Hi, Dear user:

Thank you for your answer, I'm a little clear about same question.

But when I check the ASHRAE 90.1 2007, the description of the steel
framed walls' construction, in A3.3 ,has two gypsum boards one lays on
exterior , and anther lays on interior.

But you only show the interior one in your answer, is that a mistake or
I'm no clear with that?

And by the way, I saw you use the polystyrene when there is an
additional R-factor(I'm no very clear about the different between
-factor and -value, hope it's a right description) ,is that all right ?
Could I always use the polystyrene when I meet the additional R-factor?

Thank you for your help

Best hopes form Joey in China.