insulation above or below moisture barrier?

I have found tons of information on insulating earth sheltered roofs by placing rigid foam on the outside of the membrane--my case is a pond liner. However, I haven't seen a lot of living roofs with insulation on the inside of the membrane which is strange since that is how you build non-vented conventional roofs.

Anyone have any thoughts on which is better? why?

Specifics of my build. Strawbale house with a living roof. I was going to layer it rafters--decking--cushion- 9.5" of poly-iso--pond liner--growing medium. I have found a great resource for the poly iso which is why i am using it. It cannot get wet which is why it will go on the interior and I am assuming will prevent condensation from building with such a high r-value. Any thoughts?