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R-value difference between fill power ratings


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Viewing 19 posts - 1 through 19 (of 19 total)
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  • #3590106
    Dan T
    BPL Member

    @dntimmerman

    It’s been asserted that a higher density of down, up to 2.5 times the chamber volume, will increase the insulating value without the loft depth changing. 800 fill power down is around 15% more dense than 950fp (it takes 15% more weight of it to fill a given space)…..does this mean that it will be 15% warmer for any given loft depth, assuming everything else is constant? Are there any studies out there testing this specifically? I’ve always felt that if there was a difference, it was trivial, but if it’s straight up “more density, more R-value” then 15% is not trivial. I have also seen it asserted in the complete reverse. Higher fill power (lower density) = higher R-value, which would completely contradict the finding in these studies.

    #3590116
    Ben H.
    BPL Member

    @bzhayes

    Locale: No. Alabama

    I’ve never seen studies on this; just anecdotal evidence.  Theoretically, for a given thickness, the less conductive the material is, the less heat will be lost for a given temperature differential.  Air is usually less conductive than any solid material, so the more air you get into that thickness the less heat loss.  From that regard higher fill power will be more air and less solid down and so should be better at insulating.  The limit on that theory is that air can move and convect heat.  Moving air can mix the hot air with the cold air which is much more efficient at heat transfer (and so worse at insulating).  That is why you fill a sleeping bag with down and not just air.  The down forms little chambers which effectively hold the little pockets of air in place and prevent them from moving around and mixing. The more down you get in there the better it will be at prevent the hot and cold air from mixing together.  So you can see you have two competing trends.

    Which one wins?  Let’s test! … but I think in the end we are near a cross-over point where slight test differences can tip you one way or the other and your test accuracy may not be able to differentiate.  My educated guess is that in a laboratory (with sufficient accuracy) the higher fill power always wins.  In real life with a backpacker wiggling around in the night its not so clear and probably doesn’t have a perceptible difference.

    #3590120
    Dan T
    BPL Member

    @dntimmerman

    Not in disagreement, but as devils advocate, if this is true then why would filling a chamber to a higher density increase the insulation value. After all, higher density of fill = less air.

    For reference here is the thread:

    https://backpackinglight.com/forums/topic/12505/page/2/#comments

    #3590123
    Edward John M
    BPL Member

    @moondog55

    As I understand it the R-Value depends on both the depth of the loft [ ie: the thickness of the insulation] and the size of the air pockets between down clusters. Higher FP down has more”open” air between each tiny down plumule and air can circulate with-in each small pocket.

    So while 900 down is going to loft better it also flows more easily if not filled to the optimum value.

    I can feel the difference in warmth in my deep winter bag between filling for maximum loft per gram, which was 1200 grams and the final fill which was 1500 grams although it was 850FP down not 900.

    While my experience in temperatures below -25C in minimal down to that temperature it is extensive and I always aim for a minimum of 10% “overstuffed” and if getting a custom fill I aim for 30 to 50%; I think it makes a lot of difference

    #3590127
    Dan T
    BPL Member

    @dntimmerman

    I typically aim for 20 to 30% overstuff, but this has always had more to do with down control than with warmth. According to the above study(s) it also directly increases the insulating value despite reducing the amount of trapped air…up to 250% overstuff! If this is true it should be able to be applied to fill power since all that is is density. There is more air trapped in 950 than 800…and this study is showing more density/less air = higher R-value. My stance has always been that a certain loft, or density will get you a certain R-value, more or less, regardless of the fill power. But these 2 assertions challenge that in two different directions. I don’t see how both could be true.

    #3590130
    Edward John M
    BPL Member

    @moondog55

    Well 3-M Thinsulate works because the fibres are so small and they can get a lot of fibres into the thin layer which hinders air movement [ friction??] so I think using overstuff is analogous

    This is Richard Nisleys line of expertise tho. I just used his expertise when deciding on 30% over fill past maximum loft per gram.

