Question of the day?

Maybe those webbed feet are good ground insulation? Windproof outer feathers?

I know of a PCT thru-hiker who used a WM Ultralite and was very cold on the last stretch of her trip. So she's thinking of replacing her bag with another brand. I would like to see her try better ground insulation. Just one 3/4 length closed cell foam pad, which probably smooshed over 5 months, probably had a lot to do with her being cold.

Plus, like the first poster, she may have been wearing too much in too confined a space, actually being counterproductive with efforts to be warm.

This has been an interesting thread. As I sift through the information I see the basic things to do are avoid tight or sweaty clothing, use effective ground insulation, and pay attention to where you sleep (under trees v. open, windy areas).

Personally, I've found putting quality closed cell foam (gossamergear) over my Prolite 3 insulates me from both the cold ground and any cold air coming from the Prolite and I am much warmer in the same conditions than when I had the foam and Prolite reversed.

I like to crunch numbers; I'm a retired accountant. But quite a few of the above posts were beyond me, and I will defer to the superior higher mathematical ability of those posters!

My first lightweight down bag was a Marmot Hydrogen (30*). I started getting cold in it, and having to add outer clothing, at 40 degrees. The night it got down to 24*, I had my base layer (Capilene 4) and all my outer clothing on inside it, including rain gear, on a Thermarest LE (2" thick) pad. I lay awake shivering all night. The places that got the coldest were my hips (I'm a side sleeper) and feet, even though to the best of my knowledge I was not stretching the insulation. I made my dog sleep at my feet, which took care of that problem, but not my hips. It was a three-dog night with only one dog! Two conclusions: I am obviously a cold sleeper, and the Marmot Hydrogen, at least the 2005 model, is a bit over-rated.

I sold the Hydrogen and bought a 20* Western Mountaineering Ultralight Super. There wasn't any weight penalty because WM bags come in "short" for us folks under 5'6". I also changed my pad to a 3/4 length POE InsulMat Max Thermo (now replaced by the Ether Thermo) insulated air mattress. The Max Thermo, like the BA, is rated, ostensibly, to 15 degrees. I didn't have a chance to test either bag or pad to close to their limit until a week of frosty nights in Wyoming's Wind Rivers last August. I didn't have a thermometer along, but on at least two nights my dog's water (in a nylon bowl, out in the open) froze completely solid (that was one puzzled dog in the morning!), and there was ice in my Platypus water bottles which were left under a small fir tree with thick low branches.

One thing I found out after my first below-freezing night was that my body's moisture was evidently condensing, freezing and later melting on the outer shell of my sleeping bag, part of it inside the shell. Since I had non-breathable rain gear (the Brawny Gear rain jacket and pants now sold by BPL), I tried wearing that over my base layer as a vapor barrier. It worked–no more problems with moisture in the bag, and I was comfortable, with no excess sweating and no other additional clothing except a fleece balaclava and fleece socks. There were a couple of nights (the same ones in which the dog's water turned solid) during which I woke up cold. However, once I'd made the inevitable (at my age) excursion outside, I moved around enough to get my circulation going and, once back in bed, was able to warm up and go back to sleep. The main problem was when I woke up, very groggy still, and lay there asking myself if I really had to get up! The draft collar on the WM Ultralight Super really helped–tightening that up often solved the problem. There were a couple of times I wanted to grab my jacket, but I never quite got that far–by the time I was alert enough to go that far, I had to get up anyway. I had no problems with my hips getting cold; generally it was my upper body. My last night out, the cold was underneath me, meaning, of course, that the pad was marginal. If it had been cold enough to keep me awake, I'd have grabbed my sit pad, cut from a GG Thinlight 1/8" pad (my dog uses the rest of the pad) and put it under me on top of the air mattress. I will put it there to start with on potentially frosty nights from now on.

I do recommend using the BA Pumphouse rather than blowing up your pad, to keep moisture out. However, while it's a great pump, I found the Pumphouse to be a failure both as a pillow and as a dry bag–it leaks both air and water. I use it as a stuff sack, with a turkey roasting bag inside to keep my sleeping bag dry, and the combo is a little lighter than a dry bag.

