[GrimJosef]

The problem is that the question would immediately be "how foamed ?". There's a compromise between putting as many air bubbles in as possible, in which case the beneficial dielectric properties of the air (particularly low dispersion and low high-frequency attenuation) predominate, and leaving plenty of polythene, in which case the beneficial electrical and mechanical properties of polythene (high breakdown strength, good rigidity to keep the inner centred, and good crush resistance to allow relatively tight bend radii) are retained. Since the wavelength of the RF signals will always be much much greater than the dimesnions of the foam structure the material will look simply like a weighted average of the permittivities of the two materials. You don't need to worry about the permeability by the way. All plastics and air are very close to being non-magnetic (relative permeability of 1).

Cheers,

GJ