Having NO down between layers of fabric is a pretty clear case of underfill. But even this term is difficult to define with any accuracy. Very dry down will perform differently than damp down will perform diferently than wetter down, will perform differently than soaked down. I suggest that filling the bag might not be accurate between days to say. Some manufaturors are known to consistanly underfill bags and quilts slightly. Others to consistantly overfill slightly.
The correlation between loft and insulating value is fairly well established, refering to the EN system. Measurements for both fill type and and bags have been firly well done. Highly ignored was wind resistance of various fabrics, wet performance (though this was done partially here by Ryan et al), and a host of other factors, such as compression of down when multiple layers are used (or even when in thick artic bags, ie, -20F and below, touched on in the article.) Even a super light bivy will compress a bag somewhat, but this compression is often offset by the reduced air convenction...soo, we consider a bivy to be a bit warmer (even though it reduces loft by as much as half an inch.)
In less than perfect conditions, a bit of overfill is always needed to maintain some loft. It is like paint on a car. They use enough to cover it, then use more to cover against minor scratches. Bag manufacturers don't like that methode though. It means more (expensive) down stuffing. And, weight is always a factor. 2oz in an otherwise 16oz bag will often mean the difference between a purchase and looking elsewhere. As Ultralighters, this is one of the trade offs we make for weight controll vs being warm enough. Or, adding another layer of baffeling to stop internal convection. . .again expensive.
I think this was a good atricle, and, stands firm as a jumping off point for others examining some of the finer details of synthetics vs down and combination bags.
Well done, as I said before. Thanks Jerry!