Thanks to its hygroscopic properties, raw earth can passively buffer indoor humidity. The high sorption properties of clay minerals present in earth also attract other molecules, such as carbon dioxide (CO2), this can potentially lead to indoor CO2 buffering. It would improve air quality in buildings and their resilience towards mechanised ventilation systems. However, there is currently no protocol to evaluate this potential. This paper aims to present a innovative methodology based on thermogravimetric (TG) and calorimetric (DSC) analysis to characterise the interactions between clay and CO2 at the microscopic scale by focusing on the dry state. Results show a CO2 adsorption capacity of 134 mg/kg at 5000 ppm at 35 °C associated with a reversible physisorption process and provides first evidence of a passive CO2 regulation capacity of the indoor air. Determination of the adsorption enthalpies as a function of the amount adsorbed revealed high values (from −74 kJ/mol to −40 kJ/mol) which are characteristic of a strong energetic heterogeneity of the material.