homemade SIP's?

This is a detail of what I was thinking in my original post. I never really thought it through. One thought led to another, wanted to know if there would be a problem with all the vapor barriers. I read John's post's about them and believe I have a better understanding for the first time. Thanks John.

SIP's seem to perform well. I would guess because they would be easy to make air tight. That seems to be the key for vapor movement, and also comfort.

Why 4"x4"s, I like the proportion I think they would make the interior seem larger and would help to give a feeling of being inside/outside. I guess that the posts are under sized went by the old rule of thumb that if you can use 2"x4" 24" OC you can use 4"x4" 48" OC. Maybe I made that rule up? don't think that I will do this anyway.