Environmental performance assessment of an innovative modular construction concept composed of a permanent structure and flexible modular units

To face the challenges of global warming, the building sector is currently undergoing a noticeable revolution. Buildings are tending to consume less energy, use more renewable energy sources, be built with eco-friendly materials, and generate less wastes during their construction and end-of-life stage. Yet, they could be more resilient or else capable of quickly responding to the housing demand, which may fluctuate in time and in space. Innovative concepts therefore need to be developed to allow buildings to expand and/or shrink. Modular buildings could be a solution to combine these criteria, since they offer faster construction process, provide better construction quality, allow reducing construction waste and are potentially flexible. Frames of modular units can be made of metal, timber, concrete, or mixed materials but lightweight structures do not always allow erecting high-rise buildings and generally present a higher risk of overheating and/or overcooling. To reconcile these pros and cons, a building typology called Slab was designed by a group of architects jointly with the team of the Eco-Construction for Sustainable Development (ECON4SD) research project. The Slab building is an innovative modular building concept based on plug-in architecture, which is composed of a permanent concrete structure on which relocatable timber modular units come to slot in. With respect to flexibility, the Slab building was designed to adapt to any orientation and location in Luxembourg. This doctoral thesis mainly deals with the environmental performance assessment of the Slab building but also involves the development of an energy concept for this one. In this regard, the minimum required wall thicknesses of the Slab building’s modules were determined in compliance with the Luxembourg standard although the current regulation does not yet cover flexible buildings. In this process, two module variants were designed; the first one fulfils the passive house requirements which match with the AAA energy class requirements, and the second one complies with the current building codes requirements, also known as the requirements for building permit application, which in principle correspond to low energy house requirements. Calculations showed that 40 cm wall thickness is sufficient to fulfil both requirements. The environmental performance assessment focused on the appraisal of specific CO2 footprint, which considers on the one hand the operational energy and on the other hand the building materials. The operational energy of modules was determined by carrying out energy balance calculations on LuxEeB-Tool software by considering the worst-case and best-case scenarios. Besides, a method was developed to estimate the space heating demand and CO2 emissions of module aggregation, which can have different configurations over time. The method proposed in this thesis was established for the Slab building but could potentially be applicable to flexible buildings. A comparative study of the CO2 footprint considering the embodied and operational energy showed that there is no environmental benefit in having the modules comply with the passive house requirements in the worst-case scenario (window facing north and high wind exposure). A thermal comfort assessment was also done by realizing DTS on TRNSYS software, to check the necessity of active cooling. Simulations showed that with adequate solar shading and reinforced natural ventilation by window opening, summertime overheating risk could be avoided for the normal residential use scenario for both module variants. Finally, the LCA of the Slab building consisted, on the one hand of optimizing its life cycle and, on the other hand, of comparing its specific CO2 footprint with benchmarks. LCA based on 100 years of lifetime concluded that the total specific CO2 footprint of the Slab building for a low module occupancy rate is lower than that of the Slab building bis, which is a building designed based on the Slab building. The latter would be built according to conventional construction method and thereby does not provide the same level of flexibility as the Slab building. However, for a high module occupancy rate, the Slab building does not environmentally perform any better than the Slab building bis. Some solutions could be proposed to further reduce the specific CO2 footprint of the Slab building, but these would impact the architectural aspect or even the functionalities of the Slab building