Climate-Dependent Performance of Natural Ventilation Under Continuous 24-h Mechanical Ventilation in Residential Buildings

Natural ventilation is widely regarded as an energy-saving strategy in buildings; however, under continuous mechanical ventilation in Japanese residential buildings, its performance remains insufficiently understood. This study evaluates the performance of different natural ventilation strategies for a typical two-story detached house across eight climate zones in Japan using dynamic building energy simulation. Four ventilation strategies are examined, including baseline mechanical ventilation (S0), shoulder-season natural ventilation (S1), summer night ventilation (S2), and an adaptive natural ventilation strategy with humidity constraints (S3). Annual HVAC loads, monthly variations, and the structure of cooling loads are analyzed. Results show that shoulder-season natural ventilation (S1) does not lead to energy savings and may result in a slight increase in annual HVAC loads in most climate zones. In contrast, summer night ventilation (S2) reduces annual HVAC loads by approximately 8–10% in transitional climates (CZ3–CZ5), while its effect is weaker in hot and humid regions. The adaptive strategy (S3) achieves moderate reductions of up to about 2–3% and significantly decreases the proportion of latent cooling loads. Overall, the effectiveness of natural ventilation is governed by the trade-off between sensible load reduction and latent load increase and is strongly climate-dependent. These findings provide a basis for optimizing hybrid ventilation strategies under continuous mechanical ventilation conditions.