Insulating external walls in rooms with a curved ceiling?

[sean oc]

Hi all































































Rooms have a curved ceiling and looking at fitting insulated board or battening and insulating then boarding external walls































































But it will add 25-50mm depending what Iinternal depth is there any sort of trim, picture rail or omething to hide/blend it in? As the wall be set out from the curve



Thanks

 

[kp00110]

Taper top of board so it runs into the curve? or also board the curve and taper until it runs int0 flat part of ceiling? assume that slope is also uninsulated so would benefit from insulation?