What’s the benefit of synthetic?

I was in the Pecos wilderness this June when we had a sudden downpour followed by 3 inches of hail, followed by more rain. The humidity went from low to 100% in an hour.

My nice double walled tent, sans vestibule, with two of us trying to stay dry during the night, turned into a condensation trap.

My Down bag was soaked by 11:00 pm and the temperature dropped to the high 30's. I was in trouble and knew it.

I managed to scrape together enough semi-moist wood, that with my emergency Esbit tab and a cotton tee shirt that I had packed for hot temps (cotton tee shirt strips make GREAT tinder for a fire), I managed to get a fire going in spite of the wet ground. It took amost every trick I knew, however, to keep that fire going until the moist wood managed to dry out enough to burn well.

I stood around the fire, with my poncho on and pulled up in such a way that I could dry my bag while keeping the rain off.

My son, who I shared the tent with, had an old north face 20 degree synth bag and even though that synth bag was in the same exact moisture laden tent, he stayed toasty all night long.

I'm rethinking my sleep system and for one thing, the double walled tent is out once and for all. I'll stick with my tarptent, which, by the way, my other son used in the same conditions without a problem in the world, or my tarp and I'm going to carry my Bivy with me even if I'm in a tent.

Rethinking the whole sleep system, I'm considering a more layered approach such as the one that Backpackinglight sells.

I've used a 40 degree ray way quilt with a bivy down to freezing before without much problem. It's surprising how much warmer you sleep in a breathable bivy and how much moisture you keep off your bag.

I dont know, I have been in tarptents with crazy condensation which wetted my down bag and I was always just fine and my body heat dried it out eventually.
I am suprised by the difference between your down bag and a synthetic in that situation?
As for winter use of down, I am no real winter hiker but I always thought that a vapour barrier was almost mandatory for any extended use in winter conditions regardless of insulation used?
And just too note Im not saying synthetics dont have an edge in wet conditions, just that down is not quite the cotton of insulations some seem to make it out to be.
I also have a Ray-way quilt and really liked it but for the fact that I made it a bit to short!
But my down quilt stuffs too almost half the volume, has at least an inch more of loft, and wieghts a lot less!
Worth the little extra care in most cases in my opinion.

>I have been in tarptents with crazy condensation which wetted my down bag and I was always just fine and my body heat dried it out eventually.

I think that this discussion highlights the source of a lot of the "conceptual disconnect" sometimes experienced when hikers talk about gear+situation.

I had the fortune of learning to camp in the Rockies and Foothills of Alberta. (North of Montana; probably similar climate.) Spectacularly dry is an understatement: anywhere from freezing to -40 was very safely weatherable by our poorly-equipped scout troop wearing cotton clothes and 80's era jackets in rental sleeping bags and worn-out 70's tents.

Then I moved to the West Coast in my 20's and have been slapped silly by the climate on a few occasions. I weigh 185 and I have a high metabolism and suffice it to say that I never *ever* had a problem keeping warm in Alberta. All the way to -40, even as a skinny teenager, I was never cold. (I was famous for wearing shorts to school in -20C/0F weather.)

Out here, to contrast, I can be bundled up and walking in 5 above zero weather, shopping downtown, and get chilled like a vegan 12-year-old girl with an eating disorder. What the??

If you'd asked me when I lived in Alberta, I would have told you "yeah you have to be more careful with down, but if you get it wet it will always eventually dry with body heat." But my perspective was changed by winter rainforest hiking. In Alberta, the aforementioned statement may generally be true. Out here? Maybe 1/2 the time, if you're smart and careful and have some luck on your side. Some Olympics/Cascades hikers will probably back me up on that.

My point is that our perspectives are often shaped by our "home stomping grounds." As Brett said, it comes down to METTT — and all broad generalizations should be qualified by location, season, and conditions in which they apply.

Is a bag with 1" of synthtic loft equally warm when compared to a bag with 1" loft of down?

The short answer is "generally not". A high quality 800 fill down bag provides up to 63% more warmth per inch than the AVERAGE synthetic insulated bag. The BEST synthetic insulation on the market provides approximately 30% less insulation per inch than the best 800 fill down bag.

There are a large number of different synthetic insulations. There is also a broad spectrum of down quality and bag construction quality. There can be many exceptions to the general rule.