    Certainly when looking at R-Values for wall and ceiling insulation the finer fibres of Fibreglass give a greater value per millimetre than the very coarse fibres of recycled PETE insulation

    #3590131
    Eric Blumensaadt
    BPL Member

    @danepacker

    Locale: Mojave Desert

    BOUNDARY AIR -> The one or two molecule layer on each down plumule, synthetic fiber or wool fiber, etc is that layer (usually) attracted by opposite electric charges from air to material.

    This is what makes the most insulative value. Also, naturally, the conductivity of a material is important. i.e. a 1/2″ layer of wool or polyester felt will be much warmer than 1/2″ of dry wall.

    Air alone is not the answer. Convection must be mitigated as much as possible. Down is best for this in garments. Thus an air mattress with no insulation is cold due to the constant convection of body-warmed air chilled by ground-cooled air. Insulated air mattresses both reduce convection with the insulating fibers bonded beneath their top layer and reduce radiation loss with reflective coatings inside the bottom layer, or are down filled OR, like the Neo Air, with several layers of tubes and reflective coatings.

    Synthetic hollow fibers, like hollow deer, elk or caribou hair, with their internal boundary air that cannot easily be moved is likewise a very good insulator. Polarguard Delta was one of these and there are one or more hollow fiber pile materials on the market that are also quite warm.

    What insulator had the most boundary air per cubic inch and has the lightest weight?  Yep, down, and from least to most, duck down then goose down with eider down being SUL the champ – IF you can find it.

    Howsomever… it is my personal opinion/experience that two parkas of equal construction and loft  but different fill powers are different in average warmth IN USE. A parka of say, 650 fill power goose down will be warmer than a twin parka with 800 fill power goose down B/C the 650 down is less compressible in areas like shoulders and arms and torso near the arms where compression is often taking place as well as the back if a pack is worn.

    So what about Aerogel? I dunno if we can count it as it has little resilience after compression.

    #3590144
    Todd T
    BPL Member

    @texasbb

    Locale: Pacific Northwest

    …if this is true then why would filling a chamber to a higher density increase the insulation value. After all, higher density of fill = less air.

    I thought Ben covered that.  Basically, air is a great insulator if you can just keep it from moving…no small feat it turns out.  The function of insulating materials (the ones we care about anyway) is simply to get in the air’s way.  The more stuff you cram in there, the better you hold the air still and the better it insulates.  There’s a limit, though.  Eventually, you’ll cram in enough stuff that heat conducted through the stuff itself starts to overwhelm the heat transfer you’ve stymied by stopping the air.

    And, of course, economics comes into play in multiple ways.  Is it better to cram a little more stuff into my air gap or increase the thickness of the air gap so I can use less stuff?  Which way costs less to create?  Which way imposes less burden on someone carrying it around all week?  Which way will be more durable?  Etc.

    #3590153
    Dan T
    BPL Member

    @dntimmerman

    Todd, after going back I guess my first reply was not worded well. I’m totally with you on that notion of density resisting air movement. What I was sorta meaning to ask there is how can it be true that the lower density of high fill power down result in higher R-value while the higher densities of overstuffing also result in higher R-values? One angle says that high FP has more trapped air with less mass and that’s warmer, while the other angle says that more mass resisting air movement is warmer. It seems to me it can’t be both.

    #3590154
    Edward John M
    BPL Member

    @moondog55

    I don’t have a link but I remember it being said somewhere that air can be seen to circulating in pockets as small as 0.6mm in size and that air can freely circulate in spaces of 6mm or more.

    That free “dead air insulation” between the wind shirt and our base layer is dead air only in comparison to the outside conditions by this comparison

    #3590161
    Todd T
    BPL Member

    @texasbb

    Locale: Pacific Northwest

    What I was sorta meaning to ask there is how can it be true that the lower density of high fill power down result in higher R-value while the higher densities of overstuffing also result in higher R-values?

