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Topic: Comparing warmth between filll weights, is it all about loft?< Next Oldest | Next Newest >
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PostIcon Posted on: Oct. 12 2012, 1:56 pm  Skip to the next post in this topic. Ignore posts   QUOTE

Although the increasing use of EN ratings is making sleeping bag comparisons easier, there are still a lot of bags out there w/o EN ratings.  All other things being equal, more down equals more warmth.  But how do we compare the warmth between two bags of known fill weights with different lofts?  

For example, an 800 fill bag at 17 ounces creates (theoretically) 13,600 cubic inches of loft.  A 600 fill bag requires 23 ounces of fill to achieve a similar loft of 13,800.

The question then becomes, is 13K cubic inches of loft from an 800 fill bag equally as warm as 13K inches of loft from a 600 fill bag (all other factors held constant, including bag dimensions, materials, etc.)? In other words, is warmth is primarily a function of loft, and how you achoieve that loft (600 v800) doesn't much matter?

If so, then warmth comparison becomes relatively easy, no?
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PostIcon Posted on: Oct. 12 2012, 2:55 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(lrh442 @ Oct. 12 2012, 11:56 am)
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If so, then warmth comparison becomes relatively easy, no?

Not quite, no.  Other things related to the bags' construction and fit, like insulated zip tubes (I momentarily forget what they're called) to keep cold air from drafting in from exposed zippers, the distribution of loft (whether it stays evenly distributed in baffles or "settles" with cold spots because of underfilled baffles), how the insulation is distributed between the top/bottom layers of the bag, and how much empty room/air space remains in the bag with the person inside, etc. make a difference too.  A bag's construction and design plays into all of that for a EN test... and even moreso for an actual person using it (when you get into factoring the size of the person compared to the bag, their hydration, warm/cold sleeper, etc).

To answer your question... basically speaking, yes, I think loft (which is a measure of the trapped air space in the feathers) is a good thing to measure for the basic reasons you mentioned.  But it's not the only variable.

(On a tangential side note, the math calculations aren't quite right, since fill-power assumes no loft compaction at all, i.e. not even bound inside a sleeping baffle... 17oz of 800-fill won't give you 13,600ci of loft in any actual bag in use... the constraints of a shell and baffle make it somewhat less than the theoretical maxima... which again, plays into the actual design and construction of the bag in question.)


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PostIcon Posted on: Oct. 12 2012, 4:31 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(lrh442 @ Oct. 12 2012, 12:56 pm)
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The question then becomes, is 13K cubic inches of loft from an 800 fill bag equally as warm as 13K inches of loft from a 600 fill bag (all other factors held constant, including bag dimensions, materials, etc.)?

Yes.

Loft = thickness of insulation. Thickness of insulation determines the r-value. More insulation, more resistance to temperature differences.


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PostIcon Posted on: Oct. 12 2012, 4:31 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

I compare bags by loft in regards to temp rating. Moisture retention is something I also look at when considering a purchase. I do winter backpacking and I don't want a bag that will trap the moisture inside so I prefer lighter materials that I have to care for a bit more that will theoretically allow more moisture to push out. Mind you, I still like a DWR shell. Unless the draft tube is wimpy or a crappy hood design which creates gaps, I primary "vote" based upon the loft level in regards to the temp rating it will get to.

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PostIcon Posted on: Oct. 12 2012, 5:02 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(lrh442 @ Oct. 12 2012, 1:56 pm)
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For example, an 800 fill bag at 17 ounces creates (theoretically) 13,600 cubic inches of loft.  A 600 fill bag requires 23 ounces of fill to achieve a similar loft of 13,800.

The question then becomes, is 13K cubic inches of loft from an 800 fill bag equally as warm as 13K inches of loft from a 600 fill bag (all other factors held constant, including bag dimensions, materials, etc.)?

I think what he is asking is if you have two identically constructed sleeping bags but one has 800fill and the other has 600 fill with the extra down to match the same loft as the 800fill sleeping bag would one be warmer then the other.  

I may be wrong but I always thought if that were the case the 600fill would be slightly warmer since the down would be denser in the baffles creating more smaller pockets of air.  Again someone correct me if I'm wrong but since some cottage companies offer overfill on their bags doesn't that essentially do the same thing but still have a lighter more compressible bag.  Since its still bound to the size of the baffle its not like it would gain much more loft.


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PostIcon Posted on: Oct. 12 2012, 5:17 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(crgowo @ Oct. 12 2012, 3:02 pm)
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(lrh442 @ Oct. 12 2012, 1:56 pm)
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For example, an 800 fill bag at 17 ounces creates (theoretically) 13,600 cubic inches of loft.  A 600 fill bag requires 23 ounces of fill to achieve a similar loft of 13,800.