>Is a bag with 1" of synthtic loft equally warm when compared to a bag with 1" loft of down?

Call it heresy, but I don't place any stock in the standard line that loft=warmth.

The two are strongly correlated, *all things being equal*. But all things are *not* equal.

When it comes to sleep systems, the cut, face fabric, and user's sleeping habits combine with loft measurement to determine the total warmth of a system. I've slept in 2.5" loft bags that were warm for me, and 2.5" loft bags that have just about frozen my balls to the ground a few times. (I have one of each hanging in my bedroom.) The difference has to do with draft control (I move a lot) and cut — *for me*.

I don't think that you can find two otherwise identical bags or garments that have down/synthetic fill as their only differentiator. Thus, comparing loft directly between synthetic and down-filled garments/bags is necessarily comparing apples to oranges. Or something.

Re: " (I have one of each hanging in my bedroom.)" 2.5" loft bags, or balls?

I think the real benefit of synthetics is that you don't have to be as careful with them. My primaloft sleeping bag is a snap to wash and it dries (on air dry)in a half hour. My Polarguard bags take a while longer to dry, but still easier to wash then down. The benefit is you don't have to worry about tearing a baffle or ruining the down. They are low maintenance. If you spill something on them, you know you can clean it pretty easily and not get stressed out like you would with a down bag. If you accidentally tear your fabric (not that there is much of a chance this could happen, but always remember Murphy's Law, it is much easier to repair a synthtic and you don't have to worry about insulation floating all over the place.

Sure synthetics might insulate better when wet, but I have never been in a situation where the bag got soaked. I guess you would have to maybe worry about it if you have a Black Diamond tent, but otherwise you should be alright.

Wow, now you all have me worried. I'm getting ready to make my first sleep system (similar to the Warmlite system, in that it's a "quilt" that is going to zip onto a casing for a downmat), and I've chosen down for the insulation (already purchased, too late to change my mind).

I'm making the shell (both inner and outer) from momentum90, which is said to have a great DWR on it.

I'll be mostly camping in and around New England. While I currently have no plans to use it in winter, I'd like it to be up to the task, and it HAS to work well in the spring and fall. I've got a Rainshadow 2 by Tarptent (good ventilation).

I've only camped in really cold weather once, and condensation was most DEFINITELY an issue. The bag I had at that time was synthetic, and it certainly kept me toasty enough. But I not having had much experience, I had since come to believe the condensation was due to not enough ventilation in the tent (it was a double-walled).

Having researched it (or so I thought), I now understand that condensation also becomes an issue from INSIDE the bag, not just moisture in the tent, and that condensation could still be an issue, even with a well ventilated tent. But then, I also thought that the Warmlite system addressed that as well, by having the entire inner lining of their systems be a vapor barrier…

Therefore I was under the impression that I could keep my down sleep system perfectly functional by having a vapor barrier lining that I can add to the system in the cold, using a DWR shell (so it's breathable, instead of waterproof, therefore not keeping humidity IN, if it DOES get in), and by making sure I have a shelter with little chance of condensation problems (preventing condensed water from dripping onto the OUTSIDE of my DWR-coated bag). I thought I had all my bases covered.

So, my question is, what other down failure stories are out there? And what do you think could have been done differently to avoid the problem (if anything)? Someone earlier asked how a bivy would have helped in some of these situations.

I guess I'm a just a worrier, but you guys really have me all freaked out now about using down. In situations where there's lots of moisture and not much you can do about it, is a VB lining and keeping the outside of the bag dry usually good enough to avoid disaster?

I think that your greatest fear should be situations in which your VBL does *not* apply: that is 20F below freezing or less.

If it's cold enough to break out the VBL, it's probably cold enough that the air will hold very little moisture. Also, at that temperature your perspiration can be very well-controlled and your VBL will keep it *out* of your insulation. Any excess humidity will generally create frost on the shelter walls, which may or may not rain down on you depending on the shelter you're employing.

Down will pose more of a danger at *warmer* than about 15 degrees F. At these temperatures, you won't comfortably be able to deploy your VBL but it is of course still quite chilly out. In the absence of VBL, moisture evaporated from your skin will pass into your sleep system — and probably freeze before leaving.

