We are hoping to put our trusses up in a week or so for our new sugarhouse. We spec'd the trusses to space the ones around the smoke stack at 4' (we were told to double the trusses on either side).

With our 2x6 we ran metalbestos and it worked great. A you get larger that is a challenge. I like the idea of the double wall, insulation blanket, and you could add some cement board on spacers too.