We researched extensively when we were building our underground house, but finally we determined to go with a compromise. The house is bermed on three walls, but we went with a metal roof and heavy spray foam insulation; R60 I think is what we have.
The thing about a green roof is weight, and that's what made our decisions for us. You have to allow for weight of the soil, but more importantly, for the weight of water, and that's where you need extensive structural design. Every time it rains, all that soil sucks up all that water, and it's incredibly heavy. I would never consider retrofitting an existing roof. That needs to be designed in from the get-go, and it will mean a lot of compromises in terms of space and how the house is built.