That said, how many nights do you plan to spend at these warm-but-not-warm-enough temperatures? Will you be able to dry your sleep system at all during the day? Will you be wearing warm clothes inside your quilt that you'll be able to dry during the day?

Points to consider.

Brian

> I think that your greatest fear should be situations in which your VBL does *not* apply: that is 20F below freezing or less.
I think you mean 20 F below freezing or WARMER?

Vapor Barrier Clothing will be more functional, more effective, and more flexible (in your sleep system) than a Vapor Barrier Liner inside of your down sleeping bag. You will be able to have a tighter fit with the micro climate next to your skin. You can wear less clothing and not have the perspiration enter your clothing or your sleeping bag.

You will need to experiment with the amount of clothing that you wear outside of your vapor barrier clothing though.

In my case I have opted for 2 Feathered Friends eVENT outer 800+ down sleeping bags in addition to a Cocoon UL60 Polarguard Delta synthetic fill.

I will need to experiment with the eVENT (waterproof breathable) outer sleeping bags under different conditions. But these bags should by all rights be less prone to the problems of down collapse due to condensation inside of my Six Moons Lunar Solo enhanced tent or my Integral Designs tent. Hopefully in those cases where I want to dry out my clothing while I sleep they will be breathable enough so as to let the moisture through and not collect in the down.

Rich

Sharon,

I have done a fair amount of winter camping in New England and exclusively used high quaility, fill and shell, down for my sleeping system with no adverse results.

Now, let me qualify that statement with two points.

First, I use a tent, not a tarp, so external wetting like spin-drift is not an issue. The tent I use is a Hex3 tee-pee that has top vents and can be set up off the ground/snow all around the perimeter for ventilation. That tent has a high interior volume which I think helps with speading out the humidity (from my breath, etc.) in the micro-climate of the tent. Lastly, the very steep tent walls drastically reduce the amount of drips of condensation that are more of a concern with tents with flatter roofs.

Second, I am only out for 1 to 3 nights, usually not more than 2 nights. A major long trek or thru-hike is an entirely different animal, (unless perhaps you are in a dessert). I do not, and probably never will, have the credentials to address those conditions.

Yeah, the planned use of my sleep system is going to be mostly for 2-3 day outings (so, 1-2 nights), and not usually (if at all) for flat out winter camping. Though, obviously, I would like it to be useful for longer/colder trips as well.

I'm mostly concerned about all of that unpredictable weather you hear about people encountering up in the mountains – not so much mountains in New England (though it happens here too), but more so about say, in the Rockies or in the Cascades, since I eventually plan to live out that way again.

My Rainshadow 2, well, I haven't had it long enough to test it, but from what I've read, it's good with condensation. There's not much flat space on the roof, and it pitches fairly tautly, with steep enough sides that I don't anticipate dripping to be an issue, especially if I keep a watch on the tautness of the tent when humidity changes…

And not to get too risque, but with regards to VB clothing, and sleeping in it – well, I tend not to like to sleep in anything (clothing gets all tangled up around my legs and arm-pits, etc), and while I can imagine accepting that I may have to sleep in SOME layers in the cold to stay warm enough (or to avoid having to get dressed from a naked state in the morning), I think it would be much harder to adjust to sleeping in VB clothing (as opposed to say, wool or capilene). So I guess my question is, does VB clothing really offer substantial advantages over a VB liner (which wouldn't get all tangled up around me like clothing will)?

********
Edit: Ok, gave the VB clothing thing more thought. Seems like it DOES offer lots more flexibility: say for example, I have a sleep system that keeps me comfortable down to 0F, without any clothing used, but freak weather drops the temp down to -10F or -20F. If I had VB CLOTHING, I could safely add insulating layers, like a jacket, over my VB clothing, to increase insulation, without being in danger of soaking the extra insulation (since VB is next to my skin). On the other hand, if I have a VB liner, any insulation I need to add could get soaked because the VB liner is now keeping the humidity in with me AND my extra insulation.

Sorry – didn't mean to be slow to catch on there.

Guess that means one less zipper I need to worry about incorporating :-)

That said – is there any VB fabric/clothing that could vague feel like pajamas? Somehow I doubt it…

Wow, there are alot of discussion points in this thread…

I just wanted to add that VB clothing may decrease your prespiration from entering the bags insulation, but that is the least of my worries.

