Overstuff? How much does it help?

[BillyBob58]

Yes the geometry would change but, I'm really talking about warmth.
Warmth does not actually relate to loft, but to volume (to a degree).

Let's take an imaginary example: make a quilt out of rigid material such that is is 2" thick.
Now, fill the quilt with enough down to occupy that volume without being compressed.
That quilt will insulate it's occupant like any 2" quilt that we normally think of.
Next, let's double the amount of down in that quilt.
Now we have a quilt with the equivalent of 4" of down in it, but, since it is rigid, it's still only 2" thick.
BUT, it is as warm as a 4" thick quilt.

There are thick scientific papers on this, the best of which was published by Richard Nisley a few years ago.
But the bottom line is that the insulation comes from hindering both convection and conduction.
As a down quilt is compressed, one losses ground while the other gains ground.
According to Richard's testing, the two properties essentially offset each other until the down is compressed about 2.5 times.

So, if the shape of the quilt can be reasonably controlled, which is why we put baffles in,
the warmth of the quilt is a function of the amount of down used, even if it is compressed a little.

So, does it also work this way? :
If I compress my 3" UQ to, say, only 1.6"(less than 50%) by pulling it too tight, will it be as warm according to the scientific papers? As warm as it was when at the full, original 3" loft?

[G.L.P.]

"So, if the shape of the quilt can be reasonably controlled, which is why we put baffles in,
the warmth of the quilt is a function of the amount of down used, even if it is compressed a little."


from what i got over the years Baffles are used to control the migration of Down more so then the shape of the quilt ... yes it helps shape your quilt but baffles are more for down migration

ok i'm trying to figure out how you got to this conclusion ...
my understanding of down is when compressed there is a loss of insulation properties ..down works best when it has room to fluff up or "Loft" down works by trapping hot air in between it's self
i have a summer UQ and a winter UQ... both are nice and full of down for there respected airspace but my summer UQ with 1 inch baffles does Not have the same temp rating as my winter UQ with 3.5inch baffles both have 20% overfill for there respected airspace... so what your saying is there is no difference in the 2? ... because i can tell you there is i can't take my summer UQ lower then 40F... i'm just trying to understand where you are coming from. from what your saying thickness has nothing to do with warmth?
then why is every vendor and big box Co. use thickness as a gauge for temp ratings? IE 2 inch baffles 25-30F 3inch 10F and so on ?
also if this really is the case then why are UQ's and sleeping bags not just packs full of down to increase temp ratings instead of bigger baffles?

[catalyst]

At the risk of putting words in MAD's mouth, what I think he is getting at is that according to some research, the loss of warmth due to the compression of down is insignificant up to a certain point and that down can be compressed quite a bit without losing most of the warmth created by the insulation. I found a quote in another forum that is similar to MAD's example that may help describe the issue:

"If you compress 4" of down to just 2" of thickness, then it's going to be almost 2x as warm per inch of thickness, so the total insulation value is almost unchanged. There is a slight loss (ie. it's most efficient to have down fully lofted) but the loss is quite small until you get into serious down compression.....like compressing it to 1/10th or so, which is what happens with the down in a sleeping bag when it's underneath the body."

So, according to the research, it seems that down could be really packed into the baffles rather than increasing the baffle height. My guess is most manufacturers don't do this because it's most efficient to have the down fully lofted.

[GrizzlyAdams]

So, does it also work this way? :
If I compress my 3" UQ to, say, only 1.6"(less than 50%) by pulling it too tight, will it be as warm according to the scientific papers? As warm as it was when at the full, original 3" loft?

This is an excellent point, and I would have to say that from the premise the conclusion would be "yes", at least of the compression was uniform. Perhaps the conclusion does not hold if the compression is localized, e.g., directly beneath your posterior!

[DemostiX]

Over-stuffing hurts if it impairs drape. You can brag about your extra ounces of down, like extra inches of -------, but if it results in a top-quilt out of which an open-ended tent forms by itself above you, you can as well brag about keeping yourself warm with a 4" stryofoam board. It won't tuck in and won't stay tucked in.

I was cold last night at 26F with an overstuffed down TQ, after being comfortable most nights all winter with either a 40F Hammockgear TQ that drapes,or
a WM Highlite sleeping bag, or
a WM Summerlite sb,or
a DIY double-layer Climashield TQ

because they all drape.

Overstuffing? Maybe or likely for a bottom quilt and for the top of a zipped-up and close-fitting mummy sleeping bag, with draft collars. Not for a top quilt without simultaneous design changes to restore drape and guarantee sealing.

[HappyHiker]

Ok, let's see if I understand this correctly.

I'm going to build a theroretical TQ, and aim for a 50 degree quilt. I'm going to keep it rectangular for simplicity, and use the 1" of loft (and WM standard) to achieve the temp rating of 49 degrees.

67degrees - 18 degrees = 49 Degree (WM standard 1" of loft = 18 degree reduction from 67 degrees)

48" x 72" x 1" = 3456 cuin (LxWxH)
3456" / 800 FP = 4.32 oz down (Cuin / FP)

So now to calculate the effect of overstuffing by 20%:

4.32 oz x .20 = .86 oz extra down
4.32 oz + .86 oz = 5.18 oz

Using Mad777's formula:

800FP * 5.18 oz = 4144 cuin effective loft
4144 / 3456 sqin = 1.2" effective loft

.2 (effective increase in loft) x 18 degrees (reduction in temp for 1" of loft) = 3.6 degree reduction in temp rating.

