typical cape cod style, story and a half, home located in barrie, ontario.  climate zone 6. early 50’s build. 

Roof to be re shingled shortly adding a ridge vent to be paired with already open soffits.

currently the home has a 3 attic system on the finished upper floor with decades old insulation and extremely poor air sealing. After much reading on GBA and researching different possibilities we feel the wise route would be to remove knee walls and insulate along the rafter line.  we cannot find a roofing company willing to install rigid foam above sheathing.

The rafters are only 2×4 (that measure 2 x 3 3/4) and brings us to the main challenge and purpose of this post.  

planned assembly as follows from sheathing:

-3/8″ site build ply baffles from soffit to ridge creating a 1 1/2 vent channel in each rafter bay. each foam sealed.
-3 1/2″ stone wool insulation between rafters that have been furred out with 2x2s to accomodate depth.
-2″ continuous xps over the furred out rafters as a break from thermal bridging. taped.
-1/2 drywall

with 2×4 rafters and the inability to insulate above sheathing our options seem limited and are looking for a best case without causing rot/condensation issues.  the one thing our roof has going for it is that the plumbing vent is the only penetration 🙂  gable ends will be simply batt filled and boarded as we plan on adding exterior rigid foam during a later siding replacment.

any problems seen with this or recommendations to improve?
will the collar ties penetrating into conditioned space be an issue?  should they be excluded from condition space, leaving the upper attic as is?