In exploring options for a cost effective R-40 exterior wall in a cold climate (Western Massachusetts), I am left scratching my head. As a residential designer, I have researched Building Science's website and their High R wall systems, where multiple layers of rigid insulation and a thermal break is their obvious choice. However, when clients want shingled siding and decorative trim, I want the sheathing on the exterior face of the rigid for consistent nailing. In addition, my structural engineer wants the sheathing directly nailed to the studs for shear (100 mph wind speed).

That being said, I was planning a double 2 x 4 wall filled with dens-pack cellulose. However, I just witnessed a hybrid wall, framed with 2 x 6's (or 2 x 8's) at two feet centers and rigid insulation spacers on the inside face each stud with horizontal interior nailers. This provided a thermal break, increased the bay depth and was cost effective. Any ideas on the pro's and con's of each?