When approaching the design of a new building, it is possible to define a wide set of options and to choose between them taking account of the local climate, the mini- mum performance standards, the architectural context and the landscape. However, when approaching a refurbishment project the number of degrees of freedom is obviously lower. The retrofit of historic buildings introduces further limitations, especially when regulated through the decision-making processes of different local and regional authorities. We consider different sustainable retrofit approaches for the former silk spinning mill located in Valmadrera, not far from Lake Como in northern Italy, which has been active since 1819. Due to its historic value, any physical intervention on the building should be authorised by the cultural heritage authority. For this reason, dif- ferent possible wall insulation approaches are considered: a conservative retrofit, with a lower visual impact on the external façade, to preserve the historic value of the building, and a deep retrofit that includes thermal insulation on both the internal and external surfaces of the walls.