Author: Rafał Buczek


Participating in outdoor activities during winter conditions offers a deeply satisfying and truly unforgettable experience. The snow-covered landscapes take on a unique charm, mountain trails change their tracks, and it becomes the ideal season for ski touring, cross-country skiing, or splitboarding. Engaging in winter equipment trainings can be an incredible adventure and provides valuable knowledge.

Yet, with the thrill of those initial bivouacs in sub-zero temperatures comes higher risk, mainly due to harsh weather conditions and the ever-shifting characteristics of the snow cover. Strong winds and biting cold spare no room for errors. Deep snow can significantly extend the time to reach the bivouac site and set up the tent, while the rapid onset of dusk only adds to the challenge.



Just as crampons and ice axes are indispensable tools for navigating the rugged terrains of higher altitudes, a meticulously chosen sleeping bag serves as a safeguard during rest, facilitating recovery before embarking on yet another challenging day in the mountains.


How does low temperature affect the human body and why do we need external insulation to maintain the proper body temperature?


The human body is an endothermic organism, with its optimal operational temperature confined to a narrow range of 36.4°C - 37.8°C (97.5°F - 100.0°F). Thermoregulation is the mechanism responsible for maintaining this temperature irrespective of external conditions. However, its efficacy is limited, prompting the body to initiate various warming responses when faced with cold temperatures. These responses may include:

-> cessation of sweating,

-> narrowing of blood vessels (which slows down the circulation of denser blood in the body and slows down heat transfer),

-> shivering thermogenesis (heat production by organs such as muscle shivering),

-> hormonal thermogenesis (the activated thyroid produces hormones that speed up metabolism, which produces the energy needed to maintain a constant body temperature).

When thermoregulation's effectiveness diminishes and the body temperature falls below 36°C, hypothermia sets in, which in extreme cases can result in death. Yet, to maintain the optimal temperature, we can employ insulating layers to shield the body from the chilling surroundings. Given that both clothing and sleeping bags serve as these insulating layers, it's no overstatement to assert that a carefully selected sleeping bag could mean the difference between life and death. The colder we anticipate the nighttime temperatures to be, the greater the significance of the aforementioned statement.



What factors should we consider when selecting a winter sleeping bag?


The choice of filling for the sleeping bag is the initial and most critical decision to make. It's important to understand that it's the air that keeps us warm while we sleep. Air is a poor conductor of heat, which makes it an excellent insulator... and that could essentially wrap up this topic, if not for the phenomenon of convection. Convection is the process of heat transfer linked to the movement of currents in gases, occurring when the temperature of the lower layers of the gas is higher than that of the upper layers. Cold, denser gas descends due to gravity, while warmer, less dense gas rises. It's essential to stabilize our air layer. To achieve this, sleeping bags and clothing are filled with either down or synthetic insulation.


Down is a natural material of animal origin that, considering its weight, boasts an exceptionally lofty volume when loose. Furthermore, it can be compressed repeatedly into compact sizes without risking damage. Each down feather comprises numerous micro-branches that efficiently stabilize air movement. Sounds ideal, doesn't it? Unfortunately, when down becomes wet, it loses its exceptional qualities, clumping into damp, downy balls. This poses its biggest drawback in outdoor usage, but in reality, it's a minor issue, particularly in winter when the air is dry.


During warmer temperatures, even several consecutive rainy and misty nights won't affect a high-quality down sleeping bag. Nonetheless, remember two golden rules: never compress a damp sleeping bag, and whenever conditions permit, air-dry the sleeping bag in the sunlight during packing or while enjoying a break.



The characteristics of synthetic insulation are somewhat contrary to down. It's a heavier material, and its compressibility isn't high, but even when completely soaked, it provides insulation practically on par with when it's dry. Therefore, it's a solid choice for military or racing purposes where there's no time or opportunity to tend to the gear. A synthetic sleeping bag capable of offering comfort during winter nights with temperatures hovering around -10 degrees Celsius (14 degrees Fahrenheit) will remain bulky and weighty even after compression. That's why te optimal choice is: DOWN!

