Hello, I am about to build a base for a greenhouse but I'm facing a small dilemma. One of the long sides will end up underground about 50-60 cm where I've excavated. The problem is that there will be just under 10 cm between the "earth wall" and the wall I'm building, so I thought that maybe I could coat/render the blocks before building with the same mortar I'm using? Is this doable or a bad idea? 
I then plan to lay down a type of dimpled membrane before backfilling.
I then plan to lay down a type of dimpled membrane before backfilling.