The plan is to have a vapour barrier underneath the joists which will initially support the insulation and eventually be covered by plasterboard.
I was planning to use the DPC left over from the concreting but this is pretty heavyweight stuff and visualising trying to fix this to a ceiling is not giving me good vibes.
(1) What weight of plastic sheeting should I use for this vapour barrier? Web sites don't seem to be specific about vapour barrier the way they are about DPC.
(2) Is stapling O.K. as a method of fixing?
I saw http://www.screwfix.com/prods/48677/Power-Tools/Nailers-Staplers/Tacwise-140EL-Pro-15mm-Nailer-Stapler and wondered if this would make the fixing easier - fixing anything over your head and holding it up at the same time is likely to be entertaining for spectators but not participants. £25 does not seem a lot for a labour saving device. I have a manual stapler but this is possibly a bit cumbersome. Then again I might just want a shiny toy to make me feel better.
(3) Most vapour barrier seems to come in very large sheets. Is it better to cut the sheets down and lose some coverage through overlap but gain on ease of handling?
The basic plan is: Put up the joists at 400mm spacings. Vapour barrier under 4 joists. Fill 3 gaps with insulation. Fit 1m (coverage) wide metal roofing sheet. Proceed to the next 3 or 4 joists and repeat.
The reason for doing it one sheet at a time is to allow working up through the adjacent joists when fixing the metal roofing. The metal roofing is very light and so it doesn't seem advisable to plan to work from the top of the roof. To do this would require the construction of special crawling boards.
I have ordered Plastisol Hi30 from http://www.clarkesofwalsham.co.uk /. Has anyone else worked with this kind of sheeting?