Abstract This study uses a building energy performance simulation to investigate the impact of predicted climate warming and the additional issue of building ageing on the energy performance for a library in Turin, Italy. The climate and ageing factors were modelled individually and then integrated together for several decades. Results from the climate-only simulation showed a decrease in the building heating energy usage which outweighed the increase in the on-site cooling energy demand occurring in a warming scenario. The study revealed a high sensitivity of energy performance to building ageing, in particular due to HVAC (Heating, Ventilation and Air Conditioning) equipment efficiency degradation. Building ageing was seen to negatively affect the energy performance as it induced a further increase of the cooling energy usage in a warming climate, while it also counteracted the reduction of the heating energy usage resulting from warming. Simulations on the combination of mitigation techniques showed a number of potentially retrofit measures that would be beneficial for buildings to avoid an increase in the cooling energy usage due to climate warming. The combination of these retrofit techniques showed a potential decrease of 87.3% in the final cooling energy usage for the considered building.