I am building a metal building type house. It will be a traditional metal building (metal I-beam, purlin and r-panel sheeting- muellerinc or metalmart makes these) with a 2x4 wood frame built into that metal building "shell". I was thinking about insulation and was wonder what combination of radaint barrier and batt would work best. My idea is radiant barrier on the metal outer walls and then R13 batt in the 2x4 walls. I could swap the two around, but think the first way would be better. I live in Texas and the heat off the metal outer walls is huge in the summer. Would the radiant barrier work better next to the hot metal side or in the 2x4 wood wall? In my idea the 'layers' would look like this- (out to in)- metal sheet>radiant barrier>~4" air gap (purlin)>2x4 wall with batt insulation>interior. What do you think? How would I apply a vapor barrier with this setup?