Richard,
Thanks for the very clear explanation.
One thing I might argue is that by "maximum efficiency" on this board we usually want to maximize versus weight, not thickness. Compare the points marked "maximum loft" (.32 btu in/hr ft^2 deg F for .25 lb/ft^3) and "maximum efficiency" (.25 btu in/hr ft^2 deg F for .5 lb/ft^3) on your graph. For an equal area and total weight of down, the "maximum loft" will have twice the thickness, and so the heat transfer through the "maximum loft" density will actually be less than that of the "maximum efficiency" density (.32/2 < .25). So to minimize weight (amount of down) we would want a bag or jacket stuffed at what you call "maximum loft" density, not "maximum efficiency".
If we use the "maximum loft" numbers instead of "maximum efficiency", then assuming the ratio is .32/.25 for conductivities, we get something like 0.8 m^2 K/W for 1" of down. This is close to the value you calculated for primaloft one, indicating about similar warmth:loft ratio for down and Primaloft one.
To be thorough I note that thruhiker.com lists 6 oz of primaloft _sport_ having loft of 1.2" and clo of .74. This would give .57 m^2 K/W for 1" thickness. Thruhiker's Climashield XP specs yield .53 m^2 K/W for 1" thickness. Both are less than that of either density of down.
After all these comparisons, it seems that if you use the test numbers you supplied for the conductivity of down, the down will be less lofty or at most a similar loft to equal warmth synthetic. However, AYCE's claim is based on real world experience with his garments  making me wonder if something is amiss with the calculations.
For example, if we take the following of Ayce's comments from that thread:
"Or compare two of my own kits: the Maxima and the Whitney. These are equivalent jackets, the Maxima being the synthetic version of the Whitney. They are both about the same warmth too: over three years use with the Whitney and two for the Maxima they’re comfortable for me in the 30’s just sitting around. The Maxima has a 3.0 oz basis weight layer of PL Sport with a single layer loft of 0.6” (clo: 3.0 * 0.74=2.2) while the Whitney fully lofted has on average about 1.5” of single layer loft. In other words, the synthetic jacket is as warm as the twice as lofty down jacket."
then it means that 1.5" of down loft has clo of 2.2, or for 1" of down 1.5 clo or .23 m^2 K/W  a factor of 4 or 5 different from your calculation! I wonder what is causing such a large discrepancy...
Edited by huppy on 09/25/2007 09:21:06 MDT.
