Radon gas is the second biggest cause of lung cancer after smoking and is directly linked to approximately 350 lung cancer cases in Ireland each year. It is a serious public health hazard, and the Government has published a National Radon Control Strategy

More and more single-family houses are being retrofitted to achieve better energy efficiency levels. In this retrofitting process, the building envelope's airtightness is usually improved, and a ventilation system becomes necessary to create and sustain a healthy indoor air quality (IAQ). However, in France, as in many other western countries, ventilation requirements exist for new dwellings but not for residential retrofitting. Ventilation professionals as well as inhabitants are facing a lack of knowledge, recommendations and decision tools to choose, design, and install the most efficient system in retrofitted houses. The work presented in this paper was carried out as part of the French JUSTAIR national research project (2022-2025) which aims to tackle this issue.  
A consultation of ventilation stakeholders and a review of the scientific literature were performed to identify the most commonly used ventilation systems for individual house retrofitting in France and to develop a methodology for assessing the performance of such systems. The performance is assessed by a numerical approach using the NIST’s multizone software CONTAM. As a first step of this project, numerical models of the various ventilation systems are created based on a specific test case: a 100 m² experimental single-family house of two stories, representative of the French building stock. Only the ventilation terminal devices, filters and heat exchangers are modelled to simulate the ventilation systems. The calculation is performed in winter, under a French south-eastern climate, and the indoor temperature is considered as constant.  
This paper presents the selected 8 ventilation systems with a potential for single-family house retrofitting, including commonly installed systems (balanced and exhaust mechanical systems, with and without humidity control) as well as more emerging systems (thermodynamic, decentralized and balanced without supply ducts). The methodology for the performance evaluation of these systems is also detailed, with in particular the indicators identified to assess the physical performance and the scenarios used for the model: occupation, pollutants and humidity emissions.  
Finally, the results of the numerical study are presented with the simulation of the 8 ventilation systems applied to the test case building and a performance comparison based on a set of eight indicators. In the next steps, three of the studied ventilation systems will be implemented in the experimental house for the model’s validation. The validated models will be used to perform sensitivity analysis under different configurations and climate conditions.