ok, so i'm not going to convert it to living space, but i want to make it a bit warmer so it's all about making it air tight and wind proof and insluated and shit like that so i'm after the basics

so can i just whack in some foam insulation inbetween the rafters and then cover it up with plaster board?

what do i do about strenghtening the floor? just screw some thicker joists in?

etc

any other suggestions other than "get a builder in"