Garage conversion internal wall insulation

[tvrulesme]

I'm looking in to converting a detached garage to a living space. The wall construction is 200mm lightweight blockwork rendered externally.

 

 

As the garage goes right up to the boundary my only option is internal wall insulation. I will need multiple things attached securely to the wall (toilet, boiler, sink etc) so I think insulated plasterboard is going to be too much of a pain to use. 

 

If I use internal studwork with foil backed PIR between studs does anyone have an idea of the best construction for this which will achieve a 0.28 u-value or lower to satisfy building regs?

 

I was thinking of 100mm studs with

• 75mm foil backed insulation pressed up to block work
• 25mm void
• OSB board pattress where strong fixings are required
• 300GA Vapour Barrier
• 12.5mm Gyproc Wallboard
• Plaster Skim

 

Questions are:

1. Does this sound sensible?
2. Would it be better to have insulation flush with front of studs?
3. Can I use the 25m void to run cables/pipes?