Passivhaus summer overheating: the development of an effective natural ventilation system

Concern regarding overheating potential has been growing in the UK as buildings are being built to higher standards like Passivhaus. Lack of window operation due to noise and security implications specifically at night, alongside higher expected temperatures in the future can only add to this concern. Furthermore the quality of incoming fresh air through windows in Passivhaus dwellings could be lower compared to filtered air in MVHR systems. The aim of this research is to investigate the possibility of overheating in reference Passivhaus dwellings and consequently, to examine and propose a remedial natural ventilation strategy and system for the non-winter period. The internal temperatures, indoor CO2 levels alongside frequency and duration of window openings were recorded using data loggers and sensors. A dynamic thermal model was created in DesignBuilder using data from the original PHPP model and further amended by results from monitoring, creating a base case model. A specific natural ventilation system was modelled using the base case model to increase efficiency and effectiveness of natural ventilation. The proposed system was also tested for the winter period in terms of airtightness and thermal bridging as well as forecasted future climate data. The proposed system increases natural ventilation rates compared to the original design, thereby reducing summer overheating for current and future climate by around 20%. Passivhaus designers can benefit from this system for new building designs or for refurbishment of existing Passivhaus building stock that could encounter overheating in the future. The system can be tested in the PHPP calculation allowing the elimination of all window operations during the cooling season.