To achieve circa 0.1W/m2K for my attic space the architect spec'd 500mm of Rockwool insulation.  Having upgraded to attic trusses instead of roof trusses, the proposal was to have 200mm of Xtratherm and a 25mm Xtratherm insulated plasterboard. However this is quite expensive.   Question: is spray foam infilled between first floor ceiling joists likely to be even more expensive?  (The specs suggest the spray foam would not need the insulated plasterboad)