Hello all.
Here is an interesting holiday question, with some background first:
We are in the (slow) process of building a  20’x24’ stand alone addition to our small house, which will be connected by a ‘breezeway’ (which sounds basic but will likely now contain a bathroom and HRV unit).  This build is located around 60degN in the Yukon.
The design is rather unique in that the vapour barrier (Protecto self adhesive wrap) is on the exterior of the 1/2” plywood sheathing, this will be followed by 2 layers (8” total) of Powerwool Rigidboard 80 (R 33.6).  (Also, there will be no conventional vapour barrier (poly) on the interior walls).  This amounts to plenty of insulation for the build and leaves the interior stud bays empty, as we do not specifically require insulation within the stud bays.  The reason to leave them empty or minimize insulating from the inside is to allow the sheathing to remain warm and therefore avoid any potential condensation to form on the sheathing.  Also, should any moisture develop in the walls, drying can occur either: a) from the exterior face of the Protecto out (to exterior) or b) from backside (interior) of the Protecto in (to inside).  The ceiling does have poly vapour barrier on the inside with 2×4 strapping, the attic will contain at minimum 2’ of blown fiberglass.

So, here is the question: I have options, of doing nothing by leaving the stud bays empty and covering with drywall or some other finish, or completely fill the stud bays with cheaply sourced 2×4 lumber cut offs to add thermal mass to the structure, or something in between.  Seems like there would be no harm doing either, the opportunity exists to try something interesting, any thoughts or advice?

I will be running through the thermal mass calculations to see if the 4080 square feet of 2×4 walls can accommodate an amount of timber to be useful in tempering temperature swings, make the inside more comfortable, and save a little on heating costs.  

AST.