The question then becomes, is 13K cubic inches of loft from an 800 fill bag equally as warm as 13K inches of loft from a 600 fill bag (all other factors held constant, including bag dimensions, materials, etc.)?

I think what he is asking is if you have two identically constructed sleeping bags but one has 800fill and the other has 600 fill with the extra down to match the same loft as the 800fill sleeping bag would one be warmer then the other.  

I may be wrong but I always thought if that were the case the 600fill would be slightly warmer since the down would be denser in the baffles creating more smaller pockets of air.  Again someone correct me if I'm wrong but since some cottage companies offer overfill on their bags doesn't that essentially do the same thing but still have a lighter more compressible bag.  Since its still bound to the size of the baffle its not like it would gain much more loft.

Not quite, as far as I know anyway.

The overfill just helps prevent down from shifting around inside a baffle and creating cold spots.  More down packed in there tighter leaves less room for it to shift around.  That can also make a difference as the bag ages and loses a bit of loft.

I really don't think there's any appreciable difference between a bag with, say, 4-inches of 600-fill loft and 4-inches of 800-fill loft, all else being equal, except that the 800-fill will be lighter.  I don't think the size of the respective air chambers really changes all that much.

That's my own conjecture though, easily open for debate.


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PostIcon Posted on: Oct. 12 2012, 5:29 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(GoBlueHiker @ Oct. 12 2012, 2:17 pm)
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(crgowo @ Oct. 12 2012, 3:02 pm)
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(lrh442 @ Oct. 12 2012, 1:56 pm)
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For example, an 800 fill bag at 17 ounces creates (theoretically) 13,600 cubic inches of loft.  A 600 fill bag requires 23 ounces of fill to achieve a similar loft of 13,800.

The question then becomes, is 13K cubic inches of loft from an 800 fill bag equally as warm as 13K inches of loft from a 600 fill bag (all other factors held constant, including bag dimensions, materials, etc.)?

I think what he is asking is if you have two identically constructed sleeping bags but one has 800fill and the other has 600 fill with the extra down to match the same loft as the 800fill sleeping bag would one be warmer then the other.  

I may be wrong but I always thought if that were the case the 600fill would be slightly warmer since the down would be denser in the baffles creating more smaller pockets of air.  Again someone correct me if I'm wrong but since some cottage companies offer overfill on their bags doesn't that essentially do the same thing but still have a lighter more compressible bag.  Since its still bound to the size of the baffle its not like it would gain much more loft.

Not quite, as far as I know anyway.

The overfill just helps prevent down from shifting around inside a baffle and creating cold spots.  More down packed in there tighter leaves less room for it to shift around.  That can also make a difference as the bag ages and loses a bit of loft.

I really don't think there's any appreciable difference between a bag with, say, 4-inches of 600-fill loft and 4-inches of 800-fill loft, all else being equal, except that the 800-fill will be lighter.  I don't think the size of the respective air chambers really changes all that much.

That's my own conjecture though, easily open for debate.

+1

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PostIcon Posted on: Oct. 12 2012, 5:30 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(GoBlueHiker @ Oct. 12 2012, 5:17 pm)
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The overfill just helps prevent down from shifting around inside a baffle and creating cold spots.  More down packed in there tighter leaves less room for it to shift around.  That can also make a difference as the bag ages and loses a bit of loft.

That makes perfect sense

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PostIcon Posted on: Oct. 12 2012, 5:44 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

Interesting discussion, as this is different from what I thought. The fill power essentially indicates how much air the down can trap in tiny pockets of air, no? I know the technical definition is the volume of an ounce fully lofted, but the only reason that's meaningful is the number of trapped air pockets. And the whole point of how down insulates is that the tiny air pockets closest to your body stay warm because there are so many air pockets between you and the cold that the cold can't get through very well, each air pocket insulates from one to the next. The more tiny air pockets the better the down does at retaining warmth, and thus higher fill power means more tiny air pockets. Or so I thought.

This assumes equal volume of down and that the down is fully lofted. In which case I would think that 800 fill would be warmer than 600 fill, all other things being equal. Please correct me if I'm wrong.

And this is just the theory, in reality a whole bunch of other variables come into play for any given jacket or bag. As pointed out you can stuff more or less down into the same sized baffle, different thicknesses, and issues of construction come into play as to exactly how warm a jacket or bag will be.

Also, can't even really compare two down jackets by weight unless you have the weight specifically for fill material -- the difference between an 11oz and 13oz down jacket may only be that one used a heavier material for the shell/lining.


