For the outdoor insulation of high voltage transmission lines, insulating materials made of high-temperature vulcanized silicone rubber (HTV-SiR) are used all over the world. To enhance the performance of these base polymers, fillers of various sizes, concentrations and dimensions are added. In this study, four different types of HTV-SiR materials, one unfilled and three reinforced with silica of micro/nano size were used. Followed by preparation of the samples, aging was performed by placing them in a specially designed weathering chamber with various stresses and bipolar DC voltage for 5000 h. To diagnose integrity of the aged materials, different types of measurements based on hydrophobicity classification, leakage current, mechanical analysis, thermal tests, Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) were conducted. Results of the hydrophobicity classification revealed S3 to be most hydrophobic having HC2 class under the influence of bipolar DC voltage, whereas, sample S1 was the most hydrophilic resulting in HC4 and HC5 under negative and positive DC voltages, respectively. Similarly, the lowest leakage currents of 5.56 μA and 5.81 μA were recorded for sample S3 after being aged under negative DC and positive DC voltages, respectively. The %age decrease in tensile strength recorded for samples S1, S2, S3 and S4 was 32.3, 25.32, 23.56 and 27.12, respectively, under the positive DC voltage. Thermogravimetric analysis exhibited the least decrease of %yield from 49.3% to 48.9% and 48.4% for sample S3 under negative and positive DC voltages, respectively. Additionally, according to FTIR spectroscopic investigation, hybrid composite S3 kept the highest intactness in siloxane backbone (Si–O–Si) linkages, with a drop in its peak of 37% for positive DC and 11.2% for negative DC. In contrast to the co-filled composites S2 and S3 with improved surface morphology, samples S1 and S4 indicated voids, cracks, increased roughness and structural damages.