New ceiling going up

[mylittlehouse]

Hello all
My old 1900 lathe and plater ceiling is now down and the original Cornice is still in place.
I will put some Batterns up to screw plasterboards into.
So the cornice gap between the edge and the joists is 30mm

So what size bords should I put up 2x12.5mm over each other?
I looked into insulation board but cannot find the right sizes?

Please help

Thanks

 

[1john]

If you double board it that will give you 25mm, so perhaps a tight coat of bonding over the board and then skim will get you to 30mm.