Wow, thanks for all the contributions. I will attempt to guide this tread to some sort of final conclusion(s). I hereby volunteer to be put in front of the firing squad:
At first glance, the value of the ‘overfill debate’ might seem a bit exaggerated: the added value of a smaller – read: lighter - shell vs. total down weight is more or less neutralized by the lower clo/ounce value of down in an overfill context. For example – and now I am cutting some corners for the purpose of simplicity - a down bag with a shell weight of 200g and a 100% overfill, resulting in 400g down in total, will more or less have the same insulation value of a bag with a shell weight of 300g – read: bigger baffles – with a non-overfill of 300g down in total.
Again, I am cutting some corners – the values in my example are obviously arbitrarily chosen without any scientific backup.
As there seems no difference in total weight vs. insulation, one might conclude that a non-overfill scenario is better for secondary reasons (e.g. quicker drying).
In this analysis, again for the sake of simplicy, I make abstraction of a small amounts of overfill, aimed at compensation humidity and filling up the last cubic inch of my baffles.
This is however a conclusion drawn to hastily, at least from the perspective of a lightweight backpacker. Let’s go back to Richard’s graph:
It shows that overfill will – up to a certain amount – result in a LINEAR increase of clo (and not a degressive increase). Assuming that a normal fill corresponds exactly with the volume of the baffles and making abstraction of external variables such as humidity, there will thus be no difference in clo/ounce down between e.g. a 30% overfill or a 80% overfill.
Or to put it differently (and more sharper): even an overfill of 10% will result in a not-negligible decrease in clo/ounce down (again, assuming that a normal fill corresponds exactly with the volume of the baffles and making abstraction of external variables such as humidity. In real world situations, 10% overfill might still result not result in a ‘clo/ounce penalty’ for the reasons already mentioned earlier in this thread).
If my analysis is correct, then my conclusion, from the perspective of someone how wants maximum weight efficiency, would be:
- The question a lightweight backpacker should not primarily be: with HOW MUCH should I overfill, but rather: should I overfill at all or not at all.
- And if you decide to go for an overfill, rather choose the SMALLEST shell available and overfill it AS MUCH AS possible, up to the amount corresponding with the desired minimum temperature rating. According to Richard’s own measurements, one can overfill up to at least 2,5x without breaking the linear increase in clo/ounce down.
Or to put it bluntly: from a lightweight backpacker’s perspective: it is an all or nothing question. Again, in my (proposition of a) conclusion, I am of course assuming that the baffle volume corresponds exactly with a normal fill and I am making abstraction of external variables (e.g. humidity), just for the sake of maintaining clarity with regard to the main issue of this thread. Bear that in mind before pulling the trigger.