You should aim for 0.18W/m2K for a flat roof or a pitched roof where the insulation is installed at rafter level. Insulating a pitch at ceiling level rather than rafter level actually carries more stringent levels (0.16W/m2K)
The exact thickness of insulation will depend on the necessary product for your roof and this is also affected by the construction and materials!
However, Kingspan have a really good technical helpline that will help you work out exactly which (or their) products will be most suitable for your application, I suggest you check out their website for contact details and then fire off an e-mail or get on the phone.
As a shot from the hip, making presumptions for a pitched roof with tiles and no sarking board I'd say that you are looking at something in the order of 75mm K7 board between the rafters with 62mm K18 plasterboard fixed to the underside of rafters...
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