Insulating an oak frame building or extension is something worth considering as it has a big impact on the energy efficiency of the home. There are different approaches you can take too. Let’s say you are creating a vaulted roof on an extension with an 8×2 rafter. Then you can either insulate within the structure or above it. If you go for between the rafters, then 6 inch insulation would work then over board inside with 1 inch board before plaster boarding.
Alternatively, you can go for a warm roof where the insulation is outside the structure. In these designs, you can go as thick as you want but you do want to look at the visual side of things. If it is too thick and very visual, it might spoil the aesthetics of the home. generally, air flow gaps need to be 50mm to ensure they meet building regulations and hit current recommended standards.