It's a timber framed barn basically wanting to build a timber insulated box inside an existing barn , minimal windows, one bed room /shower room. Everything else open plan, one wood burner in middle of house.
Was not sure best way to build box look at SIPs for ease of build, but cost does come in to it , or use 6x2 and osb board we have a full joiners shop at the farm so pre fab wouldn't be a problem