2 transmission through a porous material and measures the CO2 concentration decay rate. The double-chamber setup is a closed system that prevents CO2 from escaping thereupon measures the impact of CO2 retaining behavior on CO2 concentration decay rate. Joint interpretation of the data allows discussing the potentials and limitations of materials in reducing indoor CO2 concentrations. For further evaluations, this practical testing method is useful in producing reference data on CO2 reduction performances of building materials.