I think most codes now require 2x6 framing, because it's impossible to get the required R value in 2x4.  Usually, 2x6 has R19 glass, period.  You  can put up tyvek or tar paper in an attempt to keep the wind from blowing through the seams on your wall sheathing (and your glass insulation), but I don't think it's even required by code.  I put it on because I hadn't decide to foam at that stage. 

With just glass in a code compliant wall, you have R19 where there 1) isn't a stud 2) a sill or header, 3) an electrical box, 4) wiring), 5) plumbing, or 6) an air leak in your sheathing/wall wrap.  By leak, I mean that if the structure was turned upside down and used as a boat, would it leak?  Basically, you have R19 nowhere in your wall despite the printing on the insulation.  How much less depends on how complex your wall and how much your "hull" leaks. 

A properly foamed structure doesn't leak.  I have approx. 2" of foam (R14), leaving a stud bay about 3.5 inched deep.  That's followed by 3.5" of glass (R13), still the cheapest bang for the buck in glass insulation even though 2x4 exterior wall construction should be on the way out.  I loaded up when glass insulation went on sale at Home Depot. 

The stud sides have some foam on them to a depth of about 3 inches (a little overspray), so I've even reduced the heat loss through the studs a little bit.  I may actually have a little better than an R19 whole wall value.  On a windy cold night, that would be several times the effective R value of a code compliant glass insulated wall.  Put it this way, if given a chance to patch your "leaking boat" of a structure, which would you chose, foam or glass.  No fair using resin with that glass, but it's understandable that boat builders need to use resin.

There was a web site where one of the posters was a professional foamer in Idaho.  He did both residential and commercial (mostly potato sheds and frozen food storage).  He stopped doing residential because the code required such a ridiculous amount of form to compete against the phoney R value of glass.  But the potato farmers knew that a relatively thin even layer of foam over the entire storage shed worked better than stuffing pieces of 12 inch thick insulation haphazardly around the structure and taping it all together.  Plus, no condensation issues. 

Mark

Wall with sheathing, Tyvek and windows going up.