The objective of this article is to investigate the energy performance of natural ventilation as a passive cooling method of buildings within houses located in temperate countries using Denmark as a case study. The method consists in running simulations with a thermal-airflow program of a household located in Vejle, Denmark during the months of June, July and August calculating the indoor air temperatures during this period. The dwelling belongs to a Danish project of passive houses named Komfort Husene where its users claim there are periods of overheating during the summer time. Then, after the simulations are validated with measured data, and by applying a new assessment method presented in this article as the cooling rate due to natural ventilation instead of a constant mechanical ventilation rate in the thermal balance within the dwelling, the energy saving is calculated. Results show that there is a reduction of 90% of hours of a possible use of mechanical ventilation, showing the feasibility to achieve thermal comfort within the house by using passive ventilation. The conclusion is therefore that the results contribute to an assessment of the economic and environmental benefits of using natural ventilation rather mechanical one on large-scale scenarios located in temperate conditions. Finally, as a practical implication example, an assessment for Denmark is carried out.