DIY Doctor UK DIY Forums

We want to insulate our lean to so as to make it a liveable room.
Can someone tell me what the roof waterproofing membrane is in the attached picture and is it ok to put sheet insulation under it? In addition, what should I do with regards to ventilation as I hear some people say I need to have a 25mm gap between the membrane and the sheet insulation but do I need to do anything else?
Thanks for any advice given.

Apologies, attached is the image

Batt blanket insulation is perhaps the most common type of roof and attic insulation. Though mineral wool insulation batts make a comeback, these rolls are typically made of fibreglass and are among the most affordable insulation materials.