We have a shower room which has single brick external walls. It also has a flat roof which overlaps by 4 inches. We want to apply insulation to the outside two of the walls (50mm Celotex, polystyrene or similar) but do not know what to use as a cladding over it. Some sort of weatherproof panel perhaps. expanded ali sheet and render has been suggested but I cannot do rendering (or plastering) and would prefer to keep it as DIY. Has anyone any thoughts on this please.