No, I understood what you meant - install the plasterboard foil-side up against the joists, as I would have to do with a single-layer, foil-sided plasterboard installation.
The orientation of the foil isn't really an issue, if it's on the uppermost layer of a double-layer plasterboard installation.
Well, that's debatable. I've installed a loft conversion and the roof ventilation will have been compromised (although I have added the appropriate ridge vents and additional eaves vents). For a small extra outlay I can use foil-sided plasterboard to reduce the amount of moisture passing through the downstairs ceilings, thereby minimising the possibility of condensation.
Whilst the foil is not necessary directly beneath the loft conversion area, it isn't worth messing about just using foil-faced plasterboard around the ceiling perimeters. Might as well use foil-faced plasterboard throughout and done with it.
I also intend to install an additional layer of 3/8" foil-faced plasterboard onto the existing ceilings (foil-face up this time). This is primarily to recover the old ceilings that aren't in very good condition.