The invention provides an arrangement for covering and thermally insulating a basin (11), containing water, which comprises at least one jointed cover (16) consisting of an assembly of adjacent parallel slats (18) hinged together so that they can be wound up to expose the surface (22) of the water, and unwound horizontally to cover the basin, ;characterized in that each slat (18) is a length of hollow section made of rigid or semi-rigid plastic and filled with an injected or expanded plastic foam (25), ;and in that when the cover is in the unwound position there is a predetermined gap (J) between the approximately horizontal lower face (23) of the adjacent slats (18) of the cover (16) and the surface (22) of the water in order to define a thermally insulating cushion of air (24).
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