Studwork for 75mm insulation

[mdnorth]

Hello this is a daft question and I think I'm going mad with advice however I am insulating an external garage single walled to use as on office it is attached to an adjacent garage up one wall assuming double brick I have been told 75mm kingspan for the walls sandwiched between semi permeable membrane and polythene sheet with external plasterboard should my studwork be 75mm thick ? Told you it was daft I have nearly come to blows over air gap's and the fact I could use 50mm kingspan and my architec is an idiot even though I don't have one

 

[Granada 1234]

You would be better putting 50 mm of rigid insulation between the studs and cross the stud wall with 25 mm of rigid insulatio.

 

[Granada 1234]

75mm studs are fine

 

[mdnorth]

Thanks for the replie's so are you saying make up the 75mm of solid insulation with 50mm and 25mm separate sheets crossed over ?

 

[Granada 1234]

Yes set your studs at 400 centres cut pieces of 50 Kingspan or equivalent and the fit a 25 mm sheet straight to the face of the timber and the use a 12.5 mm sound proof plasterboard (blue) if it's attached to next doors garage.
This way you save 50mm of space and stop any cold bridging from the studs work.