Synthetic sleeping bags also have their niche in winter conditions, so if you're planning an expedition to the poles, for instance, don't hesitate to reach out to us. We will advise you on the appropriate equipment:




In the outdoor industry, down can originate from either geese or ducks. Goose down is generally considered superior to duck down due to its higher loft. Particularly esteemed is the Polish down, sourced from a specific breed of Kołuda geese found exclusively in Poland! The measure used to assess down's elasticity is "cubic inches." For instance, a rating of 850 cuin (or 850 FP) indicates the volume occupied by one ounce of down. In essence, the higher the number, the better, especially for those seeking ultralight sleeping bags.


-> Down with a rating of 700-850 FP is typically used in winter down models, while products filled with 900 FP down are considered exceptionally refined. It's worth noting that all our products utilize ethically sourced Polish down!

-> A quick note: you might encounter sleeping bags or clothing filled with down exceeding 1000 cuin. In such cases, is worth asking whether the manufacturer adheres to European or American standards for determining down's loft, as they can differ significantly.

-> The second important parameter is the down-to-feather ratio, often expressed as a ratio like 96/4, indicating 96 percent down and 4 percent feathers. Naturally, the higher the percentage of down, the better the quality of insulation in the sleeping bag.



When discussing down, particularly in the context of winter sleeping bags, it's essential to mention hydrophobic down. This type of down undergoes a hydrophobic treatment during production, making it less susceptible to moisture. While hydrophobic down doesn't offer the same performance as synthetic insulation, its resistance to loft loss when exposed to moisture is remarkable compared to traditional down. In Poland, manufacturers have been offering this refined raw material for almost a decade, and it has been available globally even longer. It's now a highly advanced product known for its durability across multiple seasons.

For washing hydrophobic down, I recommend using specialized washing agents provided by leading manufacturers such as Grangers®.



During sleep, the human body can perspire approximately 0.5 liters of water, which ideally should be able to leave our sleeping setup. Hence, the fabric from which the sleeping bag is made should primarily be characterized by high breathability. Does this mean you have to forego a waterproof outer fabric? Nothing could be further from the truth! For example, Pertex® Quantum Pro fabric achieves waterproof ratings up to 1000 mm. This ultra-thin membrane effectively shields against moisture while offering the highest possible breathability parameters.

Downproof properties of the fabric, is another important characteristic. This is a parameter that a high-quality material manufacturer is able to present quite accurately along with the specification of how it was achieved. Avoid fabrics coated with additional substances such as acrylic, as coatings of this type, even if they increase the level of downproof, reduce the breathability of the fabrics.



When searching for a sleeping bag intended for use in winter's low temperatures, it's definitely worth looking for one with a mummy shape, which is more snugly fitted and effective in terms of thermal insulation. It's important to remember that outdoor gear shouldn't consist of many completely separate items. You should see it as one whole, where individual elements work together. The same goes for a winter sleeping bag. If you plan to sleep in a down jacket (which, despite many repeated myths, is not a mistake at all!) or want to use a thinner summer sleeping bag inside, choose a more spacious cut. Make a similar choice if you tend to sleep with your legs drawn up (which creates tension around the hips in narrower sleeping bags). Taut fabric compresses the down inside, causing it to lose its loft and cease to insulate with a layer of warmed air.

Typically, our winter sleeping bags are wider than summer ones. The extra space inside a down sleeping bag can also be useful for storing cold-sensitive equipment items, such as all kinds of batteries or a gas cartridge.

It's also worth paying attention to the internal construction of the sleeping bag. In winter sleeping bags, V-shaped or trapezoidal chambers are most commonly used. In my opinion, V-shaped chambers are the perfect compromise between the weight of the internal construction and highly effective down stabilization. Meanwhile, double-H chambers are used in the most advanced expedition sleeping bags (like our EXUISTIC 1500), which are often filled with as much as 1500 grams (53 ounces) of down!



