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