Hi Paul,
I have recently lined the saloon on my grp Twister. We used white plastic sofit board. It's only 9mm thick, light, easy to clean and very cheap. We cut it with a jigsaw with a fine toothed blade.
The holes in the picture are for deck fittings.
Kind regards
Mark
Yacht Sycorax 
The answer to that is both yes and no!
All GRP Twisters originally were built with an integral GRP deck head moulding. We have this and there is insulation between it and the deck moulding. (There are also some voids which show up after a cold night).
After a while the internal moulding was discontinued. I suspect due to both cost and the fiddle of fitting it. ‘Shrinkflation’ is not new!
Newer boats have their deck heads lined in a variety of ways depending upon the yard/owner/builder’s preference.
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