This topic has been brewing in my head since I decided to order a new top quilt. For the purpose of this thread, I'd like to limit the conversation to down-filled gear with baffled construction (e.g., quilts, sleeping bags, etc. - but not those with sewn-through construction and not synthetic fills).
I already have a good handle on what "Overfill" means in general. Basically it's the practice of adding more down than the total calculated volume of the baffles. There are numerous reasons for overfilling a piece of gear including:
- Getting expected loft in less-than-ideal conditions (e.g., high humidity, moisture collected from vapor condensation, etc.)
- Preventing unwanted shifting of the down within the baffles
- Achieving adequate loft over the life of the gear (assuming down achieves less loft over it's life due to compression/decompression, age, etc.)
Is there a generally-accepted method for calculating the volume of the baffles (and what is it)?
I'll call this the "baseline". The reason for the question is that I've heard/read about two different methods and you really don't know what you're getting regarding "overfill" without first understanding how this baseline calculation is made.
Say you have a rectangular quilt that measures 78-inches long, 50-inches wide, and has 2-inch baffles. At first, I'd think the baseline would be 78x50x2 or 7800 cubic inches. However, I have read some people add 1/2-inch to the baffle height when they calculate the volume. In this case, the baseline of our example quilt would be 78x50x2.5 or 9750 cubic inches. That's 25% more volume than the first calculation.
Do all manufacturers calculate the baseline volume the same way?
Using the example quilt above, suppose "Manufacturer A" uses the baffle height for their baseline but "Manufacturer B" adds 1/2-inch to the baffle height. You would have to ask "Manufacturer A" for 25% overfill in order to get the same amount of down (by volume) that "Manufacturer B" provides with no overfill.
See my confusion?
I personally don't think either way is better or worse as long as I understand what I'm getting.
Maybe some of the manufacturers on the forum could comment on their own method of calculating the baseline (hint, hint)?
Is there a recommended amount of "overfill" - based on which baseline calculation?
I've heard recommendations anywhere between 10% and 40% - but, since I don't know the baseline, different recommendations could actually achieve the same results (as in the example above).
In my case, I'm not necessarily looking for increased warmth. I'm looking for those reasons I listed above (loft in less-than-ideal conditions, no shifting, long lifespan).
Do different fill-powers require different overfills to achieve the same performance?
I understand lower fill-power down requires more down (by weight) to achieve the same volume. What I'm asking is, do they require a different amount of overfill (by percentage) to reach the same goals I mentioned?
Say 900-FP down loses 10% loft given a fixed amount of moisture absorption but 600-FP down only loses 5% loft under the same conditions. In this case, I would want a higher overfill percentage for a 900-FP quilt than I would for a 600-FP quilt, right?
I know there are a bunch of really smart people out there who have been making and using down-filled gear for a long time. I'd be really appreciative if some of you could shed some light on this for me.
P.S. - Yes, I'm a dork but I've been that way for over 50 years now so I doubt that's going to change anytime soon. Something in me just has to understand these things.