Scientific Way to Compare R-Value to Down Insulation?

[John Papini]

Hello,

Still hammering out my kit for a hike with my S.O. and wondering how I might go about tackling this question:

The difference between a 10 degree quilt and 20 degree quilt is approximately 5 oz of extra down. The difference between a 3.3 R value pad and a 5 R value pad (Hyperlite Synmat Duo / Winter Duo) is 8 oz. How can I compare the warmth to weight ratio of R value and down?

My intuition is that it would be better to get the warmer pad, which will come most in handy in low temperatures when the ground is cold, but won’t heat us up when it’s warm as much as extra down will. But I really don’t know. Is there a way to determine that 1 R value = x ounces of down in terms of warmth?

Any thoughts?

[Jerry Adams]

whether you’re better off with more pad or more sleeping bag is complicated

there’s more surface area losing heat through the quilt so more R value will prevent more heat loss

it depends on how much R you have currently on your pad and quilt.  For example is your pad R value is small and your quilt large, then it would be better to increase your pad.

R value in the pad is heavy – it has to keep the R value when you press down on it with your body.  R value in the quilt is light – it only has to hold up it’s own weight.

[Eric Blumensaadt]

Look at insulated sleeping air mattresses. There are several brands out there but steer clear of Big Agnes insulated air mattresses because there have been many bad reports on them.

SUPPLEMENTING R RATING ON YOUR MATTRESS:

1. Put 6″ of dry leaves or small evergreen boughs** under your tent floor. (This works very well)
2. Lay your day clothes under your mattress (pants, shirts, vests, etc.)
3. Lay a closed cell mat like a Thermarest Ridgerest under your mattress.

**Yeah, the “small evergreen boughs” is not great LNT but they can be scattered when leaving camp and for penance you should donate $50. to Earthjustice, etc.

[Todd T]

Jerry is right:  it’s complicated.  But my experience–and I think this reflects most of the testimonials I’ve heard–is that an R3.3 pad is adequate for virtually all summer hiking.  Even if nighttime temps drop below freezing, the ground generally won’t be that cold.

If worried about overheating on mild nights, don’t upgrade the bag, and simply wear extra clothes on colder nights.

P.S.  I think the most important reason for LNT rules is preserving others’ opportunity to experience the beauty of nature, not preserving the health of the environment.  Few things a backpacker can do will have a measurable effect on environmental health, but many things, including cutting evergreen boughs, will ugly up the place.

[Eric Blumensaadt]

I’ve used my Prolite Regular in my tent, on snow at 15 F. with outer clothes (pants, etc.) beneath it and been “OK”, as in not actually cold.

But now with my new REI Insulated Air Mattress at R 3.7 I think I’ll be good to near 0 F., again with some outer clothing beneath it. I also have a mummy-shaped 2/16″ closed cell foam underlayment that I can bring. I’ll be testing that combination at about 9,000 ft. this winter IF the REI mattress alone is too cold.

Using my -20 F. down bag kinda throws off that “test” I suppose. But I am not using my 20 F. WM down bag and puffy clothes at 0 F. just to test a mattress. (I am for short-suffering, if any.)

[Paul McLaughlin]

I don’t know if there is a truly scientific way to compare the two, but if there is it would be a pretty complex equation, with numerous variables. Just for instance: let’s say you are on a late summer trip. Your campsite (a smooth granite slab) has been in the sun all day. You may find in the morning that the rock underneath you mattress is still warm to the touch. In a situation like that, the warmer pad would be a complete waste, while the warmer bag might be nice. Contrast that with a late spring trip, with the same nighttime temperatures, but you are sleeping on the remains of the snowpack. Now the warmer pad starts to look pretty good.

These are the extremes, but they point out that the ground you sleep on varies and how cold it is varies despite the nighttime temperatures being the same. Thus a scientific comparison is probably not really possible.

