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

You definitely need a vapor barrier, otherwise moisture from the room will condense in the insulation and destroy it. it goes on the side facing the living space, right before the insulation, so steam doesn't get inside. without it even good insulation will fall apart quick and start getting moldy. i see this all the time with clients when i come visit - everywhere people forgot about the vapor barrier, that's where the problems start.