The invention relates to an installation for composting organic material (1). The inventive installation comprises an assembly of cells (10) in which the organic material (1) is stored for fermentation purposes and a ventilation device (20) which blows or draws air through the organic material (1) that is stored in the aforementioned cells (10), each cell (10) comprising a base slab (11) and partition walls (12). The above-mentioned ventilation device (20) comprises at least one ventilator fan (21), which is equipped with a suction opening (211) and a discharge port (212), and a ventilation duct (22) which connects the suction opening (211) or the discharge port (212) to one or more cells (10). According to the invention, the installation comprises an apron (40) forming the base slabs (11) of the different cells (10), said apron comprising means (41) and reserves (42) which are used to install the partition walls (12) of the cells (10) at different pre-determined positions. As a result, the size of the cells (10) can be altered according to requirements and said optimised installation has also removed the need for any air siphon regardless of the ventilation mode used.