Adding insulation to single storey roof above bathroom.

[whatme]

I am redoing my bathroom in a house built in 1911.
The bathroom is at the back of the house on the groud floor and has a sloped, slate tiled roof above it. As the room gets very cold, I was going to rip down the old sloping ceiling to the joists, fit insulation between the joists, plus a new inline fan, then fit plasterboard back to the joists.
The roof is slate tile, roofing felt stuff, joists, then plasterboard at present.

Is there any particular way I should tackle this job? I was just going to fit celotex type insulation between the joists and make sure there is adequate ventilation between the insulation and felt so as to not get condensation by adding some air vents in the fascia outside. I was just going to use standard 12.5 mm plasterboard to reboard the ceiling. Is this ok, or should this have a moisture barrier incorporated?

Thanks for any advice.

 

[Alastairreid]

You will need either foil backed boards or install vcl.

And consider insulated plasterboards.