Impassable, impenetrable, impervious, impermeable are comparable when they mean not allowing passage through.
Impassable applies chiefly to stretches of land or water which cannot be passed over or crossed because of some insuperable difficulty or obstruction.
Impenetrable applies chiefly to something which is so dense or so thick that not even the thinnest shaft (as of light or air) can find its way through.
Often, however, impenetrable is preferred to impassable when implying an exceedingly dense growth that prevents passage <an impenetrable thicket> <an impenetrable forest>
Impervious, which implies impenetrability, applies basically to substances or materials which have been so finished or treated as to make them impenetrable (as to air, water, or sound waves).
When applied to persons or their minds or hearts, impervious usually implies complete resistance to anything that would affect them for better or worse.
Impermeable implies impenetrability, whether natural or artificially acquired, by a liquid or a gas and incapacity for becoming soaked or permeated; the term applies chiefly to substances (as some clays) which do not absorb water, to cloths treated so as to be rainproof, or to materials which do not admit the passage of air, light, gas, or water.