A thermal conditioning system for an off-shore wind turbine is provided. The thermal conditioning system comprises an insulating structure that reduces the thermal losses by convection of a treated air circulating through a duct inside the tower, from the base-level to the nacelle structure, for thermal conditioning purposes. The treated air is supplied by an air-treatment system located at the base level of the off-shore wind turbine. The insulating structure comprises insulating material of a greater thermal conductivity than the air and a plurality of voids arranged between insulating material so that air flow between said voids is prevented.