If it's cold and damp, it survives better than hardboard does.
If it's too bad for plasterboard, you probably ought to improve it just for the sake of the tools you'll probably have out there too.
.... I'd use OSB.
Even OSB has moisture issues... ply's probably the best bet, if you don't mind the expense (or at least, it's 3x more expensive than OSB this side of the Pond, but there's probably a good reason for that :-)
Having said that, if it's ventilated and regularly heated then you can probably get away with OSB....
cheers
Jules
What's your joist spacing? Over here all the DIY sheds sell insulation with thick paper backing (about an inch excess either side of the insulated portion) for the various standard joist spacings found here - it's still a bit unweildy to do long runs from below, but it's not too difficult to staple into place. Reasonably cheap stuff, and a lot easier than trying to bugger about with unbacked! I'd be really surprised if it wasn't available in the UK too...
I don't think so, providing you're not relying on the profile bits being structural. Unless you're dealing with some kind of blown-in insulation, it sounds like a lot of effort to taper it, though.
cheers
Jules
I would hardly describe a block build workshop on a concrete base as 'temporary'. It also falls between the '15 sq m and 30 sq m floor area within 2m of a boundary' which makes the rules quite specific about what you can build without applying for planning and/or building regulations approval. As far as I know the regulations for outbuildings apply.
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