Mixing mediums, waterproofing

I have begun planning mine and my S/O's future home and have been exploring various building techniques, etc. Which we could use. We are both particularly drawn to the soft, organic appearance of cob, though our future home is going to be "hobbit hole" in nature, for lack of a better description. That is to say, we intend to have the roof and walls covered in a growing medium and grass.

Due to this, cob obviously is not the best choice, especially living in a rainy location (PNW).

We are considering mixing two methods in order to retain structural integrity, protect the cob from water, and keep the aesthetic we are seeking. These two methods would be cob and earth bags.

Based off of the anatomy of a green roof, would it be reasonable to assume we could use layers ordered as such to protect the home?

Grass and growing medium - filter fabric - drainage layer - water resistant membrane - Earth Bag - thin insulation - vapor barrier - cob

Provided I'm looking at this correctly, layering this way would protect the cob from moisture, right? Or is this simply not possible?

All suggestions welcome, warnings also welcome.

??? S/O  and PNW??

Those layers are reasonable to use in dry areas.
Where are you planning to build and what rainfall levels are we looking at dealing with?

Cob, earthbag and mudbrick need protection from rain.
Either by design or elimination of the material. Being a hobbit stye maybe pushing your luck.

With hobbit I believe you will need to provide amazing underground drains to ensure nothing wicks across to the house, for a start.
Then a water proof membrane will be required to extend as far as you think will be necessary to exclude moisture in the ground.
That may be 6M from the structure depending on the fall of the land.

Without trying to rain on your parade, can you explain a bit more about the land shape, which way its facing, why earth covered, details of the fall of the land.
Photos will help with markers to show slope etc.
From that we may be able to make suggestions that may allow the dream to occur.

The great news is that you are surrounded by cob builders, cob innovators and cob homes to investigate.

https://globalnews.ca/news/3917861/bc-cob-cottage-airbnb/
https://werewild.co/the-art-of-cob-building-with-our-friends-dreamweavers-collective/
https://www.cobcottage.com/pics

I mostly know the Canadian builds, there are many on Vancouver Islands and the Gulf Islands.

My only warnings based off my experience with cob (helping friends with a build at Emerald Earth, and building garden beds in the Yukon) is be very careful with any vapour barriers. Be they plastic sheeting or cementatious plasters, preventing water/vapour from flowing through your cob is simply disastrous. Be sure to look into clay plasters and lime plasters.

So  far as roofing systems go, I have no personal experience but check out these other threads where people talk about that kind of thing:
https://permies.com/t/102929/Living-Roof
https://permies.com/t/29399/Cob-Underground

John: S/O = Significant Other, life partner, sexy friends.
PNW = Pacific North West, the American's name for lower Columbia country, the old stomping ground of the Salish, Athabaskan and Nootka peoples and later the North West Company's David Thompson. IE Lower British Columbia