The ability to wear all my clothing in the bag, including my parka, is essential below freezing. It is the moisture in baselyers, shells and parka that concern me… even with VB clothing, that will be pushed into the down.

Also, far and away the biggest problem I have below freezing is my breathe condensing/freezing on the shell around the hood and collar of my bag. This soaks right through Quantum (my experience, every time). A bivy helps this sometimes… but not always.

I honestly think the best system is a down bag and a synthetic overbag- protects from external mositure, yet also places the dew point farther from you, hopefully into the synethtic top. I am seriously considering making a quilt for over my WM Antelope for below zero.

Any experience with this kind of thing?

I have no first hand experience with it but there's WarmLite's "Fuzzy Stuff" fabric. http://www.warmlite.com

Hi Sharon,

Stephenson's Warmlite makes reasonably priced vapor barrier clothing with fuzzy stuff lining which helps to absorb much of the perspiration and makes the clothing more comfortable against the skin.

Here is a link:

http://www.warmlite.com/vb_shirt.htm

Here is their pricing page link:

http://www.warmlite.com/prices.htm

Rich

I have considered this idea of a synthetic quilt over a down bag in the past. The theory that prespiration vapor might condense in the synthetic vs the down is quite plausible. Though the results would vary in fact given specific circumstances of temperature and humidity, this system should improve one's chances.

I've considered the disadvantage of this system to be the extra weight and bulk caused by 1. synthetic insulation and 2. extra layers of nylon.

One more possible disadvantage is that I can picture myself in a Harold Lloyd / Buster Keaton type silent movie as I try to get into my sleeping bag while keeping the quilt over me and continuing to do so as I toss and turn during the night! (But, that might be due to my coordination level)!

I have concluded that it would be more efficient to simply bring a little higher loft, down sleeping bag in questionable conditions than what would otherwise be called for. A little extra down doesn't weigh much or increase bulk much.

On the other hand, I have a thin synthetic quilt that I made for summer camping, so this dual system wouldn't cost me anything! Hmmm..

Note: My viewpoint is from a 3 day trip as opposed to a thru-hike where minor moisture gained each night can accumulate to something significant.

Brian – Relative to Ryan Gardner's question, "Is a bag with 1" of synthetic loft equally warm when compared to a bag with 1" loft of down?

I answered his question using ASTM, C1518 derived test data. This procedure eliminates all of the variables that you mentioned.

Richard,

Ayce over at Thru-Hiker has said that for equal warmth, he would expect that down to be loftier than synthetic clothing, i.e. synthetic insulation is more warm per inch. (See the thread titled "5oz XP rating of 20* with only 1.2” loft?" under the second page of posts in Gearmaker's lounge.) However, you seem to be claiming the opposite. What data are you looking at or how did you arrive at your conclusion? I'm not taking a side here — just confused about what seems like a discrepancy.

Thanks — interesting discussion.

Harold – You asked, “What data are you looking at or how did you arrive at your conclusion?”

I have read posts by Acye in the past. In general I was 99% in agreement with him. He not only understands this subject extremely well, but he also does an excellent job of simplifying his explanations. Ayce said, “The problem of including down in these insulation discussions is that it’s a loose insulation of different qualities and can be stuffed to varying degrees. But assuming a company with responsible temperature ratings, if you compared the loft of similar down and synthetic bags with equivalent temperature ratings the down bag will always be loftier.”

My analysis was that a reputable manufacturer will fill the baffles on a down bag to the density that yields the lowest thermal conductivity (same as highest thermal resistance). I knew for example, average quality down, packed to a density of .25 lb/ft3, tests an average thermal conductivity of approximately .32 K in BTUs. By contrast, at increased density levels from about .5 lb/ft3 to 1.6 lb/ft3 tests the same at about .25 K in BTUs. In laymen terms this means the warmest down insulation will have a packing density approximately twice the highest loft/weight ratio down insulation bag. I assumed a reputable manufacture would pack their baffles to yield the highest insulation efficiency… not just the illusory efficiency derived only from loft. My guess is that Acye assumed that they would pack their baffles to yield the highest loft/weight ratio rather than efficiency/weight ratio.

