Heat balance and temperature ratings for sleeping bags

Heat Balance
Factors influence the heat balance
Comfort and Extreme ratings
Temperature standard

Science influences almost all the human spheres of activities and sleeping bags production is not an exclusion. The aim of this article to explain some key questions, connected with testing and using of the sleeping bags. It will get in touch with temperature ratings, heat balance, thermal resistance and other terms.

Applying all state-of-the-art scientific researches and achievements our BASK Company follows up with the latest demands to the thermal resistance and safety, which are absolutely necessary in polar expeditions, mountaineering and hard hiking, ski-tours. The user should have a knowledge what demands a special attention while choosing a sleeping bag, what comfort depends on.

Well, let’s go…

Heat balance
If we feel warm, then the heat that we are producing is equal or greater than the heat that we are losing to the environment around us. We generate heat through our metabolism.

We lose heat through:

• Conduction to surfaces we touch, especially the ground below us.
• Convection as air circulation carries away heat and respiration losses from heat in the air we exhale.
• Evaporation of moisture from our skin (mostly sweat).
• Radiation of infra red heat.

Heat Generated
=
Heat Lost through Convection, Conduction, Evaporation, and Radiation Science

Physiological factors
A sleeping bag only retains heat. The person inside is the generator of the power to keep warm. Next, we will look at the influence on this power output and the differences found between individuals and situations. All ratings systems are based on a standard person. However, the first problem is that there is no such thing as a standard person.

Metabolic Heat Output
A sleeping man generates between 75 and 100 Watts of heat. For an average built man, this translates into a heat output per square metre of surface area of between 47 to 55 W/m2. However metabolic heat output is a very complex issue involving body fat index, age and sex. In general, young well nourished men produce more body heat than older men and much more than women. The sensation of cold is also linked to age and conditioning. Novices feel the cold more than experienced mountaineers. People who work outside feel more comfortable in cold environments than people who work in offices. After the age of 25, it is generally true to say that heavy people feel the cold less than slim people.

Heavy or slim
Fat people often say that they have a slow metabolism. It is more accurate to say that they eat more than they burn up. Slim people have a balanced metabolism - they eat as much food as they burn. Body fat has a big impact when the food intake is less that the energy output. Backpacking and trekking often leads to a situation where you eat less calories than you burn. Polar and high mountain expeditions face a situation where it is impossible to eat enough to match the energy requirements. The fat person will be able to burn energy from their fat reserves. Also the fat person has more natural insulation than the thin person. So it is true to say that a fat person feels the cold less than a thin person.

Sex
Women are generally colder than men. In EN 13537, the comfort value for a standard woman is 5 ºC higher than for a standard man.

Age
However, metabolism is age related. Older people generate less heat and hence feel cold more than younger people. In particular, well fed young men between 16 and 24 have a high metabolic rate and demonstrate “dietary induced thermo-geneses”. In other words, their bodies pump out heat so they do not feel the cold. So for a young male, the comfort temperature could easily be 5 ºC colder than for an older man. Children, especially small children do not have the same natural heat controls as adults. Their metabolism speeds and slows as they grow. For this reason, it is exceptionally difficult to define temperature ratings for children.

Conditioning
Most historical research on sleeping bags was done on soldiers, outdoor instructors, mountain guides and expedition mountaineers. These are a group of fit experienced people, predominantly males 18 to 40, conditioned to living outdoors. Most westernised people live in centrally heated houses, work in air conditioned offices and drive everywhere. This lifestyle reduces the body’s conditioning against cold.

Experience
Experience in sleeping in the outdoors also affects the comfort temperature. Experience and familiarity with equipment helps get the best performance from it. A novice will generally feel colder than an experienced person.

Fitness
If a sedentary person goes on a strenuous trip into the wilderness or high mountains, they will quickly become fatigued. Exhaustion reduces heat output and means you feel cold. When they stop exercising, unfit people feel the cold more than fit people.

Basic Definition of Comfort and Extreme Values: Comfort Temperature is the range of temperatures at which the user gets full nights sleep. Usually this is defined as an upper and lower limit.

