ᐅ Insulating a Mortar-Covered Roof

Hello,
I have a question for you.
As the title suggests, I have a roof with mortar between the tiles. Now I want to insulate it. My original plan was to install a temporary underlay membrane between the rafters and extend it outwards at the last row of tiles.
On the inside, I wanted to use a climate membrane to allow moisture to pass into the interior.
A roofer friend of mine said this is not necessary because the roof is mortared. He suggests adding a thick layer of insulation up to the tiles, then installing the climate membrane, and that would be sufficient. (Before that, he plans to repair any damage in the mortar.) Now I have several concerns. On one hand, if moisture gets into the insulation, wouldn’t it have no way to dry out? Or can moisture also escape through the tiles to the outside?
Would it be better to create ventilation behind the tiles? And if so, what would that look like?
What do you think would be the best approach?

I wanted to add a photo, but somehow I can no longer edit the post. So here is the image: the tensioned membrane you see was left by the previous owner; it wasn’t extended outward at the bottom. The original build-up, from outside to inside, was the visible membrane, wood fiberboard, insulation (very thin, maybe 10cm (4 inches)), then 3cm (1 inch) of polystyrene, followed by OSB. This was only on one half of the roof.