[Greenbuilding] thermal bridging--letting it go

[Alan Abrams]

<I understand the benefits of minimizing thermal bridging>

I know I should let go of this and get on with my life, but this issue
plagues me....sigh!  

I understand all about stripes on vinyl siding, and linear patterns of snow
melt on roofs, and thermal imaging showing hot spots at framing members--but
where oh where is there some empirical evidence that this effect is not
caused for the most part by convection at ill fitting fiberglass batts?  As
Holmes says, "never overlook the obvious."

I also understand how an r value for wood can be substantially less that
that of insulation.  But to calculate heat loss through a 2x4 or 2x6 stud
*presumes* that heat wants to travel in a straight line, 3 1/2 or 5 1/2
inches from inside surface of stud to outside surface.

But in the case of tight fitting insulation--in particular, sprayed in place
foam--why doesn't at least some of the heat want to take a shortcut to the
left or right, a max distance of 3/4", into insulated void?  To a certain
extent, the insulation is buffering this tendency, but the effect would be
progressively less as the heat travels farther and farther away from the
interior.  

The point being, that there must be a certain point where the bridging
effect is substantially nullified by lateral dissipation of heat.  And I
suspect that with the use of icynene or similar insulation, it happens a lot
closer to 5 1/2" away from the interior than the common wisdom has it.

Alan Abrams AIBD