The relative permittivity of insulating materials is typically in the range of 2 to 10. This means that they are able to store a moderate amount of electrical energy when subjected to an electric field, but not as much as materials with higher relative permittivity values, such as conductors or semiconductors.

It's important to note that the relative permittivity of a material can vary depending on the frequency of the applied electric field, temperature, and other factors. For example, the relative permittivity of some materials may decrease at higher frequencies, while that of others may increase.

Here are some typical relative permittivity values for common insulating materials at room temperature and low frequencies:

- Polyethylene: 2.25

- Polypropylene: 2.2

- Polystyrene: 2.6

- Teflon (PTFE): 2.1

- Rubber: 2.5 - 3.5

- Glass: 4 - 8

- Ceramic: 5 - 20

These values are approximate and can vary depending on the specific composition of the material and its manufacturing process.