I was wondering indeed, I am insulating with PIR plates, but they are coated with a vapor-closed layer and one is advised to tape all panels together with vaper-closed (aluminum) tape, so no air from inside the living area can get into the insulating layer, cool down and leave condensed water. As an additional precaution, the advice is to not insulate from the inside if a vapor-closed foil is on the outside of the wooden part of the roof (where the tiles are resting on), to ensure any vapor getting into the insulation after all, can vaporize away when temperatures rise.

All that being common knowledge, it does indeed seem weird that people were just spraying foam directly on the inside of their roofs as vapor will be deposited somewhere in the open cell structure of the foam, or against the inside of the wood/roof-tiles (as the air cools down, it can hold less and less water so that condenses).

Given all that knowledge, I'm still unable to close all the vapor bridges in my house, because I can't remove certain structures. So it is still a bit of a gamble here and there, where the moisture will pile up. I just try to get it to pile up in sites that are exposed to outside air.

I can't imaging everyone being as analytical and careful as me though. Indeed I sometime here people just pasting PIR plates or mineral-wool in place and leaving them like that, no vapor screens or anything. Time will tell what happens to these houses. Old houses were made to just have a lot of heat-transport through the structure, which prevents vapor deposition. Insulating any house should come with at least some thought about where the humid inside air will go (preferably outside through ventilation holes.)