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  1. #1
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    Let's talk overfill (down-filled gear)

    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.)


    First question:
    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.
    Example:
    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.

    Second question:
    Do all manufacturers calculate the baseline volume the same way?
    Example:
    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)?

    Third question:
    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).

    Fourth question:
    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?
    Example:
    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.
    Last edited by tylojuky; 12-02-2013 at 11:52.

  2. #2

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    Excellent questions. I have had the same questions but didnt know how to formulate them. You must be an engineer of some sort.

    I am surprised no one has an answer yet. Look forward to reading some.

  3. #3
    WV's Avatar
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    I overfill more than most because I'm bothered by thin spots more than most. Motivation trumps measurement, IMO. Having said that, when I added .75 oz down to each of the 10 tubes of my No Sniveller, I assumed (correctly, I think) that the original down was evenly divided among the more or less equal size tubes, so I carefully measured the down I added. I probably would have been perfectly happy with .5 oz. added per tube, but .75 oz. gives me a cushion, as it were.

  4. #4
    Boston's Avatar
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    Quote Originally Posted by tylojuky View Post
    First question:
    Is there a generally-accepted method for calculating the volume of the baffles (and what is it)?
    You calculate the volume of the chamber. This depends on if you do a differential cut or not. (differential recommended for underquilt. Not nessesary for TQ, unless you want to allow it to double as an UQ)

    For a non-differential cut quilt it's fairly simple. L x W x H.

    For more involved there are formula approximations based on the radius of the inner shell around your body and the amount of loft vs baffle height. There are excel sheet calculators out there. I have one I modified to do a dual differential. I posted it here. It was based off CatSplat's calculator.

    Second question:
    Do all manufacturers calculate the baseline volume the same way?
    See above. Is varies based on some assumptions with the radii, and the baffle height vs loft height.

    Third question:
    Is there a recommended amount of "overfill" - based on which baseline calculation?
    Usually 20-30%. Based on down weight. See below. Using a lower fill power will result in adding more down because you're taking a percentage of weight. Lower fill power results in more down by weight for the same performance.

    Fourth question:
    Do different fill-powers require different overfills to achieve the same performance?
    Fill power is a measure of volume per unit weight. In this case it's in^3/oz. So 800 FP will fill 800in^3 with 1 oz of down.

    Determine the amount of down required is simple. Divide Baffle volume / FP and multiply by your overfill factor (if 30% it'd be 1.3. 20% would be 1.2, etc..).

    So let's put it together in a simple example. If we have a square baffle 2" high, 6" wide, and 72" long.

    V = 2[in] * 6[in] * 72[in] = 864[in^3].

    Down needed (800 FP, 30% over fill) = 864[in^3] / 800[in^3/oz] * 1.3 = 1.4[oz] per baffle.

    If the quilt has 9 baffles we need: 9 * 1.4[oz] = 12.6[oz] for the whole quilt.

    If you use a calculator it'll do all this math for you.

  5. #5
    UrsaMajor1887's Avatar
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    I go with standard fill, don't store it compressed, shake it out good when I pull it from the stuff sack to fluff it back up and make evenly distribute. I know moisture and down are a problem, but I don't know how overstuff would impact that. I am not concerned about the down breaking down anytime soon especially since I don't store it compressed and I don't use compression stuff sacks either.

    So, it would be nice to know more about the moisture issue. Of course I just ordered a 40 underquilt today with no overstuff, but the down is water resistant so maybe that will be as much of an issue.
    "When you see something wobble, push it."
    - Unknown

  6. #6

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    I'm curious where the extra half inch came from. It sounds less than honest. Translate that to I can see it in the big boys but not the cottage folks as they get looked at too hard.

    If one does not use their quilts for many day trips loft loss due to moisture should not be noticeable. OTOH if you have an old quilt that seems too thin the first step is to wash it. Dirt will also cause loft loss. ;-)
    YMMV

    HYOH

    Free advice worth what you paid for it. ;-)

  7. #7
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    @Boston,
    My example was simplified to try and explain my point. The real question about the baseline volume was, is it calculated based solely on the shell volume (actual formula dependent on the shape) or is there some arbitrary "extra" added to that so the baffles "puff". Sounds like the answer is the former - if you get zero overfill then, under optimal conditions, the down will just fill that volume with no additional humps or puffs between the baffles. Is that what you're saying?

    Regarding the question about fill power: that ones a little harder to explain. The root of the question is, does the loft of higher fill-power down degrade more or less than lower fill-power down (given the same sub-optimal conditions)? If there's a difference in degradation with different fill powers, then the recommended overfill percentage might be different depending on the fill power.

    @nothermark,
    I've read so many forums and sites on this topic that I can't remember where I heard the "extra half inch" comment. I do remember hearing it more than once but I also remember it was from some posts on forums and not stated by a manufacturer or anyone in a position of authority on the matter. If the consensus is that the baseline volume is as Boston stayed above, then I'm okay with chalking that one up to "internet misinformation" (it's not like the internet is the bastion of truth anyway)

  8. #8

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    Quote Originally Posted by tylojuky View Post
    @Boston,
    I've read so many forums and sites on this topic that I can't remember where I heard the "extra half inch" comment. I do remember hearing it more than once but I also remember it was from some posts on forums and not stated by a manufacturer or anyone in a position of authority on the matter. If the consensus is that the baseline volume is as Boston stayed above, then I'm okay with chalking that one up to "internet misinformation" (it's not like the internet is the bastion of truth anyway)
    Manufacturers use to list loft and baffle height separately under the specs for each quilt. My interpretation was that the stated "loft" was the goal and the measured height of the quilt (taken at the high point or peak of the chambers). The baffle height was a lesser value (have seen -1/2 to 1 inch commonly used (another unfinished discussion)) designed to slightly compress the fill, to help keep it from shifting, giving it a bit of over stuff. At least, that was my understanding of it.
    I am still not clear regarding the best way to calculate fill volumes as the different methods can yield similar results. Add to the confusion, tapered quilt shapes and you can see how a 3d CAD program can help. I played with the math for quite a while for my quilts, but then I spent a while with them filled before I closed them up to make sure that the real world results met my expectations--hand always beats paper.
    Sorry to further muddy the waters,
    jason

  9. #9
    edcpreps's Avatar
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    I'm the type of person that see's the overfill option and goes for it, not knowing much about how its spread out or how much it affects the warmth. It just has to increase it right? I'm the same person that's targeted in the supermarket checkout lane w/ the random stuff on the rack, sure I can use some gum, batteries, a portable MP3 player, readers digest & anti diarrhea medicine.

  10. #10

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    I agree with you edcpreps.
    I think I added 1 oz to my incubator just because I could.
    I have seen Adam's quilts in person and they are plenty filled, with great lofting, using just the standard fill amount. I know the math and have spent a lot of time calculating out fill volumes, so I KNOW these things are good to go with the standard fill. I even heard it directly from the makers mouth. But when faced with that little drop down box, there was no way I could say no.

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