Hi

I removed part of my ceiling on our mezzanine that was too low for me (only about 1.70m high). I asked a pro to remove some of the horizontal joists that were just above the plasterboard and add new ones higher up (the light coloured wood). The mezannine is just under the roof and the walls are slopped.

I now want to close this space and I have a few insulation questions. The fiberglass insulation is about 14cm thick. The joists are 17cm deep. Behind the fiberglass is a plastic sheeting (I don't think it is breathable). The fiberglass has just been pushed up against it. There seems to be no ventilation at the top or bottom of the roof space on the inside of the plastic sheet. There are vents in the tiles on the outside of the building. The fiberglass goes straight up to the top. I've attached some photos for your reference.

I'd like to plasterboard the sloping walls up to the new horizontal joists and then plasterboard the ceiling horizontally attaching to the new joists.

1) Do I need to put a vapour barrier on this small section of the roofing? There is no vapour barrier on the lower parts of the sloped walls. 2) Is the lack of a breathable membrane likely to cause any condensation issues ? I don't see any condensation on the plastic sheeting. We live in western Europe. 3) is it worth adding an extra layer of insulation of a couple of centimetres above the joists to reduce thermal bridging?

Thanks.