But my gut reaction, for sleeping on the ground and not snow, is that I’ve never felt cold from below while using any pad that was at least R2.5, while I have been cold on top at times. I will mention a specific trip, a few weeks ago in the Sierra at around 11,000 feet. All nights were around 30 degrees except for the last which was 20.  That last night I did get a bit chilly in the wee hours, but only felt that on top, not from below. Pad-wise I was just as warm as the other nights; I suspect the ground temperature was not much different despite the lower air temperature. A few extra ounces of down would have been welcome; as it was I had to slip on another shirt. Now this was late summer; deeper into the fall , with cooler days and colder nights, things might be different, and if you hike in an area where winter does not mean a snowpack, that again could be different. But for summer conditions, I suspect the ground temperature does not vary nearly as much as the air temperature.

So I’d go for the warmer quilt, as long as you have a pad that is at least R2.5.

[Jeff McWilliams]

Please don’t suggest that people cut evergreen boughs to put under their sleeping pads.

1.  Like most trees, evergreens grow from the top up (and out).  What you cut isn’t going to grow back.
2. The National Parks alone had 330 MILLION visitors in 2016.  150 MILLION USFS visitors, and 800 MILLION State Park & Rec Area visits.

If everyone who camped followed this terrible advice, designated and dispersed camping areas would be stripped bare of pine branches from ground to 7 feet high.

 

[Matt Dirksen]

“I don’t know if there is a truly scientific way to compare the two..”

Since sleeping bag ratings can be converted into a CLO value, and my understanding is 1 CLO is about .88R (or 1 R = 1.136 CLO), one could use that as the common unit of measurement.

Either way, I suspect that adding more insulation underneath is always a good thing. Remember that sleeping pads restrict the flow of your heat. They don’t actually “make you warmer”. So even a pad with a higher R value in the summer isn’t really a bad thing, because it’s not making you any warmer. It’s simply restricting your body’s heat from reaching thermal equilibrium with the ground.

Matt

[James Marco]

R values are from construction. These are for fixed walls, celings, above ground floors, etc. CLO is more for clothing. There is really no good way to make any equivalency from a downs warmth while camping (which can vary greatly, depending on the user, humidity, etc) and the fixed, dry values in a wall cavity. I will qualify that and say that I have heard of. There might be better insulations than down (aerogel for example) but none have the combination of characteristics that make down so valuable in a camping situation. In a wall cavity, cheap, fiberglass is usually used, but crush it up a few times and it isn’t very good anymore. Same for most aerogels, though they are far more expensive. It is the compression and weight that really give down the edge. Durability ranks far less in my estimation, but still down has excellent durability. These factors alone make for a good bag. Now if you need a good pad that is fairly comfortable to sleep on, an inch of CCF works pretty well (R value around 4.5 or so) down to about 20F, colder and you need something more. I like many others, use a CCF pad over a NeoAir (that has a radiant barrier inside.) You get a total R value of around 5.7-6.0 down to about 0F. Colder simply requires something more, like another CCF pad. Down doesn’t work all that well due to condensation (in any container there will be quite a bit in warm meeting cold,) compression (lowering R value a lot) and gravity (making it drop AWAY from the sleeper.) Just my thoughts…

[Eric Blumensaadt]

OK Jeff, I’ll amend my  “…cut small evergreen boughs…” slightly. I’ll say that if you absolutely must have more insulation beneath you and there is no other recourse then use evergreen boughs.

I routinely use two or three small evergreen boughs in front of my tent door in rainy or snowy weather. I do not cut them from the same tree or even the same view area but I do use them and then scatter them.

I’m not claiming to be a LNT purist, just one who, in certain weather conditions, makes a small impact. I’ve done a lot of work in my life, and still do, with groups like Friends of Nevada Wilderness, Sierra Club, etc, in habitat restoration so any tiny non-LNT thing I do is far offset by my “good works”. So if my recommendation, thus qualified, is still not PC enough well, I ain’t PC. Never  was.

