Thinking about my workshop roof, if I put galvanised box profile straight onto steel purlins, with a air gap under, then insulation, then a vapour barrier (e.g. Visqueen), then a ceiling of plasterboard or what have you, will there be condensation problems in the roof space?
I was thinking of putting 11mm OSB under the galvanised and over the steel roof framework, but I'm not sure that adds anything but weight and expense.
So, order of components top (exterior) to bottom (interior):
Box profile screwed to.... Steel angle purlins.
An air gap.
Insulation layer (Rockwool, foam sheets, whatever). Vapour barrier. Plasterboard.
Ventilation through each gable end is possible. Observations?