I'm getting a new house that's as described in my title wth two walls that are earth-bermed. I've seen a couple papers (Here's one
indicating that something with concrete on the inside and insulation on the outside with a R-value of about 15 would have a DRE of about 35) on how much less energy a home using a lot of interior thermal mass can use compared to traditional home designs with the same R-value.
Along those lines putting external insulation is a great idea, but none of the papers I've seen have looked at how beneficial it is to insulate something that's earth-bermed on the interior. Seeing as interior insulation is the worst option of those mentioned in the MIT paper, I don't think I would see a huge difference, but I'm still wondering if it would be worthwhile to stick some foam board between the masonry and drywall for a modest r-value increase.