[Matt Dirksen]

“There is really no good way to make any equivalency from a downs warmth while camping (which can vary greatly, depending on the user, humidity, etc) and the fixed, dry values in a wall cavity.”

James,

I don’t quite understand this statement.

While R values have to do with building products and CLO have to do with clothing, they both have to do with thermal resistance and are ultimately measured with m²K/W. 

Therefore, one could examine the “R value per inch” of both the pad and the quilt, and then derive warmth to weight ratio. But obviously, it has to do with what FP of down, as well as what kind of sleeping pads to compare.

Are you speaking more about all the variables affecting sleeping pads vs quilts, and how they are different from each other?

Best,

Matt

[James Marco]

Matt, No, not really. Humidity has a rather drastic effect on down as one example I alluded to. A damp, but not wet, down bag can loose as much as 40% of its loft and perhaps as much as 30% of it’s insulating value vs a dry bag. Compression makes a difference, too. In a standard, measured wall cavity, the R value is fairly well known within a couple tenths of a point. It will stay that way for years. Compression on say a jacket will change with your clothing, both under it and over it. Again, this can reduce loft/insulating value and is not measuered since it is not known how the user will be dressed…with or without rain gear, sweater under, vest under, wet rain, humidity levels from sweat, and so on. People respond differently to various heat levels/humidity levels based on your past 24-72hours of diet. There is individual heat/cold tolerances. All this tells me that R values are pretty much meaningless except as a very rough number. Those numbers are NOT fixed between individuals. Hence, logic says there is no good equivalency. Even with the same person, hiking for a week at 90degreesF and suddenly dropping to 60degreesF will make you feel cold. People adjust to temps. The dry cold warmth of down will be much higher than if he is used to 40F and suddenly goes into 60F weather. It is not fixed. I do not trust these for bags or clothing even if there WAS an easy conversion.

In the side-wall of a house, you have a vapor barrier, eliminating humidity. There is no variable compression of the wall cavity. There is a fixed value to sheathing, siding, infiltration, radiant, and other heat losses. This is a simple problem in comparison to a bag or a piece of clothing. And, there is no individual adjustment. It is straight up btu in/btu lost to give heat capacity of a furnace at any given temperature for example.(Yes I know there is occupancy, timing, usage, and overages, too.) You cannot do this at an individual level, though. Even a guestimate is going to be off by 30%-59%… something you would not tolerate if your house insulation was even 30% off. Even with a exact scientific number, it is meaningless because of the individual is going to feel hot/cold, it isn’t a simple matter of cranking up the heat a degree or so in a house.

Pads are different. Below freezing, you can generally count on snow as a good insulator. So, the area below you is going to be around 25-30F once you are in bed for a little while. It is possible to calculate an R value for that, but it will still be at least 25% off one way or the other. Some will be fine with R4, some will want an R6 or more. Again, individual preferances almost negate any real meaning, but as a standard value, CCF is pretty good.

[Jerry Adams]

R, clo, and m2K/W are just different units for the same thing, same as inches and cm

in addition to what James said, if you overstuff down, you will lose a little R per ounce but R per inch will increase.  If you overstuff by about 30% then you’ll avoid thin spots in a baffle but only lose a little R per ounce which is what lightweight backpackers care about.

R per ounce for down is about twice R per ounce of synthetic like Apex

for some reason, people don’t like to talk about R or clo per ounce, maybe because of the complications

to be more accurate, it would be R/oz/yd2 or clo/oz/yd2, not R/oz or clo/oz

[Eric Blumensaadt]

I think the new down DWR treatments like Dry-Down and Down Tech must be taken into account when discussing down’s absorption of humidity.

