Building a timber frame against a single skin exterior

[pertinky]

Hey,

I'm building an interior timber frame in the recess of the PVC window panels (window + lower in one piece) in my new house. The recess is about 150mm deep. I'm looking at lining it with foil backed insulation board and fixing 12.5mm plasterboard on top to create a windowsill.

I was wondering if I need to confirm to any regs when doing this; mainly:

- air gap between the exterior skin and the timber frame
- batten the insulation boards to stop them moving into the air gap
- vapour barrier between the insulation board and plasterboard

Any info would be greatly appreciated!

 

[pertinky]

Just realised this is probably better suited in the chippy section!

Can a mod move it for me? Or i'll re-post.