AUTHOR: Rafał Buczek, Product Manager at Cumulus®

Heading into the mountains in winter is hugely rewarding and can turn into an experience you’ll remember for a long time. A snow-covered landscape has its own special magic, mountain trails change their character, and suddenly it’s time for ski touring, cross-country skis or splitboarding. Along with the excitement of your first camps in sub-zero temperatures comes higher risk, though. Strong wind and low temperatures don’t forgive mistakes. Deep snow can significantly extend the time it takes to reach your bivouac and pitch the tent, and the early dusk doesn’t help either.

That’s why choosing the right equipment is so important. A well-matched sleeping bag will keep you safe through the night and let you recover before another demanding day in the mountains.

Choosing the type of insulation is the first and most important decision. It’s worth remembering that what actually keeps you warm at night is air. Air conducts heat very poorly, which makes it an excellent insulator – the key is simply to trap and stabilise it. That’s why we fill sleeping bags and insulated clothing with down or synthetic insulation.

Down is a natural animal-based material that is extremely light and has an outstanding ability to loft, creating a large volume when fluffed up. It can also be compressed down to a very small pack size over and over again without damage. Sounds perfect, right? Unfortunately, when down gets wet, it loses these properties and clumps into damp little balls. To avoid this, remember two basic rules: never compress a damp sleeping bag and dry it in the sun while you’re packing up or during longer breaks.

Synthetic insulation behaves almost like the opposite of down. It’s heavier and doesn’t compress as well, but even when it’s completely soaked it still insulates at nearly the same level as when it’s dry. That makes it a great choice for military use or races, when you don’t have the time or opportunity to look after your gear. A synthetic sleeping bag that will keep you comfortable on winter nights around –10°C will always be heavy and bulky, even when compressed.

Down used in outdoor gear can be either duck or goose down. One of the most highly regarded types is Polish down sourced from the unique Kołuda goose, a breed raised only in Poland. The parameter used to describe down’s loft is “cubic inches” (cuin), also known as fill power (FP). When you read that a sleeping bag is filled with 850 cuin (or 850 FP) down, it means that one ounce of a down sample occupies a volume of 850 cubic inches. In short: the higher the number, the better – especially if you’re looking for an ultralight sleeping bag. For winter down sleeping bags, fill powers in the 700–850 FP range are most common.

The second key parameter is the down-to-feather ratio, often written as something like 96/4, meaning 96% down and 4% feathers. As you’d expect, the higher the percentage of down, the higher the quality of the insulation in your sleeping bag.

When it comes to winter sleeping bags, it’s also worth mentioning hydrophobic down. This is completely natural down that, at one stage of production, is treated in a hydrophobic bath that makes it less sensitive to moisture. In Poland, this raw material has been available from selected suppliers for at least nine years, and even longer worldwide. At this point it’s a highly refined product with long-term durability over many seasons.

During sleep, the human body can release up to about 0.5 litres of water, which in an ideal world should leave your sleep system entirely. That’s why the fabric your sleeping bag is made from should above all be highly breathable. Does that mean you have to give up a water-resistant outer shell? Not at all! Take Pertex® Quantum Pro as an example: it uses an ultra-thin membrane and reaches a hydrostatic head of only about 1000 mm, while still offering top-level breathability.

Another important property is downproofness of both the inner and outer fabric. A high-quality fabric manufacturer can specify this parameter quite precisely, together with how it was achieved. Avoid fabrics with additional coatings such as acrylic; even if they improve downproofness, they noticeably reduce breathability.

If you’re looking for a sleeping bag to use in low temperatures, it’s definitely worth going for a mummy shape – more contoured and more efficient in terms of thermal performance. If you plan to sleep in your bag while wearing down clothing (which, despite the many myths, is absolutely fine!) or want to use a thinner summer bag inside, choose a slightly roomier cut. Extra space inside a down sleeping bag is also useful for stashing gear that’s sensitive to cold, such as batteries or a gas canister.

You should also pay attention to the internal baffle construction. In winter bags, V-shaped or trapezoidal baffles are the most common. In my opinion, V-shaped baffles are the ideal compromise between the weight of the internal structure and very effective down stabilisation. Double-H baffles are used in the most advanced expedition sleeping bags, which can be filled with as much as 1500 g of down.

Another essential element of a winter sleeping bag is the draft collar. It’s basically an extra adjustable down-filled baffle that you fasten around your neck at night and cinch to fit. Its job is to add another barrier that keeps warm air from escaping out of the bag – crucial on really cold nights. Draft collars can be fastened with Velcro or snaps. If it uses Velcro, there should be a way to cover the hard hook side so that it doesn’t damage your delicate thermal layers when the collar is not in use.

Sleeping bag hoods can differ a lot – some people prefer a close-fitting hood, others want more space. In any case, the adjustment should be easy to operate while you’re lying in the bag. It’s also worth considering a moisture-resistant fabric around the face opening. This helps protect the down from moisture from your breath while you sleep.

The footbox is the part of the sleeping bag responsible for keeping your feet warm, so it should be roomy enough and shaped to match the natural position of your feet when you’re lying down. The best manufacturers also offer the option of adding extra down in the footbox to increase insulation exactly where you need it most.

The zipper, which lets you get in and out of the sleeping bag comfortably, is at the same time one of the main places where heat can escape. That’s why winter sleeping bags use down-filled draft tubes along the zip, or they compromise by shortening the main zipper. There’s nothing worse than fighting with a zip that keeps snagging on thin fabric while the wind howls outside and the temperature is around –15°C. To prevent this, reputable manufacturers use various anti-snag solutions: zipper covers and stiffened guard strips that protect the inner fabric.

A good winter down sleeping bag is a serious investment, so my last piece of advice is to choose a trusted local manufacturer. Check whether they take full responsibility not just at the time of purchase, but also in the long run. Conditions in the field are unpredictable, so a solid warranty, support that goes beyond standard customer service, and the option of repairs if necessary are extremely important. Thanks to that you can enjoy your sleeping bag for decades of use.