I have an Eddie Bauer 1st Ascent vest with Dry-Down DWR and a Down Tech treated LL Bean -20 sleeping bag. The bag was aired each morning for 2 days by stuffing in its sack it as soon as I got up then pulling it out and airing it. This got much of the moisture out right away and airing it in the sun helped evaporate some more. It’s my regimen upon crawling out of the bag. So far so good on 2 to 3 night camps. No noticeable reduction in insulating value in barely sub zero nights.

The vest was worn only at camp when I was more sedentary but never seemed to get cooler over the days. Of course that’s just a subjective observation. At night I hung it on my pack which was in my vestibule. Not the driest place but a bit better than inside the tent. However sometimes I stuffed it inside a dry sack for a pillow. That too did not seem to affect the insulating effect.

Got more testing to do with the sleeping bag this winter. Trying for a 5 day test.

 

[Paul McLaughlin]

To circle back to the original question:

can we compare the relative value, with weight being the cost and warmth being the benefit, of more down in a quilt versus more warmth in a pad? Matt states:

“Since sleeping bag ratings can be converted into a CLO value, and my understanding is 1 CLO is about .88R (or 1 R = 1.136 CLO), one could use that as the common unit of measurement.

Either way, I suspect that adding more insulation underneath is always a good thing. Remember that sleeping pads restrict the flow of your heat. They don’t actually “make you warmer”. So even a pad with a higher R value in the summer isn’t really a bad thing, because it’s not making you any warmer. It’s simply restricting your body’s heat from reaching thermal equilibrium with the ground.”

I have no dispute with any of that, but I thought we were looking at which is the most weight efficient way to add warmth. And to do so we need to look not merely at the relative insulating capabilities of the materials, but also at what we are losing heat to. And so I would still make my guess on the bag side, for non- snow use. Here’s another way of looking at it: Say it is a summer night in the alpine regions of the Sierra. The air temperature is in the 30’s. The ground temperature will be warmer – I will say in the 50’s as a pretty good guess based on my experience. So heat loss to the air is bound to be more than to the ground – especially since we have more area of bag exposed to the air than we do pad exposed to the ground. So, warmer bag probably is more efficient for the weight than warmer pad – regardless of what measurement units we choose to use. Change the variables to a sleeping on snow situation, and we may have a different picture – although we still need to be careful, since sleeping on snow in the spring with the air temperature in the 20’s is a different ballgame from sleeping on snow in deep winter with the temperature below zero fahrenheit. And snowpack temperatures can vary.

As for evergreen boughs, does anybody have a good R-value for them, and what species of evergreen has the best R-value? And then we should consider the weight of the guilt (yes, guilt not quilt) you have to carry around afterward. I think I’ll stick with carrying a warm enough pad for the conditions. But dry leaves found on the ground, on the other hand, if you are in a forest situation, can be piled up and later redistributed at no cost, either environmental or karmic. Go for it.

[Eric Blumensaadt]

Paul,

I think we also must discuss fill power (oz./cu.in. or metric version). A 750 fill weight down garment will resist compression more than a 900 fill weight garment. In some situations this may be advantageous, such as the back of a down jacket where packs will compress down and insulation will be lost. Also at the shoulders, even when no pack will be worn, the garment weight itself will compress the shoulder area to a degree, especially in down garments with an “overshell” that may be BWP laminate material in a heavier parka for arctic areas and down parkas made for workmen.

So “zone” fill weight differences may be better than using all the same fill weight in a garment. This has been done in a different way in some sleeping bags were synthetic fill was used on the bottom side and down on the top side.

That means it is important to look at “compressed down CLO values” with a series of weights : surface area ratios in order to develop some uniform compression standards before measurements are taken.

 

[John Papini]

Looking at a previous post about CLO in a down jacket (https://backpackinglight.com/forums/topic/73153/) it seems that 950 fill is equal to 1.98 CLO/oz. Assuming Matt to be correct, 1 R = 1.136 CLO.