The myth suggesting that most body heat is lost through the head has long been debunked, but it doesn't change the fact that protecting the head in low temperatures is crucial.


-> In our range of down sleeping bags, we employ four hood designs. Starting from the very versatile flat hood design to the exceptionally complex one equipped with an additional down tube around the face opening. These designs are applied depending on the intended use and temperature range of the sleeping bags. The most advanced winter sleeping bags from Cumulus® feature spacious hoods with a three-dimensional construction, allowing you to sleep with an additional inner hood of a down jacket or a thinner inner sleeping bag. The unique down tube featured in Excuistic sleeping bags further prevents the loss of warm air from inside the sleeping bag. In chilly conditions, all minor operations become exceptionally cumbersome due to icy fingers and the necessity of wearing gloves. Therefore, in winter sleeping bags, we use an easy-to-use plastic toggle, even while wearing thick winter gloves. It can be opened by pulling a long brightly colored cord.

-> Since this is practically the only adjustable element of the sleeping bag, it is worth ensuring that its adjustment is convenient and possible while lying in the sleeping bag. Another worth considering option is using moisture-resistant fabric around the face opening. This solution helps protect the down from moisture coming from your breath while you sleep.

-> By the way, as we’re speaking of breathing during sleep, I would like to draw your attention to one common mistake made during camping, which is breathing inside the sleeping bag. Remember to always leave a small opening and exhale outside the sleeping bag, thus preventing the accumulation of moisture from your breath inside.

-> The footbox is the area of the sleeping bag responsible for thermal insulation of the feet, so it should be sufficiently spacious, and its design must take into account the anatomical positioning of the feet during sleep. Furthermore, in our sleeping bag configurator, you can customize your chosen sleeping bag model to your own needs... and select the additional option of placing more down only in the footbox to increase the level of thermal insulation in this crucial area.



The zipper, allowing for convenient entry and exit from the sleeping bag, is also the area where the greatest heat loss can occur. That's why in winter sleeping bags, insulated zipper tubes are used to prevent this. Another method is a compromise, which involves shortening the main zipper. Moisture settling on the zipper, which then freezes, can lead to its damage, so I recommend choosing more durable 5mm zippers.

There's nothing worse than struggling with a zipper jamming into thin fabric when it's windy outside and the temperature hovers around -15 degrees Celsius (5 degrees Fahrenheit). To prevent such situations, we protect zipper sliders with special covers, additionally using stiff strips to protect the inner material.



The thermal collar is an essential component of a winter sleeping bag. It is basically an additional, adjustable chamber filled with down that can be fastened around the neck during sleep, allowing for proper adjustment. The purpose of the collar is to further insulate the interior of the sleeping bag, preventing the escape of warm air, which is crucial during freezing nights. Thermal collars can be secured with either Velcro or snaps. The former option should ideally include additional protection for its rigid part to prevent damage to delicate thermal clothing when the collar is not in use.




As you can see, choosing a good winter sleeping bag is not easy, and I hope this guide will help you make the right choice! Finally, I have one last tip: Remember that outdoor conditions are unpredictable, so a solid warranty and assistance beyond standard service, along with the option for repairs if necessary, are very important in this case. It will allow you to enjoy your new sleeping bag for decades to come!

That's why all Cumulus® products are covered by an unlimited warranty: Lifetime Experience. We always strive to repair damaged equipment, and additionally, we offer servicing and repair for all our products. It's not uncommon for us to receive sleeping bags, that are several years old, and after our "makeover," they are yet again ready for new outdoor adventures!



Decades of experience, premium materials, and ongoing improvements make our down sleeping bags incredibly user-friendly and exceptionally lightweight. Remember, if you need to, you can customize any model to your individual needs using the configurator tool: