what to cover the 200mm recycled glass wool in the eaves?

[Strawman]

3.6mm ply is half the price of 9mm OSB but will the ply warp too much? What might be the pros and cons? Never done it before, never likely to do it again!

Cheers hive mind

[Mr Punter]

I am not sure why it needs covering?

[Strawman]

Good point! I should have explained. Because we are using the loft space as a room so the room needs a ceiling, not just bare insulation material. What do you think? thanks