My understanding is that the berme in an Earthship is for thermal mass? I'm not sure that the two ideas are compatible due to the high insulation values of Straw. For what reason are you proposing a berme? A way around the breathability issue might be to put a stud work wall between the bales and berme, with some kind of tanking on the earth side, with good ventilation, might be vulnerable to vermin though, alternatively, providing there is really good ventilation in the interior side of the straw wall (with heat recovery) that might also do the trick. With a waterproof barrier on the outside of the bales the moisture of the internal air will want to breathe through the straw, gotta get some good air circulation in there, maybe hollow passive solar floor might do the trick.
