A building element adapted to provide thermal insulation between two building parts such as a floor or ceiling slab and a balcony slab is described. The element comprises: an elongate insulating body; a plurality of reinforcing elements passing through and projecting on either side beyond the insulating body so as to be disposed in use within and serve to reinforce each of the two building parts; at least one through apertured formation extending transversely through the insulating body so as to be able to receive in use a post-tensioning tendon member. A building system including at least one such building element, a building structure incorporating such a building system and a method of building are also described.