Open spaces in Hong Kong are in short supply and they are often underused due to the adverse climate, especially in hot and humid summer. This is a missed opportunity that can be otherwise realized to promote health and social interactions for local communities. The high density urban environment makes the condition worse by raising the urban heat island effect and leaving planners with fewer mitigation options. This study aims to test the hypotheses that an unfavourable thermal environment disrupts the use of outdoor open spaces; if yes, whether such disruptions differ by age groups. On-site measurement and computer simulations were conducted in three open spaces in public housing estates in Ngau Tau Kok, Hong Kong. Thermal conditions were assessed using the Universal Thermal Climate Index. Occupant activities were recorded, together with a questionnaire survey. Results showed that an open space purposefully designed for breeze and shading was 2.0°C cooler in Universal Thermal Climate Index compared with the other two. It attracted more optional/social activities, higher frequency of visits, and longer duration of stay. The elderly activities were more susceptible to disruptions from heat stress compared with younger groups. Elderly activities largely diminish when ambient thermal environment exceed 39°C in Universal Thermal Climate Index. Findings have implications to design and retrofitting of open spaces in order to maximize their use.