Insulation of the attic in a timber-framed house

Hello everyone. I would like to insulate my attic in the timber-framed house. The current structure is as follows: bricks-battens-underlay-complete wooden sheathing. Now I wanted to continue with insulation wool. I have heard that some people put another underlay on the wooden sheathing beforehand. Is that reasonable or what purpose should that serve? Thank you very much for your answers

Pictures say more than 58 words.
Do you have an underlay membrane?

Yes, the underlayment is between the battens and the formwork. I just wanted to know if an additional one should be applied on the formwork because someone once told me that....

Here is a picture for better understanding. As already mentioned, there is already an underlayment beneath the formwork. I would be grateful for helpful tips :)

If the groove is a tongue, I would also leave 3 cm of space and then stretch another strip. The boards then don’t have any space. Inside, the [dichte sperre] is considerably more complicated and important here.

Hello no, they are completely normal formwork boards without tongue and groove. Great, then I can continue with the insulation now, thank you very much.