The best compression density packed 800+ fill power down typically tests at .025 and I used that conservative number in my analysis. Some Guarded Hot Plate tests yield as low a conductive value as .020. The following is a representative public domain Guarded Hot Plate .025 W/m K thermal conductivity value for goose down.

Guarded Hot Plate W/m K Values

To calculate the 1″ m2K/W thermal resistance of goose down from the Guarded Hot Plate .025 W/m K value, I did the following:

1. I took the reciprocal of the conductivity to calculate the resistance for a 1 meter thick block of insulation.
2. I then multiplied that value by .0254 to determine the thermal resistance for 1”
3. The resultant 1″ m2K/W was 1.016

I knew the clo per oz for Primaloft One was .84. I knew 1” of Primaloft One weighed 6 oz and so .84 * 6 = 5.04 clo for 1”. I converted this value to 1″ m2K/W by multiplying times 0.15482 to yield .780.

I then divided 1.016/.780 = 1.30. The 1” of optimal density goose down is 30% warmer than a standard Primaloft One 1” bat.

Richard,
Fascinating chart, and it is making me re-think my concepts of insulation fill. I thought the dead air with its low thermal conductivity was the key ingredient in thermal efficiency of a static system, and down or synthetic fillers only job was to fluff up the layer to maximum loft.
But, your chart and explanation shows that packing more down, past the peak loft/weight ratio, does not improve the loft, but *does* inprove the thermal insulation. What accounts for that? Is that because the additional down reduces the motion of air? Or because down has a lower thermal conductivity than air? Thanks!

Richard – On AYCE's Message Board he responds to a question by saying that commercial bags are typically overfilled by 20%
Ron

Richard,

Reading all of that (and opening up that awesome Power Point presentation) made me feel like I was back in college. Oh wait – I'm at college right now – and my class starts in 15 minutes!

P.S. Keep it coming – this gives me the needed motivation to study my calculus, physics, etc…

Richard,

Thanks for the very clear explanation.

One thing I might argue is that by "maximum efficiency" on this board we usually want to maximize versus weight, not thickness. Compare the points marked "maximum loft" (.32 btu in/hr ft^2 deg F for .25 lb/ft^3) and "maximum efficiency" (.25 btu in/hr ft^2 deg F for .5 lb/ft^3) on your graph. For an equal area and total weight of down, the "maximum loft" will have twice the thickness, and so the heat transfer through the "maximum loft" density will actually be less than that of the "maximum efficiency" density (.32/2 < .25). So to minimize weight (amount of down) we would want a bag or jacket stuffed at what you call "maximum loft" density, not "maximum efficiency".

If we use the "maximum loft" numbers instead of "maximum efficiency", then assuming the ratio is .32/.25 for conductivities, we get something like 0.8 m^2 K/W for 1" of down. This is close to the value you calculated for primaloft one, indicating about similar warmth:loft ratio for down and Primaloft one.

To be thorough I note that thruhiker.com lists 6 oz of primaloft _sport_ having loft of 1.2" and clo of .74. This would give .57 m^2 K/W for 1" thickness. Thruhiker's Climashield XP specs yield .53 m^2 K/W for 1" thickness. Both are less than that of either density of down.

After all these comparisons, it seems that if you use the test numbers you supplied for the conductivity of down, the down will be less lofty or at most a similar loft to equal warmth synthetic. However, AYCE's claim is based on real world experience with his garments — making me wonder if something is amiss with the calculations.

For example, if we take the following of Ayce's comments from that thread:

"Or compare two of my own kits: the Maxima and the Whitney. These are equivalent jackets, the Maxima being the synthetic version of the Whitney. They are both about the same warmth too: over three years use with the Whitney and two for the Maxima they’re comfortable for me in the 30’s just sitting around. The Maxima has a 3.0 oz basis weight layer of PL Sport with a single layer loft of 0.6” (clo: 3.0 * 0.74=2.2) while the Whitney fully lofted has on average about 1.5” of single layer loft. In other words, the synthetic jacket is as warm as the twice as lofty down jacket."

then it means that 1.5" of down loft has clo of 2.2, or for 1" of down 1.5 clo or .23 m^2 K/W — a factor of 4 or 5 different from your calculation! I wonder what is causing such a large discrepancy…