[Greenbuilding] futz factor

[Alan Abrams]

<Uh uh. Sorry. Nope.
The above configuration is not allowed on the Greenbuilding list.>

while I take RT's excellent point about the optimum use of dimension lumber,
as well as acknowledging the issue of using petroleum based insulation, I'm
not at all deterred by the putative futziness of such a framing system.
another version of the same system could sacrifice R-2.5 and use stock 2x2's
(optimizing forest resources) and 1-1/2" foam panels--or--sacrifice even
greater r and just blow the entire 5" (or 7", using 2x6 studs) with
cellulose (addressing the PU factor).  

however, speaking as an old futz of a nail banger, the runner system is not
as futzie as it is made out to be, and more economical than a free standing
2x3 wall (and 2x3 studs being notoriously unstable), not only of lumber
required but of space consumed.  as far as bridging at the interstices,
you're talking about overlaps of 2.25 sq inches every 4 square feet (if
studs and runners are 24" oc), or 0.4% of the wall area (not counting
corners and plates).  

But the criticisms miss the main point of the system--to bring the thermal
bridging layer to the inner layers of the assembly, where it will not
compromise the secure installation of the cladding.  

In short, foam between the sheathing and the hard working elements of
siding, fenestration, and flashing is futziness of the first degree.

-AA