There is disclosed a method of retro-fitting an external insulation system to a building, and a retro-fit external insulation system for a building. One disclosed method comprising the steps of: mounting a plurality of support elements (16) to an existing external surface (14) of a wall (12) of a building (10); providing a plurality of surface panels (22), each of which comprises an internal wall-facing surface (36) and an external surface (38); arranging the surface panels so that they cover at least a substantial part of the existing external surface of the wall; mounting each surface panel to one or more of the support elements, so that cavities (40) are defined between the internal wall-facing surfaces of the panels and the existing external surface of the wall; and supplying flowable insulation material (42) into the cavities; in which the step of mounting the support elements to the external surface comprises positioning at least one adjustable one-piece spacer (108) between each support element and the surface, the spacer comprising a first end (112) which abuts the surface and a second end (114) which abuts the support element, and adjusting the spacer to vary a space (110) between the support element and the surface.