I've read, also, that when enough air is let out of an insulated air pad to make it comfortable–I pump mine only about half full–that it degrades the insulation a bit. I don't know if this was the problem or whether the pad would have been as marginal if inflated rock-hard. But I wasn't lying awake shivering for long, so it didn't affect my sleep. Next time, I'll take a thermometer!

It does help to be able to air out a down bag every few days. I was able to do it daily in the Winds. If it's really wet, with lots of condensation in the tent, for days on end, the insulation is going to degrade. Kathleen, I also suspect that after 5-6 months your friend's sleeping bag insulation was a bit dirty, which also degrades the insulation. But you're right; it's lots less expensive to upgrade the pad first!

Hey, hikinggranny- Spot on with the dirty bag. For a while I had a thing about not wanting to wash a bag, being afraid of stripping the natural oils off the down. Well, some odd years later I was distraught because my bag wasn't as warm as it used to be. Turns out it didn't loft nearly as well, either, and there were several clumpy spots. I had a summer bag that I had quit using because there was so much clumping, etc.

Bottom line, I washed the bags. Still bad loft. I ended up washing them three times, drying thoroughly, and they were literally good as new. The weird thing to me was that I never really noticed the loss of loft along the way.

A couple of thru-hikers I know had the same problem as your friend–just gotta find a laundromat!

Mike and Jim,

Of course the Y intercept is above zero. Primarily the conductivity of still air and to a much lesser extent, the conductivity of the fiber itself defines the Y intercept. In other words, no fiber fill insulation based on still air can have lower conductivity than still air.

I suspect we are boring the majority of the forum participants with the detail this topic has drifted to. Although I am on a month long trip and won't get to a computer for many days at a time, please email me via the forum if you want to discuss this topic further off line.

Posting to be able to return to this great discussion in the future. (This post will be listed in "My Account–Change My Forum Profile". Is there another way to do this? "Watch this" doesn't quite work, if nobody posts again.)

So this doesn't get too academic, is the bottom line that we should have Nunatak's Tom overfill an ounce or two or three?

Density makes sense for warmth, but I've always been focused on loft. This has been a helpful string.

James –

'Bookmark' it.
And give it a meaningful name.

g.

I've thought for a while that some UL bags are under stuffed. With really good down, and very light shell fabric, they can loft so well, with so little down, that there are essentially big air gaps running through the baffles allowing convection.

On a different point. I need a warmer mat when using a down bag than when using a synthetic because the down compresses underneath more than synthetic insulation. This caught me out when I first switched to down.

I asked in another thread: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=37434
about multiple sleeping bag layers. I'll give a few more specifics to see if I can really muddy things up. Let's say I'm looking at a Marmot Lithium. This is a 0 degree bag, and weighs 3 pounds.

I'm also considering a WM Highlite (1 pound, 35 degrees) and a Marmot Helium (2 pounds, 15 degrees). I should be able to get to ~ -3 F just off of the insulation if I understand correctly (better than the Lithium alone at the same weight). Is there also a benefit to the fact that I have two surfaces next to each other (the outside of one sleeping bag inside the other)? If so is this significant enough to matter?

I hear lots of statements that a silk liner will improve the functionality of the sleep system by ten degrees (approx). What kind of R value (or col) would that make silk have?

I've also seen people say that bivies (I have an Integral Salathe) will give you 10 degrees more warmth as well… Is this actually science?

If anyone reads this, and answers, I thank you in advance for any answers provided, or heckling for my obvious lack of understanding of thermodynamics (although I do like the math formulas, and, if this is a function, wouldn't a calculus formula be better suited to identifying it)?

Everett,
I understand the concept behind the down compression thing, but I'm not really too much of a numbers guy to steadfastly work the numbers out.

However, I can comment on silk liners. IMO, a 10 degree boost is VERY liberal.

Many manufactures of any product like to inflate their numbers. Especially when it comes to the issue of warmth. The worst offender is the Sea to Summit Reactor liner that claims a 15 degree boost. I had the liner and promptly returned it. I have NO clue how they came up with a 15F boost. 3 degrees at most.

