Increasing fp and dwindling returns
I was thinking about my down top and under quilts today and the effectiveness of the 5.5" of measured loft with 900 fp down.
I read somewhere in the web (Thru-Hiker?) that 4" of down loft is rated at -20° F.
This got me to thinking about the fill power, fp. At what fp was that rating measured, leaving aside all consideration of warm/cold sleeper and what the rating actually means in terms of comfort.
Consider a quilt with a volume of 20,000 cubic inches , 80"x50"x5". Here is a table of the weight of the down of varying fp needed to fill that quilt:
- 600 = 33.3 oz
- 650 = 30.8 oz
- 700 = 28.6 oz
- 800 = 25.0 oz
- 900 = 22.2 oz
So in going from 600 to 900 fp down, we "lost" 11.1 oz of down or 1/3 of the fp and weight. That is a very worthwhile weight savings.
But at what cost?? Putting aside the economic considerations, I want to consider only the insulation value of the down.
As is obvious from the above, as the fp is increased, the amount of down decreases. That means that the volume that was occupied by the down is now occupied by air. The only way for that to happen is to have fewer air pockets of greater size.
As you get fewer air pockets and they get larger, the heat transfer by conduction and convection gets more efficient and hence more heat is transfered, i.e., lost.
In the extreme, you have the case of a large simple air mattress with, say, 4 or 5, large air chambers. In this case, for a quilt, the heat is conducted efficiently from your body across the shell fabric to the air chamber. In the air chamber, convection air currents are set up which carry the heat to the outer shell fabric, where the heat is efficiently conducted to the exterior environment. The large air chambers allow stable and very efficient air currents to be established to maximize the transfer of heat from your body.
That is why air mattresses make poor top and under quilts in comparison to closed cell foam pads or down or synthetic filled quilts which have a very large number of isolated air pockets. The air isolated pockets greatly impede the transfer of air between the pockets and hence greatly impede the transfer of heat through the mass of down.
As another example, consider large air pockets filled with crumpled up newspaper or leaves. You are closer to the situation with down, but you still have larger air pockets and a whole lot fewer of them. That is why such a method of providing insulation under a hammock is better than nothing, but far from adequate at or below freezing temperatures.
So by going to down with a higher fp, we have decreased the number of air pockets and increased their size. The air pockets are still isolated, but the heat has to be transfered between fewer pockets across the mass of the down filaments. The larger and fewer air pockets allow more efficient heat transfer via convection within each air pocket and conduction between air pockets.
The question is: how much more efficient is the heat transfer with increasing fp?
If the increase in heat transfer is minimal (define minimal in this context) in comparison (define your method of comparison) to the weight decrease and cost increase, then the trade off is probably worthwhile.
Does anybody know the fp of the down at which the various temperature ratings were derived?
If the temperature ratings were derived with down rated at 600 fp, then what is the rating for 800 fp or 900 fp?
I strongly suspect that the temperature ratings were determined for 600 fp down since the temperature ratings seem to have been around for many, many years and never mention fp - maybe because fp wasn't even considered when the temperature ratings were determined? In looking at fp and it's history, it seems that the drive for higher and higher fp is a relatively recent phenomena and if the temperature ratings were derived before fp came into vogue, then I suspect that the down used was not treated and prepped to get the best fp possible. All of which leads me to believe that 600 fp down was used.
My meandering thoughts have lead me back to the temperature ratings I have seen for which 4" of down loft was rated at -20° F. So my 5.5" of loft is rated at considerably below that temperature. I seriously hope and doubt that I will ever inhabit an environment where the temperature rating of my down quilts is challenged, but I now have even less of a notion of what the temperature rating for 5.5" of loft with 900 fp down would be.
What I would really like to see is a determination of temperature rating for various fp and varying loft. Has anybody ever seen such a table??
I really would not be too concerned about how the temperature rating is defined so long as it was defined identically for the various fp and lofts thus enabling a means of comparison and a possible means of determining if weight savings was really worth the cost increase with increasing fp and the decrease in insulation value.
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