Tromano, all for simplified models, but not sure the conduction formula is relevant here. Isn't it primarily for calculating transfer of energy across solid, uniform non-living objects, like heating one side of a piece of aluminum and seeing what the other side's temp gets to? Living systems and their boundaries are all about convection, not conduction. Particles moving through fluids, whether air or water or something else. Think about how massive and dynamic the space is between skin and parka, then between inside and outside of parka, relative to the molecules that are carrying heat away from the body. Not a lot of direct conductance.
IMO, the convection diffusion formula is more what this needs: As you know, that's stochastic differential calc if I dimly recall where my college math slewed to a halt. But it's used a lot in fields where heat transfer and convection are the issues. Here's an example from architecture: http://www.bse.polyu.edu.hk/researchCentre/Fire_Engineering/summary_of_output/journal/IJAS/V1/p.68-79.pdf, and here's an example from exercise physiology: http://ehakem.com/index.php/IJoT/article/viewFile/231/254
Bob, I suspect research groups at companies that make down/syn fills/vapor barriors routinely do computer models that use the above. I doubt that North Face et al. think about it when they actually make a parka. They just follow the simplified guidelines that filter down from the research groups. But this stuff is real; when I'm wearing a parka, its performance is all about convection, not conductance. That's probably the reason that loft doesn't fully explain why one parka feels different than another.
Convection means heat transported by flowing air. Your shell is wind proof, down jackets are wind proof. When skiing in an normal outfit with a shell over an insulation layer, not much air is flowing through your insulation. That means that convection is not an important heat transfer mechanism inside your insulation layer.
That leaves only conduction and radiation of heat. Heat transfer in an insulator (in heat or electricity), is all about reducing conduction.