Vernacular dwellings have demonstrated the ability to create desirable indoor thermal environments with nominal energy consumption through a proper passive design. This study has explored the key factors influencing the thermal environments of vernacular dwellings during summer. Field measurements were conducted in a typical house and its nearby alley for approximately one month. An analysis of a typical room adjacent to the alley on a hot day showed that the indoor net heat gain through the inner surface of the external walls, ceiling, infiltration and natural ventilation together, accounted for 34%, 26%, and 40% of the total net heat gain, respectively. The net heat loss of the day passed through the ground. Long -term measurements revealed the thermal buffering effect of the alley: The alley absorbed heat from the indoor space when the daily-averaged outdoor air temperature was between 25.6 degrees C and 31.1 degrees C. When the daily-averaged outdoor air temperature exceeded 31.1 degrees C, the alley reduced the indoor heat gain, with a stronger effect observed at higher outdoor temperatures. The rooms situated adjacent to the alley or with soil-adjacent structures met the standard for adaptive thermal comfort on up to 95% of the measurement days. It should be noted that the rooms unaffected by these factors only achieved the comfort standard on 58% of the days. This study also reveals the vulnerability of vernacular dwellings, and estimates the potential of improvement strategies including enhancing airtightness and nighttime ventilation.