From my own personal experience, liners made from silk are going to be your best option considering weight, bulk, and warmth. Really though, do not rely on any liner for warmth. What I mean is, if you expect temps of 10F, the get a bag conservatively rated for that. Do NOT rely on a 20F bag and hope to get a boost from a liner. That's asking for trouble. Yes, liners will add some warmth, but only like 2-5 degrees. Their main purpose is to help keep your bag clean, not keep you warm.

I don't use bivies, but I can be pretty sure they'll help "boost" the temp rating of your system much more than a liner will.

There's been a fundamental misunderstanding of Richard N.'s original data on this thread for some time.

1) For a given quantity of Down, the maximum warmth will be achieved if the Down is allowed to fully loft. Compressing it will reduce its warmth (period).

2) However, in a Down-filled finished product (such as a sleeping bag or jacket) a lot of the weight is in non-Down components. So, the warmth per weight ratio of the finished product can be improved (or at least not diminished) by "overstuffing" and increasing the density of the Down. In this case, overfill still reduces the total Clo of the Down compared to what it would be uncompressed, but the finished product will still be warmer than one with less (and therefore less dense) Down fill. Depending on the weight of the shell, zippers, baffling, etc. the overstuffed product may have a higher warmth per weight ratio as well.

3) As the weight of the non-Down components of a finished product decreases (e.g. through lighter shell and baffle materials), the Down density has a greater importance on the overall warmth per weight ratio. If the shell and baffles were weightless and you could stuff the Down to any density you wanted, the highest warmth and warmth:weight ratio would be achieved with the largest loft and least compression.

Michael,
All equations and extrapolations aside, what does this mean in the practical world? Given the final product (two sleeping bags), can they be layered without much concern of insulative effectiveness dropping? It would still seem to me that the compression created by layering sleeping bags is negligible concerning temp ratings.

> do not rely on any liner for warmth.
True.

> Their main purpose is to help keep your bag clean, not keep you warm.
Mostly true.

What a silk liner can do quite well is to block cold drafts. There are times when this can be very helpful, and may give the impression of a greater boost.

Cheers

Hi Roger,

Would you say it can be beneficial for a quilt user in that situation?

I haven't tested two bags in the field, so this answer won't help you much. :(

Nevertheless, in theory, there are several factors in play with two bags:

1) The combination will cause some insulation compression, reducing the total Clo over that of the sum of the two separate bags.

2) The additional shell materials in the middle of the bag sandwich reduce air permeability, which can add some warmth via a partial vapor barrier effect. (Unfortunately, this may lead to increased moisture accumulation inside the inner bag.) The additional shell materials can also create a small air pocket between the bags adding more warmth.

3) Compared to a single heavy bag, two bags can be separated for drying and will dry faster, which can reduce moisture accumulation on long trips.

4) Offsetting the moisture issue in (2), above, the dewpoint will problably move into the outer bag which will reduce condensation/moisture buildup in the inner bag.

Again, I'm sorry this doesn't directly help you or answer your question. I haven't field tested a two-bag sandwich, so I don't know whether these warming or cooling effects are larger.

"There's been a fundamental misunderstanding of Richard N.'s original data on this thread for some time.

3) As the weight of the non-Down components of a finished product decreases (e.g. through lighter shell and baffle materials), the Down density has a greater importance on the overall warmth per weight ratio. If the shell and baffles were weightless and you could stuff the Down to any density you wanted, the highest warmth and warmth:weight ratio would be achieved with the largest loft and least compression."

Good post. I think this will clear up a lot of confusion in this thread.

Mike, what Richard actually says is:

"Western Mountaineering and other manufactures targeting UL backpackers manufacture their sleeping bags with the lowest possible density of down. Their density is 2.16 kg/m^3 which yields ~ 2.556 clo/inch of loft….The optimal fill density for 800 fill down is 24 kg/m^3 which yields ~6.562 clo/inch of loft. Some manufacturers offer "over stuffing" options of 2 -3 ounces for a nominal additional fee. Most UL backpackers don't realize that with the 2 – 3 ounces of "over stuffing" they have moved the down density toward the optimal 24 kg/m^3. Yet they are still no where close to down’s optimal density for warmth."

