All electrical insulators are dielectrics "without exception". Some are highly polarized like water (εr >80), or conjugated polymers (εr < 100k) , some are low like epoxy (4), transformer mineral oil (2), air, (1.0006) and even a vacuum (1.0000...) is still a dielectric.
Generally conductors radiate Electric Fields, not insulators, but if you have a sharp variation in the dielectric constant in a constant E Field, the voltages drops will be uneven like a small cap in series with a big one will have most of the charge or dynamic voltage on the smallest cap in series.
Charge build can be tribo-electric from friction of polarized insulators or bombarding accelerated particles in a high E field or simply by conduction from an external source. Double charge cloud layers form on the electrodes which is enhanced in "super-caps" with alternating layer of +/- charges.
You don't know the charge potential inside a series of insulators unless you can measure it or know how it is generated.
For example magnetic iron molecules on insulating silicate dust can generate higher potentials than the E field because they are not only partially conductive clouds but excited by high current at very low voltage, such as >1 or 2 Tesla inside laminated steel cores (if contaminated) and create partial discharge (PD) arcs in distribution transformers which breaks down oil into hydrogen like a slow time bomb.