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Abstract In recent years, the quantity and proportion of heating defects have increased significantly with more composite insulators used in transmission lines and deterioration of early composite insulators. End heating is the main characteristic of this kind of defect in high‐temperature and high‐humidity areas. In this paper, microstructures of the housing material were observed through scanning electron microscopy and the key factors leading to temperature rise were studied through water absorption and power loss experiments. The results show that polarisation loss is the main cause of temperature rise and power loss increases exponentially with higher electric field intensity. The microporous structures of the housing material not only enhance the moisture absorption effect, but also affect the uniform distribution of the electric field. Due to the concentration of the local electric fields at high‐voltage end, the effect of porosity on temperature rise is more significant. By adjusting the shielding depth (i.e. installation height) of grading ring or adding a racetrack ring to restrain electric field intensity at high‐voltage end of composite insulator, the temperature rise in high‐humidity areas can be effectively reduced.