So from this I take–and would welcome any correction– that down "fully lofted" is not necessarily "optimally lofted", and in fact may contain a relatively high percentage of unconstrained air, which can move heat by convection. If you do a thought experiment, and imagine a given weight of down floating around in a zero gravity environment, you can see that, if the volume the down is trying to fill is large enough, the down is doing almost nothing to impede the flow of heat. Of course we don't live in a zero g environment, but we can't assume that the compressive force of the gravity that we do live in somehow optimally compresses the down.

I imagine that, in a bird, the "guard" feathers help to constrain the down optimally, compressing it when the temperature drops, and letting it fluff when the bird needs to dissipate heat.

Elsewhere I seem to remember Richard saying that down only starts to lose insulating efficiency when it is compressed to the point that the fractal endpoints of each cluster start to touch those of their neighboring clusters. Again, could be wrong about that.

So in sleeping on a pad, though the down directly under us might be compressed to the point of relative uselessness, as one moves laterally up the sides of the sleeping bag or quilt tube, one might be in a realm where the down is somewhat, or even quite, compressed, but still useful as insulation. Sleeping on ones side would minimize the "useless" zone.

There is a question in here somewhere, and I'll take my answer off the air :>).

So the debate goes on….

The thermal conductivity vs density graph in this post:

http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/12505/index.html?skip_to_post=94791#94791

from Richard supports my prior statement that for a given amount of down, fully lofted provides the highest total Clo. You have to look carefully at the data in the graph, though. Thermal Conductivity "k" is given PER INCH THICKNESS in the graph. So doubling the density has the effect of halving the thickness for a given amount of Down.

Taking the "max loft" and "max efficiency" data points at 0.2, 3.1 and 0.5, 2.7 respectively shows that the "max efficiency" point has 2.5x the density (and therefore 40% of the loft) of the "max loft" data point. Adjusting the thermal conductivity for the reduced loft shows that the "max loft" data point has much lower thermal conductivity than the "max efficiency" point for a given amount of Down.

Another one of Richard's charts (sorry, I don't have the link handy at the moment), shows a "generic" mummy with 3, 6, 9, and 12 ounces of overfill. It appears to show that the Clo/Kg ratio remains at 4.0 regardless of the overfill. But, if you analyze the data closer, you find that this is only true due to the weight of the non-Down components. Adding a column to Richard's chart to show the Clo/Kg of just the insulation shows that the insulation becomes less efficient as the bag is overstuffed:

The bottom line is that the highest warmth:weigth ratio stuff can be made by letting Down fully loft in the lightest shell materials possible. :-)

"So from this I take–and would welcome any correction– that down "fully lofted" is not necessarily "optimally lofted"

I think this is wrong. Down 'fully lofted' is 'optimally lofted' in terms of getting the most warmth out of that down.

The discovery Richard Nisley has been saying is that you can compress that down somewhat and not have a huge loss in warmth because the clo increases *almost* as fast as the down is compressed. So somewhat compressed down isn't way worse that 'fully lofted' down. Essentially, compressing the down by half will increase the insulation value of that down by almost double, so the net result is almost as warm. Conventional wisdom is that compressing your down sleeping bag by half would result in much colder bag. This relationship holds to a certain point, but not forever. Compression the down by 100x wouldn't lead to an R-value nearly 100x higher.

So as an example, lets say a fictional sleeping bag has 4" loft and it's packed loosely so it allows the down to fully loft to 2.16 kg/m^3. As we know at 2.16kg/m^3 the clo value is 2.5 per inch. That gives our bag a total clo of 10 (4" x 2.5 clo/inch).

Now if you were to compress this sleeping bag to half it's loft (ie. stuffing it in a small bivy) the loft would now be 2" but the clo value would nearly double but not quite. It would be somewhere around clo 4.5. Your new total clo for the sleeping bag would be roughly 9 (2" x 4.5). As you can see, it's not as warm as it was when fully lofted, but it's not way off.

