Nights are long in the cold season, so you’ll likely be snuggling into your bag for at least a few extra hours in your tent. And winter’s bone-chilling temperatures demand an ultra-toasty bag—if you overestimate your model’s warmth in the summer, you can probably still make it through the night all right. Not so in winter. Plus, you’ll need a sleeping bag with the features to deal with the unique challenges of snow travel. Here’s what to consider when you extend your camping season into the cold months.

In this article, you’ll learn:

• What temperature ratings to look for in a winter bag
• Pros and cons of synthetic versus down insulation
• Features that protect against winter moisture
• How to choose the right shape and size
• Which special features to look for

Temperature Rating

Winter sleeping bags, like all sleeping bags, have standardized warmth ratings called ISO or EN ratings. In milder climates, a 10- or 15-degree bag can be enough to get the job done. A 0-degree bag will probably be comfortable in a wide range of winter conditions. And if you’re heading into high-alpine terrain, frigid temps, and/or polar regions, look in the minus 20- to minus 40-degree range.

Insulation

Your choices are still synthetic fill and down, but your calculus might change a bit when you compare winter options.

Synthetic insulation is warm even when wet and the most affordable option, but it makes for heavier, bulkier sleeping bags. If weight and size aren’t a problem on your winter excursions—say, for car camping, hunting, or shorter hauls into a base camp—then synthetic bags can be a good choice.

Down insulation is more compressible than synthetic fill and warmer for its weight, reducing bulk. And most down is treated for water resistance these days, so down bags can handle moderate moisture. If you’ll be carrying all your gear on your back for backpacking, mountaineering, or ski touring, a down sleeping bag will stuff more warmth into an easier-to-pack size.

Fill power is an important stat for down bags. It measures the “loftiness” of the plumage used: The higher the rating, the less insulation you’ll need to achieve the same warmth, and therefore the lighter the bag will be. On the flip side, the higher the rating, the more expensive the sleeping bag. For serious cold-weather trips, consider a bag with 800- to 1,000-fill down.

Moisture Resistance

Water is a particular concern in the winter because you’ll be dealing with snow (specifically, melting snow), plus increased condensation on the inside of your tent. The best winter bags will protect the insulation (and you) with hydrophobic down, DWR-treated shell fabric, and sometimes, a waterproof/breathable membrane.

Shape & Size

Most cold-weather bags use the mummy cut, a somewhat claustrophobic shape that’s nonetheless the most thermally efficient option.

Getting the right size in your bag is key: Too much wiggle room inside the bag will chill you, as it’s more space for your body to heat up. But in winter, you might need to bring extra gear inside your bag, such as extra layers, water bottles, or boots (to prevent them from freezing). Make sure the bag you choose strikes the right balance.

Features

You’ll appreciate special warmth-boosting features on any cold-weather trip. Look for big draft tubes and draft collars that block icy breezes from sneaking in at the zipper or hood. Some models boost versatility with zippered vents: Open them on warmer evenings and close them when it gets colder. Arm vents let you use your arms and hands while still wearing the bag, a plus if you want to brew tea or play a card game in your tent. And some of the warmest bags have a snorkel hood that looks a bit like a tube extending up from your mouth; this protects your face from the wind and keeps frost from your breath from building up around the hood’s edges.

All articles are for general informational purposes.  Each individual’s needs, preferences, goals and abilities may vary.  Be sure to obtain all appropriate training, expert supervision and/or medical advice before engaging in strenuous or potentially hazardous activity.