[Greenbuilding] rigid foam inside Larsen Truss?

[RWT]

 Reuben;
In the Olde Days before sim programs existed (ie before Win-Doze)  if one didn't want to bother graphically determining the dew point for a double stud wall assembly, the rule of thumb was to place the thing behaving as a vapour retarder/air barrier membrane within the first 1/3 (from inside -> out) of the total R-value of the wall assembly, in primarily heating climates like mine (Ottawa Ontario Canada). 
 For  11" wide double-stud wall that used the same insulation in the three layers (ie 2x4 interior  bearing wall + 3.5" cavity + 2x4 exterior curtain wall) this meant that the VDR/AB membrane could be placed on the outside of the bearing wall (usually backed up by structural sheathing) and the outside 2/3 of the insulation placed in the walls before the walls were tilted up  ... meaning that the walls were very well insulated and air-sealed before the interior was sheathed (same procedure for ceiling) so that the interior was toasty-warm in the middle of winter before any HVAC was installed.  
 It also meant that services could be installed in the bearing wall cavity without having concerns (nightmares) about sub-trades compromising the integrity of the VDR/AB membrane.
    On Thursday, January 4, 2018, 11:34:48 AM EST, Reuben Deumling <9watts at gmail.com> wrote:  
 
 I have a line on 3-1/2" rigid foam panels for pretty cheap. My 1894 house has (of course) 2x4 walls into which I've blown dense pack cellulose. For stage 2, I was going to add a Larsen truss (another 2x4 wall with a big cavity between inner and outer walls) and blow cellulose into the rest for an eventual 11" thick wall. But with these panels showing up I'm wondering if there are reasons not to place these as the middle layer of a cellulose sandwich? Seems like a quick and in this instance cheap way to get a lot of 'r'. But I also don't want to do anything stupid when it comes to moisture. 

Thoughts?
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