The Upper Limit of Comfort is the highest temperature that you can sleep without sweating profusely. Usually this is defined with the ventilation zippers open, arms outside the bag and the hood open.

The Lower Limit of Comfort is usually defined as the lowest temperature at which a person has eight hours of sleep without waking.

Extreme Temperature is the lowest temperature at which the sleeping bag protects the user from hypothermia. This is subject to a variety of conditions but is usually defined as six hours of uncomfortable sleep without diminishing the body core temperature to a dangerous level.

EN 13537 Standard
EN 13537 was approved by the European Outdoor Group to standardize the temperature ratings announced by the manufacturers. Every effort has been made to ensure that the information contained in the temperature testing reports is correct and up to date at the time of publication.

The standard was published in 2002 and has now been adopted in most European countries.
Since January 2005 another amendment came to a force, which regulates testing, labeling and publishing the results of testing. It means that all sleeping bags adhering to the standard will have the temperature ratings set by a standard laboratory test.

All sleeping bags delivered to shops after 1st January 2005 should conform to EN 13537 and reputable Manufacturers are trying to ensure that all 2005 consumer catalogues, websites, hangtags and consumer adverts communicate temperatures to EN 13537. A sleeping bag which fully complies with all aspects of EN 13537 is entitled to display the CE symbol on the product and hangtag.

The standard is used in the following countries:-
Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Luxembourg, Malta, The Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland, and United Kingdom.

Unfortunately Russia is not included in this list. Mostly the reason is a low technological development level. Among other companies now days only BASK Company is able to make the EN conformity testing.

What is new with EN 13537
The biggest change that consumers will see is the way that temperatures are defined and displayed. EN13537 requires a thermal manikin test which produces four temperature results - upper limit, comfort, lower limit and extreme. These temperatures were worked for normal consumers.

There will be a diagram showing the temperatures on the sleeping bag label. It will be similar to this example. In this example, a ‘standard’ woman will have a comfortable night’s sleep if the air temperature is between +22°C and +4°C and a standard man will sleep comfortably between +22°C and -1°C.

In catalogues where one temperature value is used prominently, the value is the lowest limit of comfort, in this case, -1°C.

The sleeping bag should provide enough insulation to prevent serious hypothermia down to -18°C but between -1°C and -18°C a strong sensation of cold has to be expected and you should be aware that there is a risk of health damage due to hypothermia at the lower temperatures.

How to understand the EN 13537 Temperature Ratings
All of the temperature values are based on the air temperature outside the sleeping bag. Weather forecasts usually give two temperatures, the daytime high temperature and the night time low temperature. The night-time low temperature will be the air temperature outside the sleeping bag as you sleep.

The EN 13357 temperature tests use a thermal manikin which is a full size humanoid dummy with heaters and temperature sensors. The manikin is placed inside the sleeping bag and both are placed in a climate chamber. The manikin is heated to simulated body warmth. The air temperature is measured in the climate chamber and on the skin surface of the manikin. From these measurements, the insulation value of the complete sleeping bag is calculated.

The temperature recommendations are defined by EN 13537 based on the measured insulation.
EN13537 produces four temperature results - upper limit, comfort, lower limit and extreme. These give ratings for a standard man weighing 80kg and for a standard woman weighing 60kg.

The EN 13537 Upper Limit or Maximum Temperature is the highest temperature at which a ‘standard’ adult male is able to have a comfortable night’s sleep without excess sweating.

The EN 13537 Comfort rating is based on a ‘standard’ woman having a comfortable night’s sleep.

The EN 13537 Lower Limit is based on the lowest temperature at which as ‘standard’ adult male is deemed to be able to have a comfortable night’s sleep.

The EN13537 Extreme rating is a survival only rating for a ‘standard’ adult woman. “In the risk range a strong sensation of cold has to be expected and there is a risk of health damage due to hypothermia.” This is an extreme survival rating only and it is not advisable for consumers to rely on this rating for general use.

The best guideline temperatures for purchase decisions are the T Comfort and T Limit ratings.

• The article is prepared with help of Mammut (Ajungilak brochure "SleepWell", www.mammut.ch) and official European Outdoor Group material
  www.europeanoutdoorgroup.com