    Ah, I missed that.  My guess would be that the lower density of higher fill power down results from eliminating feather shafts and chunky stuff like that, not the light fluffy parts of the down cluster that actually help stop the air movement.

    #3590164
    Dan T
    BPL Member

    @dntimmerman

    That is a good point. Although once you’re at 800fp there isn’t much chunky stuff. Maybe on a microscopic level. At some point I’d like to put together some ghetto test and measure heat loss between different fill powers for a given loft and density. Then also at different overstuff amounts. My perception from my stuff is that the R-value difference between fill powers at a given loft and density is very small, if noticeable, and could go either way, but that the overstuff will increase R-value dramatically…as was found in the past studies……although just talking about this has proven to be more of a distraction than I probably need.

    #3590184
    James Marco
    BPL Member

    @jamesdmarco

    Locale: Finger Lakes

    Yeah, there is a lot of stuff going on. To say that 1000FP down is a better insulator than 500fp down, you have to find out the optimum insulation or Rvalue. Backpackers do not worry so much about optimum Rvalue, they are more interested in weight.

    Conduction has a lot to do with it. For example, if you take a 2″ waterproof shell and fill it with water, it won’t have a lot of warmth. Water will conduct the heat away from you very quickly. Replace the shell with a 2″ airtight one and blow it up, and it is better. As I remember and Rvalue of 2. Now we fill the shell with a low conductance material (like down) and the Rvalue goes up again, mostly due to the air being subdivided into smaller air pockets and the low conductivity of the fill material. We start focusing on air pockets, stiffness, and loft (convection) instead of conduction within the fill. But, internal conduction is never eliminated.

    The main job of any fill in a sleeping bag is to make small, non-connected air pockets out of a larger air space. In down, and synthetics, the pockets are connected so, resistance is a better term. The down will resist air movement. Stems are very dense compared with down plumes and weigh more. Immature down feathers are simply strands that are not fully developed in length or in barbules which form “clingyness”. More like synthetics in structure. Synthetics are far larger tubes (about 20x larger on average.)

    Higher FP down generally performs as well as low FP down. But, low FP downs weigh more. 500FP down weighs about twice as much as 1000fp down to perform the same job of making smaller air pockets. There is more air resistance to higher FP downs because thy have more, longer and more mature down plumes. The same WEIGHT of 1000fp down will do a better job of air resistance than 500fp down. It will double the number of air pockets, very roughly speaking. Convection should therefore be halved, again very roughly speaking.

    But, conduction between the down plumes will also increase. The conduction between the plumes is low, though, so it will not increase as rapidly as the air pockets convection resistance increases. In a confined 2″ shell, overfill will make a greater impact on Rvalue than the actual FP rating of the down. You end up with two competing variables. Sorry, I do not know where the curves two cross. I can guess that 50% overfill will be about the limit from a practical standpoint as far as Rvalue is concerned. Without full lofting, you cut back on the amount of air space to divide up, though.

    Using Western Mountaineering as an example: A Megalite bag with 4″ of loft and 11oz of down is rated 30F. A Verslite bag with 6.5″ of loft and 18oz of down is rated for 10F.
    Notice that an approximate 50% more down resulted in a MUCH warmer bag. I would guess that there is no difference in weight or volume between the two, but the reduced loft (4″) from overfilling the Megalight will yield a colder bag than the Versalight.

    This also accounts for the rather high himidity tolerance of down. In 100% himidity, down collapses. But, it makes smaller air pockets (unless you are sleeping in a puddle.) So, even if it looses 50% of it’s loft, it still will retain 75% or more of its warmth. If you hike a lot in humid weather, overfill will help considerably in this scenario. You can expect the 75% to be closer to 90%. I don’t think anyone sleeps in a mud puddle.