When you consider the weight of the sleeping bag shell, it may then become optimum to have down somewhat compressed because the weight savings from having a smaller, lighter shell may overcome the small drop in down efficiency. For example, my GoLite Ultra 20 sleeping quilt weighs 19oz and it contains 9.5oz of down, so the down is just 50% of the total weight. This is roughly the ratio most high end sleeping bags have.

Lets say winter is coming and I want a much warmer sleeping quilt. My two options are:

1) Buy a much warmer fictional 30oz quilt with 15oz of down
2) Stuff more down into my existing quilt.

Stuffing more down into my quilt will result in the down not being optimally lofted, but it also allows me to avoid the extra shell weight associated with buying a higher lofting sleeping quilt. Stuffing my quilt to a total of 15oz of down wouldn't be as warm as the sleeping bag containing 15oz of fully lofted down because it's a little bit less efficient, but it wouldn't be far off. I might need 17oz of somewhat compressed down in my quilt to equal 15oz of fully lofted down. So if I stuffed another 7.5oz of down into my bag to get to 17oz it would bring the total weight from 19oz up to 26.5oz. That's lighter than fictional 30oz sleeping bag with 15oz of down and it should be roughly as warm.

Buy a much warmer fictional 30oz quilt with 15oz of down

Not exactly fictional.

http://katabaticgear.com/shop/sawatch-sleeping-bag/

Dan, excellent point about the weight reduction one would get from compressing the down somewhat and reducing the total area of the outer shell (and baffles). It would be interesting to see some numbers for that, even if they were the result of a theoretical analysis.

And yes, the Katabatic Sawatch does give the impression that it's quite well stuffed–it manages to include the same amount of down as other very good products in a quilt with a smaller surface area, with the result that it has a very high down weight/quilt weight ratio. Haven't tried it out in cold weather yet, but looking forward to fall.

Yeah the Sawatch is a super nice quilt. Katabatic is making awesome products. It's really impressive that 64% of it's weight is down, but I guess higher ratio's are easier to achieve with warmer bags/quilts. You'd never get to 64% with a 40F quilt since you need almost as much shell material but way less down.

The shell of the Sawatch is just 8.5oz (23.5oz – 15oz down). The 15F rated Katabatic Palisade shell is 7.8oz (17.0 – 9.2oz down) so there isn't a big increase in the weight of the shell. I guess this does make sense because the inner fabric would be the same and the outer fabric wouldn't be much more either…just the baffles would be a bit taller.

Since the Sawatch shell is only 0.7oz heavier than the Palisade shell, it's likely more efficient overall to use the larger shell (rather than overstuff the Palisade with 6oz of down) but it's probably close.

I'd love a Katabatic quilt. I just need to thick of a way to justify one :)

Thanks Michael! I noticed this discrepancy too.

Using the data that 2.16kg/m^3 (~1oz/800in^3) of 800fp down yields 2.556clo/inch (what area is this number based off Richard, m^2?) I found the loft of the generic mummy to be ~1.76 inches (I know this doesn't factor in the clo of the shell material, but I'm working with the data I have at hand to make a ROUGH estimate). I extended your chart to show various % gains/losses in clo.

I used a modified quilt weight calculator that I found on the enlightenedequipment.com website. I double checked it using the specs for the 17oz quilt instructions on thru-hiker.com I came up with some measurements and a taper that gave the approx the same amount of fill and loft as the thru-hiker quilt, using same weights of materials. The enlightenedequipment model gave me a weight of 18.08 oz for 10.125 oz of down and 2.5 inch baffles, which for these purposes verified the model accurate enough.

Using that model I got a a fabric weight of 7.96 oz (1.1oz shell/liner, .7oz nanoseeum baffles). Increasing the loft to 4 inches and down to 16.2oz gave a total weight of 24.43 oz and a fabric weight of 8.23 oz.

If we were to overstuff a 2.5 inch baffled quilt with 16.2oz of 800fp down, we would be 160% the FLD density, which according to my chart above would give us an clo increase somewhere around 20%. I'll be using Richard Nisley's chart on clo from this thread:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=9378&disable_pagination=1

So according to that chart, a Fully Lofted Down quilt of 800fp at 2.5 inches should give a clo of 2.5*2.556*.98=6.26. This is subjectively warm to ~30F (consequently the thru-hiker article claims 20F). Increasing loft to 4 inches gives us a clo of 10.02, subjectively warm to roughly -10F (thru-hiker claims 0F). The overstuffed bag is ~20% warmer by clo, this gives us ~7.5clo which is warm to 18F.

