The original thread covered a lot of topics. This new thread is just intended to just focus on why the Chouinard, used as an inner bag, was so effective at preventing wetting of the down bag in winter conditions. Mike Clelland, Dewey Riesterer, and Mike Martin all provided valuable insights that aided me in my hypothesis which is: The Chouinard acted as a buffered vapor barrier primarily because of the coating on the insulation used.
The insulation that sounded like Thermolite was probably Thermore T37. It has a coating on the insulation that transitions to a vapor barrier between 50F and ~0F. The reason that insulation was so warm for its thickness is that it has a formulation very similar to Thinsulate without the coating.
Thermore T37 was introduced to the marketplace in 1985. Production around the world was handled through a web of joint ventures and technology licensing arrangements, which include Hobbs Bonded Fibers, Du Pont Co. and Mitsui & Co. and ties with companies in Bangkok, Korea and Bangladesh. Du Pont Co owns the Thermolite trade mark and they have used this trademark for the Thermore product during this period. Thermore now has about 100 different types of insulations.
Both Thermore T37 and Thinsulate used very thin olefin rather than polyester for the fine fibers. The thinnest version of each is about 100 g/m2, is .3” thick, and an R-Value of 1.14. The T37 coating was primarily intended to act like the Primaloft scrim so that it could be used with fabrics that weren’t down proof. It is comprised of unique formulation porous cross-linked urethane that is calendared to be very thin. This thin coating has pores that close as the temperature drops and open as the temperature increases. When used in very cold weather it becomes the equivalent of a vapor barrier.
Silnylon in combination with Primaloft One for a buffer and added warmth will accomplish the same thing with less weight and more efficiency when wet. Any light synthetic quilt or bag inside a down bag will not work the same way.
