This is a tough installation to complete effectively and you still have a 1/2" void that allow for convective air currents. You'll also have alot of waste with the custom cuts, which is very costly garbage.

Below is a comparison of two systems...Noting the different R-values, I think you'd be better off in installation time, $, and stress level to fill the bays with unfaced batts. and sheath the wall with 1/2" foil-faced polyiso before you sheetrock. The foil-face with taped seams and penetrations will give you the air and vapor barrier needed while greatly reducing the thermal bridging that the studs create.

A whole different approach is to sheath the exterior of your house with nail-base foam panels. This would require re-siding, so I'm not sure if this is even in your deck of cards.

OOPS...done quick and without great review, the titles of the charts are backwards. The cut foam scenario is on the right...

 

Below is a comparison of two systems...Noting the different R-values, I think you'd be better off in installation time, $, and stress level to fill the bays with unfaced batts. and sheath the wall with 1/2" foil-faced polyiso before you sheetrock. The foil-face with taped seams and penetrations will give you the air and vapor barrier needed while greatly reducing the thermal bridging that the studs create.

Is this technique agreeable? I know this is an old post but I've been considering this idea, the one in the original post and plenty of others for insulating.

 

can you not just stuff the wall with Fiberglass or Rockwool?

 

Of course that is an option, but air sealing seems to be kind of a big deal it seems and using these techniques makes the idea of air sealing much easier to fulfill. There is a video titled "Attic Insulation Done Wrong… and How to Do It Right!" (unable to post links) that might help illustrate this idea a bit.

 

Air seal with expando foam, THEN put batts of insulation in The way whitehouse suggested with unfaced & foil ontop might give you better insulation.

However, faced batts of insulation will give you plenty of insulation for likely a much cheaper price. If we are going the "poor mans" route, that would likely be the cheapest & work just fine.

I understand fully about insulation. Hell my walls have outer sheath, 3/4" foil board, tar paper, inner sheath, R-13 insulation, sheetrock & cedar slats.. Dunno the exact R value but I am sure it is high. Overkill but...I stay cool in summer & warm in winter. I tried the poor mans insulation route for under my floor but was more trouble than it was worth.