By overstuffing we save ~.25 oz and gain ~12F in warmth; however, by allowing full loft we would take a quarter oz weight penalty but potentially gain 15 to 25 degrees in warmth. The situation is even worse when looking at lighter materials (Momentum shell/linger @.9oz and cuben for baffles).

A few caveats, this is based off the clo/kg results from some proprietary research that Richard has access to, therefore without all the raw numbers I can't be sure how the clo truly changes as density changes. Overstuffing does have a few common sense benefits that the data can't represent. One example is that an overstuffed bag is less likely to form cold spots from down shifting, which is especially useful for side sleepers that notice their quilt/bag gets thin directly above their hips and shoulders. Finally I could have made some egregious error of calculation/statistical manipulation that I didn't catch and if so, chastise me as soon as possible.

Richard, I've followed a lot of your posts and you have some amazing insights into insulation. I just have to disagree with you on the topic of compressing down. I think you took the 5 data points and made a linear regression based off them as per this thread:

http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/12505/index.html?skip_to_post=94791#94791

This works well (R^2 of ~.983). However there are better regressions. Namely I did a quartic regression (4th degree equation for those confused) and got an r^2 value of 1 on my calculator (can't get closer than that!). The CUBIC regression even shows that clo increases until around 1.08kg/(generic mummy volume) (~2.4X density) before it starts declining rapidly which reinforces some of your statements on the benefits of compression, and that curve has r^2=.999871.

The problem is clo/kg. The data shows that clo/inch increases slower than density increases. For the lightweight backpacker, it makes more sense to compare on based on performance/weight instead of inch. Warmth/inch has it's purpose, to find how much insulation a particular item provides. It's should not be used as a comparative benchmark on performance though.

EDIT: Cubic regression predicts 2.4X density knee in thermal resistance, not quartic as previously stated.

If you have one data point, an infinity of lines of all sorts will pass through the point and there are no errors in the fit between the line and the point,

If you have two data points, you can pass a single straight line between them and there are no errors to the fit. The correlation is perfect.

If you have three data points, and you pass a quadratic line through all three of them, there are no errors to the fit. The correlation is perfect.

If you have 4 data points — a cubic equation passes through all of them and you have no errors and a "perfect correlation".

If you have 5 data points and you pass a cubic equation through all of them you have no errors and a "perfect" correlation.

I remember an exercise in the MatLab training manuals on the perils and limitations of ever higher degree of polynomial fitting function. The higher the degree of the fitting polynomial, the more wild the extrapolations beyond the limited experimental data range become. Also, the higher the degree of the polynomial used for fitting, you can easily pick up false wiggles of the fitting line as it passes through the data.

The danger point is reached when you get a "perfect correlation" and fail to actually plot the data and the fit together. You will most certainly not like the funny behavior that can be often seen.

Regarding this issue of massive compression of down being possible with no loss of actual warmth. It is a remarkable claim.

There is a saying in the sciences perhaps a hundred years old.

"REMARKABLE CLAIMS DEMAND REMARKABLE PROOF".

Carl Sagan said it this way : “Extraordinary claims require extraordinary evidence”.

We haven't seen REMARKABLE PROOF for the REMARKABLE CLAIM.

The amount of down data presented during this discussion has been limited, and only a couple of points are of the 800 loft down, and none of them are a graph of the thermal conductivity of 800 fill down vs the density resulting from compression.

The discussion has hinged upon, at times, the very details of the curvature and shape of the 800 fill power down Thermal Conductivity vs Compression (density) curve, and the curve has never been presented.

I have friends who would call this preposterous, and others that say, "Well, what do you expect of those untrained in science research?"

After all, most are here because they love backpacking. Few are here for the "science" details.

There is little data. There is no "Remarkable" or "Extraordinary" Evidence. The claim was "Remarkable".