The research aimed at moisture buffering of clay plasters modified with organic materials. The tests involved conditioning plaster samples in variable humidity conditions and weighing them to determine the sorption capacity and release of moisture into the environment. Clay plasters examined in this study showed that unmodified plasters have the highest sorption capacity shortly after their installation. Almost two years after their manufacturing and storage in conditions similar to real ones, modification with 1% organic fiber mass content increases their ability to absorb and release moisture to the environment. At this stage of research, it cannot be clearly stated which modification brings the most benefits. Among those tested, samples modified with wood wool had the lowest sorption. It is worth noting that the CF modification resulted in the structural durability of the plaster samples.