<p>Reducing the carbon footprint of cement-based materials is a crucial step for a sustainable cement industry. CO₂ emissions from the decalcification of lime during Portland cement production cannot be avoided. Thus, alternative binders such as calcium sulfoaluminate (CSA) cements are becoming more relevant, as they contain a reduced amount of cement clinker and therefore result in lower emissions than Portland cement. In the case of CSA cement, ettringite is primarily formed during hydration, which is susceptible to carbonation over its lifetime. This paper investigates for the first time the carbonation behavior of a porous insulation plaster consisting of micro hollow glass spheres embedded in a needle-like ettringite matrix with an open porosity of approximately 45%. A series of samples were produced in a field trial. On the one hand, the samples were taken from a façade that had been exposed to environmental conditions for two years. On the other hand, comparative samples were stored for two and four years under laboratory conditions. The samples were analyzed for their crystal structure and ratio, thermal conductivity and carbonation rate. Environmental conditions led to 97% carbonation of the ettringite phase after just two years, while a controlled atmosphere led to a carbonation of 76%. It was shown that effective moisture control can significantly slow down the carbonation of highly porous ettringite structures.</p>