    As backpackers, we are also looking for LIGHT WEIGHT and PACK VOLUME. Roughly 50% of a bags weight is the shell. For example if your bag weighs 25oz, about 12.5oz will be down in a good bag. Down also packs very small compared with other materials. High FP down has improved compressability (low volume,) meaning you can carry a smaller, lighter pack.

    #3590195
    Dan T
    BPL Member

    @dntimmerman

    James, your description is very clear, and consistent with my views and opinions on the topic. My general view has always been that different fill powers insulate as well, or close to as well as another over a given loft depth, but that it takes more weight of lower fill power to do so….as you stated. I’ll add one bit on shell weight. Only because I’ve had to quantify this for every, single variation…..take one of my 40 degree bags. The shell weight is around 6oz with a fill weight around 7oz. Move up to a 0 degree bag. The shell weight is about 7oz with a fill weight around 17oz. The only dimensions that change on the shell are the differential cut in the outer shell and the height of the baffles, so the weight of it changes in very small increments as the loft and fill volume changes…..you’re in the finger lakes?

    #3590201
    Jerry Adams
    BPL Member

    @retiredjerry

    Locale: Oregon and Washington

    To get maximum R value per weight, don’t overfill at all.

    If you overfill up to 100% or 150%, that extra down gives you about half the R value per weight.  I think this is because there are more down plumes in the volume so there’s less convective air flow inside the garment.  It doesn’t really matter though, it’s in Richard’s data.

    I always overfill by about 30%.  Otherwise, in real use, the down tends to shift to one end in a baffle so there the other end of the baffle has no down.  And if the down gets a little humid and starts losing some loft it will still fill the baffles.  And if I screw up in my measurements there’s some extra.  I don’t have to be as diligent fluffing up the garment each time before use.  And that 30% of overfill weight gives you 15% increase in R value so it’s not totally wasted.

    You can figure out the original question from Richard’s data.

    If you compare the R value of 800 FP fully lofted, with the same weight of 950 FP in the same size baffle, that will be about 20% overfill.  The R value will be about 10% higher.

    #3590204
    James Marco
    BPL Member

    @jamesdmarco

    Locale: Finger Lakes

    Dan, Yes. I live in Ithaca…well outside the city. Spend a lot (most) of my time in the ADKs, though. I Lived in Remsen (about 45 minutes from Old Forge) for several years. Ice out on most of the lakes, yesterday. Headed up next week for a week or two.

    #3590213
    Dan T
    BPL Member

    @dntimmerman

    James,….huh, weird….I live in Ithaca…..well, Trumansburg/Hector to be more precise. I build out of an off grid cabin out in Tburg during the summers. Then move everything down to southern AZ for the winters….things just worked out that way.

    #3590318
    Dave G
    BPL Member

    @dapperdave

    I haven’t read it, but this PhD Thesis will probably have some useful information

    Edit: Section 7 is the most relevant part.

    Baffled 31cm squares of 30d nylon filled with varying weights of 700fp goose and duck down (18g readings are unfilled) measured on a sweating hotplate. It is stated that thickness does not increase beyond 10g of fill (Sample mass 28g)

    For clarification this graph is insulation per gram on the y-axis so the roughly flat portion at the right hand side shows that, for these conditions, up to 60% overfill is linear return in terms of warmth per gram.

    ‘The structure and properties of down feathers
    and their use in the outdoor industry’

    https://core.ac.uk/download/pdf/30268287.pdf

     

    #3590326
    James Marco
    BPL Member

    @jamesdmarco

    Locale: Finger Lakes

    Dan, Wow, Yeah, Tburg is on the other side of the lake, a bunch of farms up through there. I used to pick up hardwood lumber up there and my wife used to make me head to the craft fair at the school when they had it. I live in Varna, about 5 minutes away from Cornell’s Game Farm. Short trip to work, I used to ride my bike every day without snow. Ha, hey…uphill both ways, of course.

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