This study examines the effectiveness of natural plasters, and in particular clay-based plasters, in improving indoor air quality by measuring their CO2 adsorption capacity. The experimental approach consists, at first, to characterise the interactions between the material and CO2 to better predict its behaviour under real conditions. The results for a clay-based plaster show that it has the potential to buffer CO2 through physisorption of CO2 on clay minerals. This research provides the first protocol for determining the adsorption capacity of indoor CO2 concentrations of coating materials already known for their good hygrothermal properties, which is the case of clay-based or bio-based materials.