Introduction: Why performance-based assessment methods? Overview of the needs and the possibilities

In future building regulations 2020, building performance is going to be extended to global performance, including indoor air quality (IAQ). In the energy performance (EP) field, successive regulations pushed for a "performance-based" approach, based on an energy consumption requirement at the design stage. Nevertheless, ventilation regulations throughout the world are still mostly based on prescriptive approaches, setting airflows requirements. A performance-based approach for ventilation would insure that ventilation is designed to avoid risks for occupant’s health. 
Given the European context with the generalization of nearly zero energy buildings, envelope airtightness is often included in EP-calculations, frequently through single-zone models with uniform air leakage. Because more consideration is often given to EP than to IAQ, impact of several zones interconnected by unevenly distributed leaks, on the envelope and on internal partition walls, is a rarely investigated issue, which is investigated in this work. 
Faced with this issue, we conducted an experimental study on multizone air leakages of 23 detached houses and developed an innovative database. The analysis of this database reveals that internal air leakage can become significant at door undercuts and that the type of building structure has a great influence. We propose airleakage values and dispersion input data for multizone IAQ models. Then, through a multizone modelling of a low energy house case study, we quantify impacts of these airleakage distribution data on IAQ. We model CO2, humidity and formaldehyde with two type of ventilation (exhaust-only or balanced). We highlight strong impacts and conclude that detailed airleakage distributions should be used in IAQ performance assessment methods. 
An extensive review work combined with complementary analysis allows us to come up with the development of a performance-based approach for house ventilation to be used at the design stage in a regulatory calculation. We select the use of five relevant IAQ performance indicators, based on CO2, formaldehyde and PM2.5 exposures, and RH-based indicators assessing both condensation and health risks. We propose also pollutant emission data and occupancy schedules to be used. Lastly, we describe the multizone modelling laws, the physical models and associated assumptions, and the boundary conditions. 
Importantly, we demonstrate that our proposed performance-based method was applicable, applying it to a low-energy house case study.  We assume being at the design stage of a house which should comply with a hypothetical regulation, requiring IAQ performance indicators and associated thresholds. We also demonstrate how such an approach could help at the design stage in key choices as the type of structure (regarding its impact on airleakage distributions), the type of ventilation system, the level of pollutant emissions. Indeed, in the case study, only the balanced ventilation combined with low or medium-emission class of formaldehyde emissions allow to fulfil the IAQ requirements. We show also that such an approach could help in the ventilation design, notably the distribution of the air inlets and/or outlets, or even the airflows, in order to secure the fulfilment of IAQ requirements. 
At the end of this work, we highlight the needs for such performance-based approaches, from the robustness of performance indicators which should also be based on acute exposures, to the lack of data to be used to assess the pollutants emissions at the dwelling scale.