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To reduce the heat transfer between outdoor and indoor environments, phase change materials (PCMs) were introduced to building envelopes. Research has shown that the position of PCMs is a key factor that influences the thermal behaviour of building envelopes containing PCMs. In the present paper, experiments were conducted to define the optimum PCM position to enhance the cooling and heating energy performance. PCMs with a melting temperature between 27 ℃ and 29 ℃ were encapsulated in spheres, arranged in an insulation layer. An electric heating film was adopted to simulate the solar radiation that was received by the outer surface of building envelopes. The thermal performance of the envelopes with PCM spheres was analysed in terms of temperature reduction and elevation, surface temperature amplitude and PCM phase transition temperature and time. With higher heating rate, the temperature reduction was higher for the PCM spheres at same positions. With same heating rate, the temperature reduction increased and then decreased when the PCM spheres moved from outdoor to indoor. The maximum temperature reduction using PCM spheres was 13.03 ℃ by 23.0％ when the solar radiation was 200 W/m2 and the maximum temperature elevation using PCM spheres was 7.8 ℃ by 56.7％. The allowable thermal resistance between PCMs and outdoor environment under different heating rate was given. The PCMs are recommended to install on the inward-most layer of the wall if they can complete their melting process.