Using these numbers, there’s no question that adding 5 oz of 950 fill has a better warmth to weight ratio than the warmer pad. 5 oz * 1.98 = 9.9 CLO (at a weight penalty of 5 oz). Adding 1.7 R would yield 1.7 * 1.136 = 1.93 CLO (at a weight penalty of 8 oz). This is a huge difference!

Do we agree, or am I misunderstanding the way to calculate CLO? I recognize and agree as stated above that conditions will impact both actual and perceived warmth, but I’m just trying to get a general idea of what would be more weight efficient. Sounds like the warmer bag is the winner?

Thanks for your help,

[Jerry Adams]

clo/oz is actually clo/oz/yd2

a quilt has maybe 3 yd2

adding 5 ounces would add maybe 3.3 clo

but then when you calculate the benefit of adding R value, R on the quilt results in about 3 times the benefit because the quilt has about 3 times the area as the pad, so your conclusion is correct

except heat loss = area * temperature difference across the insulation / R value of the insulation

the temperature difference for below you depends on the ground temperature which warms up a little over the night because of body temp, usually less than temp difference for above you

and you want the R value for below you and above you to be somewhat balanced.  If the R value below you is small, a lot of heat will be lost.  In that case you’d be better off adding to the R value of the pad.

But, like you calculated, R value of a pad is much heavier than R value of a quilt, so you would want a smaller R value in the pad than above you if you were optimizing weight.

Since it’s so complicated maybe better to just have a heuristic, like choose a lighter pad for summer, heavier for winter, then chose a quilt that keeps you warm, get a heavier quilt if you’re not warm enough.  If you sleep on snow get a much heavier pad.

If you add down, you should increase the baffle size so you don’t compress the down more than about 30%.

[Eric Blumensaadt]

So down “fill weight” in terms of resistance to compression has no effect on CLO value??

[Jerry Adams]

fill power just means it takes fewer ounces of down to fill the same volume.  I suspect the clo/inch is a little less for higher fill power down, but clo/oz/yd2 is still better for higher fill power.

Maybe if the fill power is 10% higher, the clo/oz/yd2 is only 5% bigger.

[James Marco]

Yeah, I agree with Jerry. But I think the overall difference might be more with higher fill downs (900+FP) than with low fill power downs (<650FP.) Low FP downs do not suffer as much from compression(assuming dry compression) and humidity(RH) changes. I believe Ryan Jordan did a study on this several years back. Looking at ONLY loft at a fully dry value (say 5% RH) you find that compression from shell material (and other clothing in the case of jackets/vests/rain gear/etc) will more quickly reduce lofting with high fill power downs. A 500FP down(about equivalent to top quality, new synthetic) is little effected. Again, as I have said in the past, loft is not the ONLY measurement for the insulating value of down, but comprises perhaps 60% of a downs insulating value.

The treatments for down, such as dry-down, do NOT effect condensation within the down. For example, a windshield on a car can be treated with rain-guard, but it is still susceptible to fogging over from condensation. And, dri-down does not make the down water-proof. It just makes down turn water a bit longer, like about 45min or something. Water will still saturate the down, especially as vapour. Dri-down only introduces a delay in saturation.

[Eric Blumensaadt]

Like my forlorn hope for solid fuel tablets with 2X the heat of ESBIT, I have a hope that synthetic fill can someday at least equal 600 fill goose down.

The best advantage I can see (and notice in field use) is that DWR treated down dries faster than untreated down. Studies indicate around 60% faster. To me that’s a big difference.

The real horror story about down retaining body vapor is the ill-fated Scott Antarctic expedition. Their down “sleeping robes” began to freeze after several days and worsened to such an extent that they recorded having to literally melt their way into the “robes” with their body heat. I believe Amundsen’s Norwegian group used reindeer “robes” – and XC skis as well, which helped them make a faster dash to the South Pole.

With longer winter trips (over 5 days) the best answer is wearing a VBL suit to keep virtually all of the perspiration vapor out of the bag and in the suit. Temps need to be near or below the 0 F. mark for this to work best.

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