How does a sleeping bag keep you warm?

A sleeping bag does not create any heat of its own accord. It is the body inside it that creates all the heat; the sleeping bag just makes sure the heat stays close to your body as the air trapped in the filling provides insulation. The amount of air in the filling directly affects how effectively the sleeping bag insulates. And the mass and volume need to be as small as possible.

Nowadays, choosing the right sleeping bag requires professional knowledge of several fields of physics. Is it really that difficult?

Assessing a sleeping bag’s quality is definitely not that difficult, but it does require a certain amount of reliable information.

What information or values are needed to assess a sleeping bag’s quality?

The sleeping bag’s mass, packed size, lower limit, absorption capacity, breathability, and how water resistant the shell fabric is.


A sleeping bag’s mass depends mainly on the quality of the used materials. The lightest sleeping bags are made of soft, yet sturdy materials (such as Colibri DWR) with highly expansive (high loft) down filling. A synthetic sleeping bag, in comparison to an equally effective down sleeping bag, will always be heavier and bulkier.

Packed size

One useful factor when assessing a sleeping bag is how much space it takes up in your backpack. Again, down sleeping bags are among the most ‘packable’. Not only because they take up the least space when using a compression sack or stuffing it right at the bottom of your bag, but also because they quickly return to their original loft when unpacked.

Lower limit

According to current standards, the basic, and often only, information available to help you choose the right sleeping bag, is the results from a lab test (in Europe EN J), where the assumed user is a regular healthy person, whose age, weight, metabolism and other parameters are completely standardised.

The tests are carried out at room temperature without a real human inside the sleeping bag. The test dummy does not sweat and is merely a sensor without any personal feelings or any specific requirements regarding warmth. The test results show four values that have been calculated by the computer along a thermal scale. The most interesting of these values is the so-called limit temperature, i.e. the standardised lower limit of thermal comfort. This value can provide an approximate assessment of the sleeping bag’s insulating properties when dry and can be used to compare several different sleeping bags from different producers in the same category.

These results regarding the sleeping bag’s insulating abilities are one of the factors we use in our quality assessment. Note: This value is only indicative and is not guaranteed (it would only be guaranteed if your body complied with all the standards, if you didn’t sweat and if you only slept in a lab...).

Absorption capacity

This is the sleeping bag’s capacity to absorb a certain amount of physical moisture without diminishing your comfort. It is measured in grams and refers to how much water can be stored in the filling’s microstructure without the user experiencing any kind of moisture or cold.

A healthy person loses approximately 750 grams of water a night. This moisture needs to be diverted through the sleeping bag’s inner fabric so that it can be absorbed and does not bother you. 


Without the ability to divert water vapour, your sleeping bag would lose its insulating properties the first time you used it; the condensation of body moisture would gradually build up in the filling turning your sleeping bag into a sweat sauna.

Breathability is measured using the globally accepted Ret test. The results are expressed in Pa.W.m2 and refer to the amount of energy needed to transfer the water vapour from one side of the fabric to the other. The lower the Ret number, the higher the breathability.

Water resistance

The sleeping bag also needs to be protected from outer moisture, which means that the shell fabric must be water resistant. As a rule of thumb, the higher the water resistance is, the lower the breathability will be. It is therefore important to choose a material that balances these two parameters. We use a fabric with increased water resistance for our sleeping bags that are likely to be used predominantly in moist environments.

BACK UP keyboard_arrow_up