Double skin facades (DSFs) have gained increasing popularity worldwide for potential building energy savings. Such energy advantages are widely thought to be attributed to the ventilation feature of the DSF cavity. Keeping the cavity open to the outside, however, practically causes noise problems, dust pollution, and safety issues and thereby raising the maintenance cost of DSFs. This paper attempted to bring up this issue for more attention. We first numerically examined the thermal performance of DSF windows based on the climate of Hangzhou City featured by hot summer (>30℃) and cold winter (~4℃). Then we discussed the potential energy benefits of DSFs and the ventilation design of the cavity. Results from our simulations showed that the DSF window was more energy efficient than a double glazing window in summer regardless of the cavity open or closed. Such energy advantages were more due to the additional pane of the DSF window to reduce the solar transmittance than due to ventilation of the cavity. Although ventilation is beneficial in summer, the annual energy gain may be limited. Our simulations showed that ventilation can save annual energy by no more than 8% under Hangzhou climatic conditions. Therefore, to justify the use of ventilation in a DSF, we recommend a comprehensive evaluation to be performed by balancing the annual energy gains and investment increase associated with the open cavity.