Mini-Study: Measuring Sleeping Bag Performance

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Testing the performance between 650-fill and 800-fill down sleeping bags

By Christina Heilman, PhD, ATC, CSCS


Although manufacturers use a theoretically sound standard to test the performance of sleeping bags, the testing standards are inconsistent (McCullough, 2009).  Manufactures of sleeping bags adhere to the standard of EN 13537, which was created by the European Outdoor Group, companies around the world who make and supply camping, hiking, and mountaineering gear.

The EN 13537 standard states:
“For EN 13537 temperature tests, a full-size mannequin with heaters and temperature sensors is dressed in one layer of long underwear and a hat. It is placed inside the sleeping bag being tested. The bag is laid atop an insulating sleeping pad inside a climate-controlled chamber. The mannequin is heated to simulate body warmth and measurements are taken of the air in the climate chamber and the “skin” surface of the mannequin. From these measurements, the insulation value of the sleeping bag is calculated.”

There are five sleeping bag testing centers around the world that perform this test. The problem, according to Dr. McCullough who is one of the textile scientists at the Testing Centers of Sleeping Bags at Kansas State University, is that each lab’s mannequin differs and there is no standardization of the thermal properties of the long underwear, hat, socks and sleeping pads. Thus, the results may vary between testing centers. For example, Dr. McCullough reported that highly insulated sleeping bags could vary as much as 20 degrees (F).

Where does that leave the consumer?

At MTI we don’t have the equipment to test sleeping bags to the EN 13537 standard, but we are interested in better understanding sleeping bag performance. Some question include:

  1. What is the insulation effectiveness of sleeping bags?
  2. Is there a difference in sleeping bags with the same rating among different manufacturers?
  3. What is the difference in sleeping bags with different temperature ratings, from the same manufacturer?


Mini-Study Design

To compare sleeping bag ratings we started experimenting with two identical 15 degree F sleeping bags: Marmot Sawtooth and Marmot Pinnacle. Each bag was approximately 5 years old. One difference between the bags was the down-fill: Sawtooth = 650-fill and Pinnacle = 800-fill.

Other differences include weight and cost:

                    Sawtooth: $250, 3lbs, 1 oz.
                    Pinnacle: $320, 2lbs, 8 oz.

We placed two Nalgene water bottles filled with boiling water into the middle of each sleeping bag.

For two hours, sleeping bags rested on a Thermarest Z-Lite pad in a 37 degree F restaurant beer cooler.

We measured heat loss by taking the water temperature inside the water bottle with a VeeGee digital thermometer every 30-minutes for two hours.



As a preliminary finding, we found that the 800-fill bag lost more heat as compared to the 650-fill bag over two hours, 55.3 and 50.9 degrees F respectively.

The 800-fill bag lost 4.4 degrees more compared to the 650-fill bag over the two hour testing period. Stated in another way, for every 30-minutes the 800-fill lost one degree more than the 650-fill.

In addition, we had a control water bottle that was not insulated and sat on a Thermarest pad. Interestingly, the control water bottle had a 122.3 degree F loss of heat during the two-hour trial.

That’s over two times the amount of heat loss compared to the water in the sleeping bags. 


Next Steps

This mini-study had a few issues. First, there was some variance in the rate of heat loss among the different Nalgene water bottles. Before we conducted the study, we tested the heat loss of each water bottle sitting in the cooler. The water bottle placed in the 800-fill bag lost 1.1 more degrees than 650-fill when sitting with no insulation in the cooler for two hours.

Is it possible that some heat loss from the 800-fill bag may be due to how the water bottle losses heat?

To better understand this question, we are wondering if the material in the water bottle isn’t consistent due to age and heat tolerance. We’re exploring if brand new Nalgene water bottles will be more consistent with heat loss or if there is a better vessel made with better material, such as aluminum.

Second, bags change over time. The fill-in down may lose it’s loft or get dirty, which can impact temperature ratings. We could wash the bags to remove dirt, but washing may reduce performance according to the EN 13537 standard. Therefore, the data is preliminary and difficult to imply any results.

The data does spark an interesting question, do you want lighter gear (800-fill) or stay warm in your sleeping bag (650-fill)? We will further develop the test design and continue testing sleeping bags.

Questions, Comments, Feedback? Email





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One thought on “Mini-Study: Measuring Sleeping Bag Performance

  1. We received the following comments from a newsletter reader:

    I read with interest your article on sleeping bag ratings and heat loss.

    I’ve worked in the Outdoor Industry since 1986, and have sold (at a small climbing/backpacking shop) many brands of sleeping bag, including Marmot, Western Mountaineering, TNF, Moonstone, Sierra Designs and Mountain Hardwear.

