Public health buildings contribute significantly to UK carbon emissions. New build initiatives have received more attention than the considerable opportunities to reduce carbon emissions within the retained health estate. The research reported here has considered the environmental performance of a typical medium rise, medium depth, concrete-framed, late 1960s acute hospital following low energy environmental design interventions. The interventions are made to optimize daylighting and natural ventilation/cooling whilst reducing overheating caused by summer time solar gains. Three options are investigated: advanced natural ventilation using plena and exhaust stacks; fan-assisted natural ventilation in which fans are used in the exhaust stacks; and mechanical ventilation/cooling with heat recovery. Computer simulations have been carried out to predict the influence on thermal performance (overheating risk) and energy consumption of each of these options on the original design. For each case, current weather data, and future weather data for the years 2020, 2050 and 2080, have been used.