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PostIcon Posted on: Oct. 12 2012, 6:13 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

I'm kind of dancing around both the theoretical and the practical.

Theoretical in the sense that I'm saying that "all other factors are held constant".

But practical in this sense.  JR posits that warmth is reated to air pockets, and that's true.  And, 8" of cellulose loft isn't going to provide the same warmth as 8" of 850 down loft.  So, technically, it's not 100% about loft.

But, I'm thinking that when comparing between 600 and 850 fill down, it pretty much is all about loft (again, holding all other factors constant).
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PostIcon Posted on: Oct. 12 2012, 7:01 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(JRinGeorgia @ Oct. 12 2012, 3:44 pm)
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Interesting discussion, as this is different from what I thought. The fill power essentially indicates how much air the down can trap in tiny pockets of air, no? I know the technical definition is the volume of an ounce fully lofted, but the only reason that's meaningful is the number of trapped air pockets. And the whole point of how down insulates is that the tiny air pockets closest to your body stay warm because there are so many air pockets between you and the cold that the cold can't get through very well, each air pocket insulates from one to the next. The more tiny air pockets the better the down does at retaining warmth, and thus higher fill power means more tiny air pockets. Or so I thought.

This assumes equal volume of down and that the down is fully lofted. In which case I would think that 800 fill would be warmer than 600 fill, all other things being equal. Please correct me if I'm wrong.

I don't suspect there's much to that theory.  I don't think there's an appreciable difference between "the number of air pockets" in a volume of 800-fill vs 600-fill down.  The reason 600-fill down is heavier is because it includes more feathers (with their heavier stems that provide little or no insulation) than higher-power down, which is just the lighter, softer clusters.

I.e... lower-power down contains more of the feathers on the left, while higher-quality down has more of just the clusters on the right, which are more efficient insulators, weight-wise:



The higher-fill down costs more because it's refined to pull out the feathers and leave more of only the breezy-light little clusters.  Takes more processing (and more geese, frankly) to get a bag with just the lighter stuff.  The leftover feathers get put into things like cheap down pillows and the like.

That extra weight doesn't result in "more and smaller air chambers"... it just means more weight for the same amount of insulation.  You're not getting any extra bonus insulation with those heavier feathers in there.


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PostIcon Posted on: Oct. 12 2012, 7:48 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

I understand what makes a down xxx fill power, resulting quality and commensurate costs.

If we're talking about a fixed volume that allows 800 to fully loft to fill that space, the same weight of 650 would fully loft and leave extra in that same space so fill it with more 650 until it's fully lofted volume matches the space, then of course it's heavier. And you will have created more insulating warmth value for having that extra 650, yes.

Which actually is the same as my point, but in reverse -- it takes more of a lower-number fill power to achieve the same amount of fully lofted down in a given space. If a space only partially fills with trapped air pockets and you add more air pockets, you are adding more insulation.

But is it linear? Is the amount of insulating value constant for any given fill-power, as long as you are comparing fully lofted to fill a fixed volume (without regard for weight)? It seems you're saying that the difference in fill power is weight, but I am under the impression that it's about more than weight.

As your pictures well illustrate, the lower power is a lower quality. You can just see in the 800 photo how the little fuzzies will trap air. Conversely, quills don't much to trap air -- they are more than dead weight, they are dead volume too. Replace all that with better-insulating fluffy 800 and I think you've done more than lighten up. Ounce-for-ounce higher fill power clearly wins. But I'm thinking that volume-for-volume it wins as well.

So basically, I would think that the higher the fill power, not only will it be lighter but also have increasingly higher insulation value.


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PostIcon Posted on: Oct. 13 2012, 3:22 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

You know what JR, I think I misread your previous statement, and thought you were arguing the same point crgowo was above.  I think we're completely agreeing, and debating nothing. :p  Mea culpa.

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PostIcon Posted on: Oct. 13 2012, 3:40 pm Skip to the previous post in this topic.  Ignore posts   QUOTE


(JRinGeorgia @ Oct. 12 2012, 6:48 pm)
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So basically, I would think that the higher the fill power, not only will it be lighter but also have increasingly higher insulation value.

Maybe a tiny bit, but not appreciably. It has to do with the thermal resistance. 6 inches of fiberglass or 600 fill down will be as warm as 6 inches of 900 fill down.

Overfill keeps there from being settled spots that there is no insulation. Plain air offers no resistance making for more thermal transfer. (Cold air outside to your warm backside;-)

I have been dealing with it in the construction industry all my life. The principles are the same.


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