This article aims to provide a critical analysis of the work of the Smithsons, with a focus on its interpretation of architecture from the standpoint of environmental commitment, an aspect that is barely touched upon in the extensive literature analysing their work. The main aim is to reveal the strategies of environmental conditioning and the energy behaviour of their major work both from an architectural and environmental standpoint, as well as that of their own home and experimental laboratory: the Upper Lawn Pavilion. These features make it a building of great heritage value, classified as a Grade II listed building according to the Statutory List of Buildings of Special Architectural or Historic Interest. Energy models reproducing the case study (in its original construction, occupancy and thermal conditions) have been simulated using DesignBuilder software in order to carry out a quantitative assessment of the house’s environmental conditions, perceptively described by Alison Smithson in her diary. Indoor thermal conditions obtained from the energy simulations were studied, identifying the environmental benefits and deficiencies caused by the strategies applied by the Smithsons to their pavilion. It is concluded that its environmental behaviour is far from the current standards of adaptive comfort.