Do you need vapor barrier when insulating a roof from the inside?

Don't skimp on vapor barriers, thinking ventilation will solve everything. Here's an important thing people often miss: you need to lay the vapor barrier on the room side, not on the outside - that's how it blocks moisture from the air from getting into the insulation.

If you put it the other way around or forget about it altogether, then yeah, the insulation gets wet and stops working.

We had to redo the roof at our dacha - we made some mistakes with the waterproofing at first, then we had to sort it all out.