Elderly people usually spend more than 80% of their daily lives in apartments that mostly belong to obsolete buildings with reduced spaces and inadequate indoor environmental quality, which may lead to tiredness and other adverse health symptoms. In an attempt to evaluate indoor environmental quality and identify the main influencing factors of social housing occupied by the elderly in the Mediterranean climate, this research develops a monitoring campaign covering different seasons to characterise temperature, relative humidity, and carbon dioxide concentrations, which are directly associated with the presence of potential air pollutants, in multi-family apartments that only depend on natural ventilation, representing the social housing patterns in Spain. The results contribute a detailed diagnosis of the indoor environmental quality under diverse scenarios, and highlight that elderly occupants frequently suffer from unhealthy carbon dioxide concentrations, above the 900-ppm recommended average value in indoor air guidelines, and from temperature values outside the established comfort range. The discussion shows the advantageous character of ventilation patterns during sleeping periods, with a 2,000-ppm reduction between certain scenarios. Additionally, retrofit opportunities are identified by diagnosing the influence of the building typology, occupation, climate conditions, air infiltration rates, and occupant behaviour, and holistic implications are provided to promote efficient urban regeneration. The conclusions indicate that ventilation habits and future energy renovation strategies should deal with the sick building syndrome by avoiding high airtightness of insulation solutions through moving towards healthier housing stock and should provide policy implications that promote efficient renovation proposals for ageing in place.