Nowadays, with the tools we have available to us, it seems super obvious that using total fill weight to assess warmth of down items is highly inaccurate. So why do we do it? Like most things like this, I think it was born out of a different time when maybe it made more sense than it does today. A lot of this is me speculating, but this is my interpretation.
I’m pretty certain that the practice of using total fill weight as a warmth metric predates fill power testing. I think in those days, builders were essentially just stuffing down into chambers until it looked and felt “right”. I believe that, in this time, measured loft was likely the most used metric for warmth. At some point we realized that we could stuff more and more down into a chamber, which would make it warmer and warmer, but without a corresponding increase in measured loft. So that warmth could vary, without showing up in the primary warmth metric. Therefore, we desired a quantity metric. We wanted to know, not only the measured loft, but the amount of down being used in that loft space. Since they were just stuffing chambers without any calculations, the only available quantity measurement to know was the total amount. Back in these times, it is also likely that there was less variability between items, both in types and dimensions within those types. If there is little variation in type and dimension, then the total amount could serve as a ballpark comparison.
At some point, the garment industry advanced and started testing down fill power. Knowing the down’s ability to fill a volume meant that there was no more guesswork. Chamber volume could be measured and the correct volume of fill could be inserted into it. This meant that each chamber could be filled consistently to the same density and amount as the others, and each item would be consistent day to day. Having a baseline specification aided in innovation since builders could adjust specs around and record what worked well and what didn’t.
Of course, in modern times, the types of items, the features on those items, and the dimensional variability within those types is all over the place. This has made total fill weight almost completely irrelevant as a warmth metric. Knowing a total amount, without knowing the volume it’s spread into doesn’t provide any relevant information. The original intention of wanting to know, not only the measured loft, but the amount of fill, still holds true. It’s just that the total amount is no longer relevant in the variable items. The fortunate thing is that, since the advent of fill power testing and filling chambers with a formula, builders know the quantity of fill for any given area. The variability of item types, features, fill power, dimensions no longer makes any difference if you know the fill quantity per area or per chamber. If you know the amount of fill being used, per area, and the loft space it occupies, you have a very clear picture of R-value…..as clear as it could be without actual R-value testing. We just have yet to understand this, as a community, and make a shift towards it. I think this shift could only happen if consumers understand it and ask for it. Otherwise, there really isn’t any incentive for manufacturers to change anything.