Attic Wall Insulation

I have an internal solid wall within my attic eaves which backs onto an upstairs bedroom. I would like to fit insulation to this wall by fixing battens to the wall, install 150mm roll Knauf insulation and then 60mm solid insulation board (xtratherm/etc).

Within the remainder of this attic is a stud wall backing onto the bedroom with 150mm insulation and I will fit 60mm solid insulation board.

As the wall is inside the attic do I need to install a membrane first?
On the photo the wall I want to insulate is to the right, with the house external wall on the left.