Is there any good way of insulating a cathedral ceiling? I am think of building some loft rooms that would make use of a cathedral ceiling to make the height of the room right for the lofts. I realize that this is probably not the most practical way to build but I am trying to squeeze out room in limited spaces so came up with the idea of doing it that way. Building codes dictate I can not do a full second story so using the roof as a loft is the work around.

Why not consult an Architect about the options/possibilities ?

I'd finish the inside ceiling with Sheetrock (gypsum board) flush with the existing rafters and sandwich installation with paper/foil on both sides, at least R6, between it and the roof. You will most likely have to add additional rafters on 24' (60cm) centers to attach the Sheetrock to.

Why not consult an Architect about the options/possibilities ?

How much of a DIY project would this be then?

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You might want to consider using the Thermaline gypsum: http://www.gyproc.co.th/en/gyproc-thermaline/