Upvc clad porch timber framing questions

[otbrett]

I have an old rotten wooden clad porch that has a concrete roof (ex council house) and wooden door. One wall is brick and supports the roof and is connected to the neighbours porch. There is a post in the opposite corner to support the roof.

I am looking to rip out the rotten cladding and post and replace it with a timber framed, upvc clad version with a upvc door. What size timber framing is recommended for it? I am planning on cladding the exterior with upvc cladding and plasterboard the interior with insulation between. I presume I need to put a vapour barrier under the cladding. What insulation is best in this use? Cellotex or rock wool?
Thanks

 

[23vc]

Not sure if it needs to satisfy building regs but something like 5x2 framing with 100mm PIR in between, cladding on battens on tyvek type membrane on outside, vapour barrier and plasterboard on the inside, probably wouldn’t be a million miles off

 

[otbrett]

Not sure if it needs to satisfy building regs but something like 5x2 framing with 100mm PIR in between, cladding on battens on tyvek type membrane on outside, vapour barrier and plasterboard on the inside, probably wouldn’t be a million miles off

This is my thinking. I contacted the door company and they have said I would need 70mm width for the door to fit. I was going to go 100mm width and fit it flush to the front.