Rubber is a kind of materials making insulators, and it is silicone rubber, so the answer of ‘Is rubber an insulator of electricity’ is yes.

You will understand that is rubber an insulator of electricity through the following rubber properties:
Injected housing moulded directly to the rod and the fittings forming an impenetrable protective housing.
Housing: Thickness Conforms to International Standard
Tests on Silicone Rubber: Tracking & Erosion Test, Volume Resistivity, Tensile and Tear Strength, Hardness (Shore A), U.V and Weathering Test and Dielectric Strength.
Silicone rubber is the fastest-growing, most dominant polymeric insulation material for high voltage products.

Orient uses only high quality, high temperature vulcanized (HTV) silicone rubber to ensure the highest possible durability of sheds, as well as outstanding tracking and erosion resistance. Now, I think you have known is rubber an insulator of electricity.