    I have also worked in house at Sierra Designs and The North Face (I no longer do), and since then have represented two other sleeping bag companies to key national retail accounts. I am not a product developer, but have worked closely with developers for decades (and have used sleeping bags on backpacking and climbing expeditions since the mid-70’s).

    I completely agree there is lack of consistency across brands/testing sites, etc.

    Adding to the problem is…

    Hydrophobic down (down treated to be water-resistant). Many companies use this claiming it increases loft…since moisture inside the bag will not absorb into the down itself, which causes loss of loft until it dries. However there are a number of problems with this. First, the waterproof treatment wears off over time, with washing. A bag washed even a couple of times loses some/most of the waterproof benefit. Also, the fill power of the down is increased when the treatment is applied…that is, 650 fill down can be treated, causing it to read as 700 fill down (this is an approximate example). So the bag is sold as a 700 fill down bag. But after washing, the down fill power returns to its original 650-fill. This may be occurring with the bags in your test, if the down in them was treated. Incidentally, the most respected/hand built/expensive bags in the market refuse to use waterproof down…for good reason I believe. I’m thinking of Western and Feathered Friends, as well as various European brands.

    The EN test itself is not designed for bags with ratings below zero…there is no standard test for these bags. The reason for this is that the test gives wildly inaccurate results even at the same test center once the ratings drop below zero. There is also no test for bags without a hood. This means all bags on the market without hoods, or with ratings below zero, have ratings based on the manufactures best guess as to what the rating should be, with no consistency across brands. (My own belief is that even at zero degrees, the EN test is starting become less consistent).

    The test also does not accurately mimic how humans feel inside a sleeping bag. Anecdotally, I can tell you from experience in many bags on many trips that a sleeping bag with a Goretex shell is warmer than the same bag with a standard nylon shell (even a “windproof” nylon). However, the EN test does not pick up this advantage.

    Another problem with the test is that the mannequin is a standard size, and temperature ratings change based on the amount of extra space inside a bag that needs to be heated by the mannequin itself. In other words, two bags may have the same amount of fill, but one has more internal volume than the other. If the first bag “fits” the mannequin perfectly, then that bag will seem to be warmer than the second bag, since heat from the mannequin needs to heat up the additional dead air inside the second bag. This does translate to the end user: assuming the sleeping bag user is the same size as the mannequin, the first bag would feel warmer as well. But if the user is larger than the mannequin, then the second bag would likely be more efficient. If a bag is too tight against the user, loft is actually reduced and the bag is less efficient. All of this means that two bags with the same rating…and the same true thermal efficiency…can be warmer/colder for different users.

    For this same reason, when looking at your water bottle test, I think a lot of the heat loss will depend on how closely the bag’s insulation is wrapped around the bottle and how consistent this is between the two test bags. Kind of like if you wrapped a hot water bottle in 1” of foam insulation and measured the heat loss vs. putting a bottle into a larger cooler that had 1” walls and measured that bottle’s heat loss. The second bottle needs to warm up the air around it so will lose heat faster. I wonder if even very small differences in how the water bottle lays inside the two test bags changes how closely the insulation surrounds them, and therefore how much dead air the bottles lose heat to.

    Also…your test compares 650 and 800 fill down, I assume because down loft is what gives sleeping bags most of their insulation properties. However lots of sleeping bag attributes other than the down fill power affect loft. A heavier shell fabric will press down on 800-fill down, compressing it slightly. 650-fill down has more structure to it, so resists this compression more. The exact patterns and cut of the baffles has a huge impact on overall bag loft. Getting down to loft to its potential in a sleeping bag is actually pretty difficult and varies across all bags/manufacturers. I guess my point is that comparative tests may give an idea of which of two bags is more thermally efficient, but it is difficult to say this has much to do with the differences in down fill power unless the bags are identical in all other ways. Increasing fill power is generally just a way to make a given bag lighter, not warmer (again, given the shell fabric is also lightened so it does not compress the 800 down).

    My final thought on how all of this is confusing to sleeping bag users: many brands round the EN ratings up/down to the closest 5-degree mark when marketing their bags. So even in their own line, they may have a bag with an EN rating of 19 and another with a rating of 22, and they might call both of these “20-degree” bags. The customer can always look into the details to find the actual test ratings…though those also may not tell them which will be a warmer bag for their personal use.

    In any event I appreciated the study and always find this topic interesting. I hope you don’t read my comments as critical in any way, I’m just riffing. You may already know all this too, so apologies if so.

    Thank you,


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