View Full Version : Sleeping Bag Loft vs. Temperature Rating?

04-04-2005, 06:52 PM
I know we've got many experts out there that can help me with this question. This is not intended as a sythetic versus down debate (I've read plenty here on the relative merits of each, I'm just trying to compare them for comfort) and I do not wish to start any heated debate.

Is loft a fair way to compare temperature ratings of sleeping bags?

Is it apples and oranges to compare loft of synthetic to down bags (i.e. does X amount of loft in down equal Y amount of synthetic)?

I'm going to be trying my different options as best I can in the field but would like to get an educated guess going in the mean time.

Current choices include:

Synthetic with 5 inches of loft rated at 0 degrees
Down with 5.5" to 6" (variable fill) of loft rated at 15 degrees
Synthetic with 3" of loft rated at 25/30 degrees

I'm not sure how accurate my measurements are as compared to any "standard method" that may exist, but each bag was measured the same way so they are comparable to each other.

If loft is a fair comparison, it would seem that either my 15 degree bag is closer to a 0 degree bag or vice versa unless the insulating properties of one is superior "per inch". Hopefully I made that more clear than confusing but in any event, I'm sure some of you can straighten me out! Thanks.

04-04-2005, 07:08 PM
To quote Jerry in the infamous "down v synthetic"

"I appreciate very much your in put.

Based on my many years of experience in the outerwear and sleeping bag areas of the textile business I have learned a great deal from those who preceeded me. I consider Gerry Cunningham who owned Gerry Outdoor Products to be my mentor. I also consider him to be the "father" of the camping industry. In a booklet he wrote and published in 1971 he states the following;
These figures represent the top layer of the bag, and the insulation is down. He does not state the down fill capacity, however I would guess the quality used at the time was 550/650 down. Gerry does use my Nautilus and has used it for 15 years on his boat."

04-04-2005, 07:13 PM
I do not profess to be an expert on this... I too have read some of the detailed threads on this topic. IMO, I can't see initial loft being directly proportional to a bags rating anywhere but a laboratory. In the field, moisture content in the air and the bags (including insulation) ability to breath would greatly impact the insulating capability / loft. I think you're on the right track with regard to field testing. I depend on the many who post here to share their experiences - good and bad
as I just don't have the time to do all the testing myself. Good luck and keep us posted if you find a combination that works for you.

04-04-2005, 07:47 PM
EmmersonFamily, right... I wasn't so much concerned with the manufacturers' ratings as using loft to compare the bags I have.

What I'm trying to get at, regardless of what the manufacturer rated them, is whether my "15" degree bag is similar in actual temperature rating to my "0" degree bag. Alternatively because of the type of insulation is loft not a good comparison?

As bbright pointed out field testing, particularly using both bags in identical conditions will be the best way to compare but I'll confess I'm unlikely to schlep two bags into the mountains to try that out and it's gotten a bit warm down where I'm at to test things out on the porch.

Thanks for all the quick input!

04-05-2005, 08:23 AM
I have to say that as long as all the bags are evaluated dry, that loft is a very good way to compare warmth, much less so than comparing the rated temperature.

All of the sleeping bag insulations work by two mechanisms; limiting free convection through the insulation, and by having low thermal conductance. In this case the insulation can be modeled as "still air" where the heat flow is by molecular diffusion. More loft equals more warmth.

Rusty Hook
04-05-2005, 08:26 AM
This whole topic is very variable and individualistic, depending on conditions, ground insulation, how hard you have pushed yourself, physical condition and diet.

Generally, loft is loft - dead air space - irregardless of the insulating material. However, and as Jerry stated, the loft that counts is the top half of the bag because your body weight eliminates most if not all of the loft beneath you except for your pad. I have been comfortable in a 15 degree bag in below zero temps with clothes on and been cold in the same bag in the middle of the summer do to higher exertion levels and a diet lacking fats and protiens.

The only valid advice I can give you is to try it an see. Over time and many different conditions, you will eventually arrive at what works for you most of the time. I know this is not the answer you are looking for, but it is the best I can give you after nearly fifty years of trying to answer your seemingly simple question...Rusty.

bill s
04-05-2005, 10:39 PM
Somewhere I have seen a report from the Army Quartermasters Corps at Natick MA., saying that the material is largely irrelevant, that 2" of loft has about the same warmth whether 800 fill down or shredded newspaper. Could be. One thing I know, when you look at a bag in the store, and it says its "rated" for zero or minus 10 or whatever, and its a thin little rag about 1/4 in thick, they're lying, and you're going to be cursing some cold night. IMO, as a "down" man, Wiggys and a few TNF synthetics are the only ones which seem to have enough loft to keep you warm.

