Kitchen Flat roof to pitch + Insulation

[Lexifir]

Hi guys. This is my first post here.

I've been poking around websites about insulating a pitched roof, but I always end up with loft conversions, or insulating a loft.

I had a flat roof on the kitchen, which is an extension to the original house. The flat roof leaked, and we decided to upgragde to a sloping roof. (Slopes from one high side, down to the window, rather than having a point and sloping both ways) The property is terraced, thus sloping one way only, the neighbours have a part pitch, part flat roof.

Can you guys tell me;

1. The insulation that I need to put in.... Can it go directly under membrane that was installed as part of the build, or does it have to have a vapor barrier plastic sheet installed first?
Currently have tiles on the roof, next layer down is this waxed fabric sheet (don't know correct name for it),

2. Is there a minimum R value that must be installed in a roof like this?
I currently have Foil backed Rockwool type insulation that has a depth of 150mm and an R value of 3.81. £50 a roll :cry: but I wanted decent stuff, I'm hoping I don't need more than that. (Roof inside will then be plasterboarded, skimmes and painted)

3. Does a vapor barrier add anything to the R value insulation of the roof? (I'm guessing not, but it's worth checking...)


Cheers guys, hope you can help me. :lol: