foam layer characteristics reg

Nobody can. There are many foam materials with many features
Foam is used for the advantage that bubbles are filling the volume, with a minimum amount of material that can have a high permittivity and high loss.
To answer your question you must measure a particular material.

untreated foams are mostly air, so the dielectric constant is near 1. However, if they are loaded with something, like carbon, etc, all bets are off.