04-06-2005, 03:56 AM
Manufacturers frequently use the most optimistic figure when rating bags. Loft is loft and can be objectively measured. Comfort ratings are subjective since there are so many individual variations. Some sleep hot, some sleep cold.

The army assumes soldiers are not going to strip naked so army ratings assume a good amount of insulating clothing worn in the bag.

Using a bag without an insulating pad underneath also changes ratings dramatically and the rater has no way of knowing what, if any, pad you are using.

Loft is also not linear. 4" of loft is warmer than 2" of loft, but not necessarily twice as warm.

Rusty Hook
04-06-2005, 10:36 AM
Added to the complications already listed above by myself and otheres is the fact that as we age our metabolisms slow down; as you get older, you get colder. This is not something that usually requires a bag upgrade, but more or thicker ground insulation to keep you comfortable...Rusty.

04-06-2005, 10:55 PM
After reading those numbers Ken posted relating loft and warmth I decided to test them against my heat transfer modeling skills, I'm lucky to currently be in a graduate level course in Environmental Biophysics, where it seems we are constantly calculating heat and vapor fluxes from plants and animals. On second thought maybe I'm not that lucky /images/%%GRAEMLIN_URL%%/confused.gif . Assuming a still air model, I calculated the heat flux density (energy per unit area) for the top of the sleeping bags with the given loft.

Now I could give you boring details (and I will if anyone asks /images/%%GRAEMLIN_URL%%/wink.gif ) but the short version is that all three of those loft/temperature combinations resulted in nearly identical values of heat flux (within about 1%) and equaled about 0.76 Watts/m^2. This heat flux density should be related to the comfortable metobolic output of the sleeping individual.

Thusly armed with a heat flux value that represented "comfort" I calculated a realionship that is plotted below:
Pretty neat, at least I think so.
A couple of things to keep in mind\
1) This is only for the Top of the sleeping bag, if you are not well insulted from the ground you will still be cold.
2) This is only for that single metobolic rate from the three example values from Gerry Cunningham posted above.

Hope this helps or is at least amusing

04-07-2005, 07:11 AM
Joe, nice graph. I think loft is a fair way to compare bags so long as you take into account other variables such as shell material, etc. It appears it could also be used to estimate temp rating if you know where you are in relation to the given Comfort Rating. Thank you for the 'visual aid'.

jerry wigutow
04-07-2005, 08:59 AM
I have Gerry Cunningham's booklet How to Keep Warm, the graph you are showing does not exist in that booklet so where did the graph come from?
The graph I have in the booklet shows +50 to -60. 1 inch to 4 inches. I do not accept 4 inches above you for -60, 8 inches is more like it.

04-07-2005, 09:19 AM
Jerry, he put the graph together himself using the three data points from Gerry's booklet listed above--as such most of it is theoretical. I would bet, though, that if you provided more of Gerry's data he would be able to put together a more accurate graph.

jerry wigutow
04-07-2005, 09:49 AM
The problem with Gerry's data is that it is based on down as the insulation. I do not know but can only surmise the down used was 500 to 600 fill power. Down is technically obsolete since continuous filament came about. Down can only be used in ideal situations, synthetic fiber if used properly nullifies the weather.
I'll have more info on the subject later.

04-07-2005, 09:52 AM
I am not convinced that the equation should be linear.

04-07-2005, 09:53 AM
I don't have Gerry Cunninham's book, I calculated the relationship myself, using 1-D Fourier's law. To relate rating temp to loft I assumed those three points above defined the maximum comfortable heat flux. As an interesting aside I am realativly certain that those three values must have been calculated the same way originaly, because calculated heat flux for each was identical aside from a little bit of round off error.

I bet you are right about extreme cold, I have never been out in weather below -20 F so I have no first hand experience but I guess that a couple things cause the linear relationship to break down.
1) The insulation from the ground becomes more and more critical, if thermal conductance to the ground is higher than to the air, the loft of the bag has less and less impact on your comfort. In my opinion synthetic bags are great in that they insulate from the ground better than down.
2)In the calculations I assumed molar density and thermal diffusivity of air at a constant average air temp, and though there is little error down to about -20 F, there may be more in extreme cold.

