Slate roof insulation questions

[CortinaV8]

My 1930's semi still has the original slate roof, with what looks like horsehair type mortar on the underside (for whatever reason)

I am wanting to insulate the roof and plasterboard the walls out and use the space for storage.

I am wondering if it would be acceptable to staple lenths of tyvek between the rafters to form some kind of vapour barrier before I fit some kind of solid insulation.

 

[PaulJR]

vapour barrier needs to be on the warm side of the insulation - in your case it's insulation first.