of diameter and forming a rigid distribution element in the embodiment in fig2 the second member consists of a sort of plug 11 which has an internally threaded tubular part 12 which interacts with the rod 7 and which is surmounted by a rigid flat head 13 constituting a bearing washer or rigid distribution element similar to the rigid washer 9 lastly the fixing device comprises a flexible plate 21 which is placed between the rigid washer 9 or the rigid flat head 13 and the insulating panels 3 plate 21 is preferably circular of diameter d and of large dimensions flexible plate 21 is for example made from a metal of small thickness and can comprise a surface coating compatible with the material constituting the cladding it is also possible to use a material comprising a reinforcement which is woven or nonwoven made from glass polyester organic material or from a mixture of these components in which case the plate is coated with a material identical to or compatible with that of the cladding the abovementioned plate 21 can advantageously be fastened beforehand firmly to the washer 9 or to the rigid head 13 by adhesive bonding or welding in the embodiment in fig1 comprising a first member consisting of a screw it is also possible to provide for the flexible plate to be crimped between two rigid washers corresponding to the washer 9 the rigid washer 9 or the flat head 13 can have a diameter a of the order of 80 mm the flexible plate 21 must have a mechanical strength under tension which conforms with the equation r t u03c0a gt 5000n and preferably of the order of 8000n the mechanical strength under tension r t being.