insulate sloped roof, which board between insulation and air gap?

[kanis]

I am going to furnish and insulate the attic in my 1 and ½ story house.

I plan to have a 30 mm ventilated air gap so that the insulation does not rest directly against the raw wood boards of the subroof.
I assume I should have some kind of board that marks the boundary between the air gap and the insulation, but is there a specific board that I should use?

 

[sixten88]

Either you take masonite or you stretch windy, but it can be difficult to do afterwards. Masonite is definitely recommended.

 

[Dzollero]

make the air gap 45mm, by attaching a 2x2 you will have an air gap and a stud to fasten the masonite with roofing nails.

Best regards, Alex