Is a vapour barrier required?

[mattshaw]

I am renovating a attick room.
The room has an existing latte and plaster ceiling. The roof is slate and does not have any felt.
I plan to batten over the existing plaster ceiling and add foil insulation, batten over (so an air gap either side) and then board and skim.

My question is, do I need a vapour barrier and if so where should it go, and what material? There is no barrier at the moment and everything seems ok.

 

[pinenot]

If the void created is suitably vented then a vapour barrier would not be required.
The original lath and plaster, whilst airtight, allows a certain amount of moisture wicking, meanwhile the tiled- no felt roof covering, provides a certain amount of air permutation i.e. venting, this is in a state of balance. Wherever insulation is added it alters the ability for internal moisture to wick trough the ceiling to be vented. You must consider if the end venting you describe will sufficiently replace this old venting technique of building?
You might be better to fix Plusboards (bonded insulation and plasterboard)
directly to the existing plaster face ensuring there's sufficient room ventilation to solve the implied problem...pinenot