So by adding a 20% overstuff I increase the rating of the quilt from 49 degrees to 45.4 degrees. Or roughly from a 50 degree to a 45 degree quilt.

Sound about right?

So IMO what is really being discussed is insulation density and not loft. The 20% overstuff figure would leave room for compression of the insulation (another 30% or so) through various means where if overstuffed by 50% any additional compression would result in loss of warmth (if the 50% compression figure is correct).

Loft being used by manufacturers seems to be the most easily conveyed to consumers (loftier bag is warmer) for comparison, far easier than trying to convey insulation density, even though it doesn't always hold true IME.

[Risk]

Great discussion! Mad, thanks for pointing me to this old thread.

[BraveSaintStuart]

I still don't quite know if I should order any overstuffing in my hammock after all this...

[BillyBob58]

Yes the geometry would change but, I'm really talking about warmth.
Warmth does not actually relate to loft, but to volume (to a degree).

Let's take an imaginary example: make a quilt out of rigid material such that is is 2" thick.
Now, fill the quilt with enough down to occupy that volume without being compressed.
That quilt will insulate it's occupant like any 2" quilt that we normally think of.
Next, let's double the amount of down in that quilt.
Now we have a quilt with the equivalent of 4" of down in it, but, since it is rigid, it's still only 2" thick.
BUT, it is as warm as a 4" thick quilt.

There are thick scientific papers on this, the best of which was published by Richard Nisley a few years ago.
But the bottom line is that the insulation comes from hindering both convection and conduction.
As a down quilt is compressed, one losses ground while the other gains ground.
According to Richard's testing, the two properties essentially offset each other until the down is compressed about 2.5 times.

So, if the shape of the quilt can be reasonably controlled, which is why we put baffles in,
the warmth of the quilt is a function of the amount of down used, even if it is compressed a little.

So the 2" thick quilt will offer the same thermal resistance as a 4" thick quilt with the same amount of down but half of the entrapped air?

So, does it also work this way? :
If I compress my 3" UQ to, say, only 1.6"(less than 50%) by pulling it too tight, will it be as warm according to the scientific papers? As warm as it was when at the full, original 3" loft?

OK, I never got an answer on this question from a year ago, but I think the answer is apparent. That is, if Mad77 is correct - and I have no reason to doubt him and he says "There are thick scientific papers on this", then the answer to my question is "Yes, a 3" quilt pulled to tight and compressed by half will be just as warm". It appears then that you can choose either maximum loft from a given amount of down or half the loft it is capable of giving, and the warmth will be the same. That is the implication of the scientific papers Mad77 mentions.

And possibly with the lower loft gain a benefit from a bit of added wind resistance and less problem with possible down "drift" leaving spots that are not as well filled as they should be? Maybe. And possibly losing some draping ability like Demostix mentions? Maybe.

I would have thought it warmer to get as much loft as you can from a given amount of down, but maybe not. This would explain something I was surprised at when I 1st got a PeaPod. I remember I had settled in one night to realize that the loft was compressed under my butt, and I felt I needed to get out and adjust to hand it more loosely. Then I decided to heck with it, there will be times on the trail when I don't hang it perfect and I'll be too tired to fool with it, so I'll just let it go. I pushed the temp rating so I expected CBS. But I did not get any at all, I was surprised at how warm I was even at the final low of 10F with a space blanket added, but with compressed loft. I probably did not have much more than an inch - inch and one half of loft under my butt, which should not have been warm at those temps, but it was. So maybe these theories explain that? Although I would think in that pod, because the down is so free to move around, you could not end up with an "overstuffed condition", but just a plain old low loft condition? I don't know. Here is the quote from that night: http://www.hammockforums.net/forum/s...ead.php?t=2661

Before I closed the Velcro all the way, I felt around from the outside. Once again, it did not feel like I had full loft under my butt. I'm going to have to work on that some more. But for the sake of simplicity, and looking towards days on the trail when I would be too tired to fool with it, I just let it be, even though this was going to be one seriously cold night..........After about 10 or 20 minutes, I was plenty warm on the bottom, starting to worry about over-heating on top. I was very warm, so I lowered my jacket hood, and was still plenty warm but not overheating.

Maybe it was a lower loft but overstuffed situation?

And so does this also imply: it is more important, even with non-differentially cut quilts, to have the quilt snugged against you, even if you compress by hakf, than to risk even the tiniest gap?

[dakotaross]

...Using more down than that is essentially replacing air pockets with the down, and the air pockets are where the insulating function derives.

When ordering a TQ or full length UQ I always have them add 2 extra oz's of down. It is so cheap ($8-$10 bucks per oz) compared to the overall cost of the quilt.
What I think I'm getting is quicker loft, less chance for voids, a little better performance in windy conditions.

I still don't quite know if I should order any overstuffing in my hammock after all this...

Basically, overstuffing works, otherwise makers like JRB wouldn't do it as a matter of routine. My guess is that down is just more efficient at separating from itself than syn materials, thus making many small pockets of air. The smaller these are, the more quickly they heat up.

Plus, the smaller pockets seem to add more structure. This is important because as you move around, you're contantly pushing air in and out of these pockets. With more structure, it takes more effort to reduce the volume of air. As I could vouch for with my Golite Ultra 20, the thing will puff up to a necessary loft, but it will only remain that way for any length of time if I am relatively still.

So, you add more down, you get smaller pockets of air, but not necessarily a lesser volume of air. Some loss of volume for sure, but negligible up to a certain point where I suppose the down can't expand from itself as well. So clearly, overstuffing has a point of diminishing returns.

However, Its been 30 years since I had Thermodynamics, so I could be wrong.