ᐅ Insulation of the attic in a timber-framed house

Hello everyone,
I would like to insulate the attic of my timber-framed house. The current structure is as follows: tiles - battens - underlay membrane - full wooden sheathing. Now I want to continue with insulation wool. I have heard that some people install another underlay membrane on top of the wooden sheathing before adding insulation. Is this useful or what purpose would that serve? Thank you very much for your answers.

Pictures say more than 58 words.
You do have a breathable membrane, right?

Yes, the underlayment membrane is installed between the battens and the sheathing. I just wanted to know if an additional membrane should be applied on top of the sheathing, because someone once told me that...

Here is a picture for better understanding. As mentioned before, there is already an underlay membrane installed before the formwork. I would appreciate any helpful tips 🙂

If it is a tongue and groove profile, I would also leave 3cm (1¼ inches) of space and then stretch another strip. Then the boards will have some air.

The airtight barrier on the inside is much more complex and important here.

Hello, no, these are just regular formwork boards without tongue and groove. Great, then I can proceed with the insulation now, thank you very much.