Insulating a unlined pitched roof above a bay window

[gull]

Hi, I've recently bought an old house with bay windows on the front.

I'm getting the room plastered and as the ceiling above the bays are in poor condition, taking the plasterboard down. Above them is just fresh air, battens and the original roof tiles - no lining, no ventilation to the exterior.

I want to insulate them so wondered in layman's terms if someone could spell out exactly what I need to do as I'm getting confused about the amount of ventilation I need and the types of membranes and vapour layers, types of insulation, how to attach it etc.

I thought I could just add a breathable membrane underneath the tiles, attached it to the battens then stuff some kind of insulation between the joists or hold in place with wooden battens?

Then put new plaster board back and job done?

thanks in advance

 

[noseall]

Is there room for quilt type insulation whilst maintaining some kind of eaves ventilation?

If not it may be wiser to use rigid foam board insulation.