Confused about insulation and vcls

[crispy_wafer]

Probably done myself no good whatsoever and have got myself a little confused reading various threads about roof buildups, condensation, vcl's.

 

As was built by the contractor my roof buildup is as follows

 

Dormer roof

Slates

batten

cromar vent 3 membrane

225mm deep rafters.

 

Now I've started filling the rafters from the wallplate all the way to ridge with PIR leaving as recommended a ventilation gap.  

 

On the underside where the PIR is flush with the rafters I've foam filled where necessary  and taped over joints, and taped over the rafters to the next board.  Where the insulation meets at the ridge, I've joined the best I can, foamed and taped over.  I'm due to add a layer of insulation, and plasterboard to the underside of the rafters.  

 

The inner layer of insulation I was going to fit from wall plate level to ridge level again.  Plasterboard will be following the room outline.

 

I'll be attempting to fit an avcl at the wallplate junctions and where the first and last truss meet the blockwork in an attempt at mitigating draughts, and leakage.  Is it worth installing the vcl all over the insulation.  Are there any other obvious problems with my approach?

 

Thanks in advance.

 

[Mr Punter]

You can either tape the final layer of insulation with foil tape or add polythene over the lot.  The polythene is a bit more cumbersome and it is not so simple to hold in place with staples.  Double sided tape may be better.

[crispy_wafer]

Next question regards the installation/overlaying of the final layer of insulation and plasterboard.  There's going to be no purpose built service void, as I'll use the roof space and angled voids for services.

 

I've got 60mm PIR + PB to go up.  What fixings would/could I use to fix the though the insulation to the timbers, then the plasterboard through the insulation to the timbers?  Or is there a better way?