The environmental design and integration of new technology in Victorian buildings demonstrates a profound understanding that was a combination of experimental scientific advances (e.g. fluid dynamics) and the intuitive application of professional knowledge by architects and engineers. Architects at this time retained direct control of the dimensions of windows, room volumes, air inlets, and chimneys that together constituted the chief variables determining the success or failure of building services. Arguably much of this tacit knowledge was lost in the first half of the twentieth century. This included the architect’s understanding of the role of transition spaces, such as porches and lobbies, assisting in the process of adaptation from outside to inside, or between spaces with different environmental (thermal, lighting, or acoustic) properties. This article examines three significant examples of transition spaces (in the House of Commons, the Natural History Museum, and the Glasgow School of Art) and identifies step changes in their purpose and design. The environmental role of these spaces is described in relation to the wider evolution of the design and the developing discipline of building services in the nineteenth century. The article concludes by reflecting on the purpose of transition spaces and the ways in which the development of modern heating, ventilation, and air conditioning (HVAC) systems have replaced their key role in providing thermal comfort. In recent years, adaptive comfort theory has contributed to a rediscovery of the importance of these spaces.