Today,there is a growing interest in developing energy efficient buildings since it is estimated that buildings account for about 40％ of the total primary energy consumption in the world.In relation to existing buildings,energy efficiency retrofits have become an important opportunity to upgrade the energy performance of commercial,public and residential build-ings that may reduce the energy consumption,demand and cost.In this paper we cover the energy efficiency deep retrofit process that has been carried out for Nottingham Playhouse theatre building for the aim of enhancing its environmental performance and analysing the en-ergy efficiency gained after implementing certain proposed modifications.It is a nationally protected historic building,listed as Grade Ⅱ* on The National Heritage List for England (NHLE).The building has had insulation enhancement,doors modifications,solar energy installations,energy-saving lights,in addition to improved heating and air conditioning system.The paper presents a novel methodology;and its results indicate significant improvements in the build-ing\'s energy performance which is demonstrated using infrared thermographic images and data logger sensors where significant energy savings to the building\'s thermal performance are ob-tained.The energy saving measures have been completed while maintaining the heritage building\'s general appearance and architectural features,which have received a Commenda-tion Certificate from The Nottingham Civic Society for this achievement.