Establishing a simplified correlation between a proposed double skin façade and a thermally equivalent single skin façade for dynamic building energy modelling

Advanced façade systems such as double skin facades are increasingly common. However, evaluation of performance in terms of energy and comfort is not always possible. Limitations and constraints of current design tools lead to the introduction of a number of necessary simplifying assumptions. In-depth understanding of both software tools and the function and the performance of double skin façade systems are essential in solving these problems. The paper presents a case study and starts to develop a methodology for establishing an appropriate correlation. An energy simulation model of a commercial high rise building in London was created for the purpose of a Building Regulations compliance check. An extremely high number of thermal zones in conjunction with the need to represent double skin façades (DSF) in the simulations, significantly increased the complexity of the simulation model. Moreover, the Building Regulations (specifically ‘Part L’ which deals with energy performance) do not permit sufficient reduction in the number of modelled zones to scientifically ease the process. Consequently, a correlation between the proposed DSF and a thermally equivalent single skin facade (SSF) was established with the aim of reducing simulation time and - more importantly - facilitate the modelling process for the building. In order to derive this correlation, a study was carried out of the impact of the ventilation rate through the DSF cavity and the position of the shading device (between inner and outer skin); subsequently, a thermally equivalent SSF was defined. The correlation agreement was evaluated for results obtained for the heating and cooling demand on an hourly, monthly, and annual basis. The modelling of the building was carried out in ‘IES’software, which was also used for assessing the achievable correlation between the simulated single and double façade performance. Additional tools were also used in order to achieve the correlation as mentioned below.