I would trust experience over calculations any day, but somtimes calculations are usefull for comparisons and to help provide insite into why experience is what it is.

I would be happy to email a copy of my calcs in an Excel spreadsheet to anyone who wants one.

04-07-2005, 10:11 AM
Here is the basics of why the relationship is linear:

H = g * (Tb - Ta)
H : Heat flux (W/m^2)
g : Thermal conductivity (mol/m^2/s)
Tb: Body temperature (C), Constant = 37 C
Ta: Air temperature (C)

If one cm of still air has thermal conductance of about 0.089 mol/m^2/s, then you can calculate the thermal conductance of any thickness by:

g = 0.089 / Loft
Loft: Loft thickness (cm)

Putting this all together yeilds:

H = (0.089/Loft)*(37 - Ta)

With this relationship I evaluated:
<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> INSULATION NEEDED FOR SLEEPING
</div></div>This yeilded a constant value for H = 0.76 W/m^2 for all three points. and this relationship:

0.76 = (0.089/Loft)*(37 - Ta)

with Ta in C and Loft in cm.

jerry wigutow
04-07-2005, 10:31 AM
In the event you did not know it I am dead set against lab testing except in the case of what is known as a "guarded hot plate". A heated surface in a box. It will tell you if one form of insulation is better than another if the same weight. As a previous posted stated the material is not really relevant if thickness is equal, partially true. That aside a human in a sleeping bag can tell you if they are comfortable or not. Then you look at the weight of a bag. As an example I have a bag with 3 inches of loft top and bottom. Bag one is made with Lamilite L-15, 15 ounces of fiber per linea yard 45 inches wide, our zero degree bag. Bag 2 is made with two layers each top and bottom Primaloft and according to the sales people at Primaloft this bag will give you a zero degree rating. The weight of the fill is 4 oucnes per square yard or 5 ounces per linea yard 45 inches wide. The fiber has to be quilted in order for it to stay tegether in use and laundering. The quilting immediately compromises the insulation and the fiber fill has significantly less density than the CF product so it will not even come close to being as good a insulator. In general there are a number of things that need to be taken into consideration when making a sleeping bag. See my newest newsletter currently on my site.

04-07-2005, 10:58 AM
Ingenaire, I understand that the insulation values are linear with thickness, although I avoided thermo in engineering school. (EE)

But you're dealing with a living breathing heat producing unit inside, and I suspect output is not linear with temperature. Cold air going in also affects how much you can put out at a certain point.

Hence, when Jerry comments that he thinks you need 8" of loft at -60f, rather than 4", I think he's right. Intuitively, I haven't gone to the lab nor dusted off the textbooks. I quit being an engineer, or at least dealing with engineering, 11 years ago.

04-07-2005, 11:40 AM
As I was under the assumption the graph was merely for comparisons it would seem to have some value regardless of other variables. But I believe we are all in agreement that reality trumps theory--if you are cold at any given temp with any given amount of insulation, you need more insulation.

Beyond that, I will leave this discussion for the engineers and experts.

04-07-2005, 12:17 PM
...stay tuned as the Kifaru message board prepares to build its first space shuttle...

/images/%%GRAEMLIN_URL%%/laugh.gif /images/%%GRAEMLIN_URL%%/laugh.gif /images/%%GRAEMLIN_URL%%/laugh.gif /images/%%GRAEMLIN_URL%%/laugh.gif /images/%%GRAEMLIN_URL%%/laugh.gif

Only kidding fellas...keep it coming.

04-07-2005, 12:29 PM
Only if there are things to hunt in space.

04-07-2005, 12:58 PM
That was the best laugh I've had in a while /images/%%GRAEMLIN_URL%%/laugh.gif

All good points, I agree with you. Quilting is such a conduit for heat, I think this is the main reason that down bags (with baffles instead) work as well as they do, similar to your bags. I scanned your article, and will read it in full tonight. At first glance it looks interesting. When theory and experience/observations disagree it means the theory is wrong or incomplete, but I suspect that your avertion to lab test results stems from people doing tests that conclude things contrary to your experience. If someone told me that water runs uphill I would have a hard time trusting them again.

One thing I have retained from reading one of your old newsletters was the importance of loft. You convinced me that loft is the most important part of the bag, without an adequate amount it simply can't be warm enough. CCH asked if loft was a fair way to compare temperature ratings of bags, and I believe it is, at leat it is better than believing the various manufacturers' ratings. So I laid out how I would compare temp rating and loft. I tried to put in plenty of warnings that it wasn't perfect.

If someone knew a values for loft and the air temperature where they were comfortable, they could make their own chart for their own body's metabolic output. That person specific relationship would almost certainly be better than manufaturers' ratings in helping that individual compare temperature ratings for themselves.

I look forward to reading your article in full tonight.

04-07-2005, 01:10 PM
I think you are right, to a point. My understanding is that the human body is very well thermally regulated to maintain a tissue temperature of 37 C /98.6 F, and that it will do a good job of it until you reach the point where your body simply can't produce enough heat energy and you get hypothermic. I think that critical point is represented by the H value above.

I also trust Jerry when he says you need 8" of loft at -60 F, though I hope I never have to find out if anything can be comfy at -60 F /images/%%GRAEMLIN_URL%%/eek.gif Trying to sleep in weather that cold is like skydiving, its nice to know I won't die if I try it, but I have no urge to try.

just for you:

jerry wigutow
04-07-2005, 01:18 PM
aside from loft the density of the fill is very important. as an example if i used a 900 fill down as some claim but i think is bs it would not have the same density as a 700 fill down which i believe would perform better. you would have more weight for equal loft, greater density for a given area. the same holds true for synthetics to a point. however, you can not equate a chopped fiberfill with continuous filament since in reality each is used differently in manufacturing. chopped fiber must be quilted and cf is but should not be quilted, as I use it. the lamination is simply a sewing aid, the trick here is to be able to laminate the beast economically, and that is why my laminating machine is so valuable, I built it to laminate CF. as long as it is not quilted it will perform far better than any quilted fiber fill. as an example the 3-d or delta polar guard bags are lighter than mine and do not perform at the stated temperature rating because the manufacturers are buying to light a weight of product. these bags also lose their loft very quickly for that reason and a second reason the fibers are resin bonded thru and thru so they are weakened. as you can see a great deal does go into a sleeping bag.

jerry wigutow
04-07-2005, 01:20 PM
a antarctic bag is comfortable at -60

04-07-2005, 01:24 PM
Ingeniare, thanks for shedding some humor into a sleeping bag thread. That's something this board hasn't seen in a long time.


04-07-2005, 01:43 PM
Joe- Thanks from me also...I haven't seen this much math here since the infamous Catenary(sp) tarp thread. I really like to see all the interaction here. Some of the math smolders the brain cells a little, but I'll be allright. It is some great reading.

...come to think of it...I think a guy down the road from me has one of those shuttles...maybe that is the same one...

04-07-2005, 01:44 PM
This is all good stuff. Thanks for all the insight.

I'm really not so much caught up in the temperture rating of the bags as my own tolerance will really determine that, just whether despite their manufacturer ratings I should be looking at the bags I have as comparable based on loft. As was pointed out earlier, that is not as simple a questions as I thought!

04-07-2005, 06:35 PM
I just read your article, and I couldn't agree with you more about Gore-Tex. In my opinion it is better to have a well vented waterproof jacket than a W/B.

I confess that I had trouble understanding what in the world clo is relating to the KSU, though apparently more is better. It seemed as though it was a poorly designed experiment, which did a poor job of replicating the natural system. I speculate that the most significant reason is related to the fill density you mention.

Insulation is made of two things: void space and solids. I think that the density that you describe is related mostly to the porosity of the insulation, which is the volume of void space divided by the total volume. I think that the effects you are observing are the results of the porosity of the insulation. Where if you have too little porosity you have a lack of the ability of the insulation to breathe and to transmit water vapor, but on the other hand, too much porosity allows for too much air flow through the insulation and it doesnít insulate as well. Hence 900 fill down has too much porosity compared to 700 fill down. I would assume the density of the solids in the down would be about constant.

So in the experiment the manikin was apparently not transpiring water as he slept. I believe that the thermal conductance of a personís skin is larger the wetter their skin is, so in this case the unnatural bone dry manikin was warmer in a bag with lower porosity (and less breathable) bag than a real person, who would have gotten damp and chilled.

Judging from the reports I have read from people on this board, I guess that Lamilite happens to have about the right porosity, allowing for both insulation and breathablity.

jerry wigutow
04-08-2005, 07:27 AM
actually they do inject moisture into the test.
i believe if you read through many of my newsletters you will ultimately find out all of the reasons that Lamilite performs as well as it does.