AMC Outdoors, October 2002

By Michael Lanza

We were camping above 6,000 feet in the backcountry of southern Utah's Capitol Reef National Park on the weekend after Thanksgiving. The night was clear, calm, and cool, and the temperature dropped to about 30 degrees Fahrenheit overnight — which just happened to be what my friend Bill's sleeping bag was rated. As we all rolled out of our tents the next morning, Bill looked groggy. "I figured out what that 30 rating for my bag means," he announced. "It means you won't die if it gets that cold."

What Bill really discovered, of course, was that the bag he finds warm enough for summer nights doesn't cut it for him in autumn temperatures. The truth is that finding a bag that's right for you has a lot to do with your metabolism and where and when you plan to sleep in that bag. Finding the right bag — or bags, if you camp in multiple seasons — comes down to knowing your own needs.

Feeling Hot, Hot, Hot
Sleeping bags keep you warm by trapping your body heat in tiny air pockets in the bag's fill, or insulating material. Your body is the furnace and needs fuel or you'll get cold regardless of your bag. Consume enough food and water before going to bed or while in the sack. Dress comfortably, wearing enough layers for warmth without cramping your space too much; or pad the space around your body with extra layers, which will come in handy if you cool down during the night and want to pull on more clothing. The other big component in keeping warm while sleeping outside is your pad (see a related article on sleeping pads).

Bag temperature ratings are not the final word on warmth. They're set by the bag maker, so a 20-degree bag from one manufacturer might be warmer than a 20 bag from another. But ratings do provide a general guideline. Summer bags are usually rated higher than 30 degrees, three-season bags between 10 and 25, and winter bags under 5, but there's great variation within those ranges. In the mountains of northern New England, I'll rarely sleep out in winter with anything less than a minus-20 bag, while at lower elevations in southern New England a 0-degree bag will do.

I don't get cold easily, and rarely have to put on extra clothing when the temperature outside is close to my bag's rating. My wife, who runs much colder, needs a bag rated 10 to 20 degrees lower than the coldest temps she'll encounter. Conversely, I usually find, for example, a 20 bag too warm for nights above 35. Don't get a bag that's too warm for most nights you'll sleep out: You'll be too hot inside it and too cold atop it.

Get Down (or Synthetic) Tonight
Bags are filled with one of two types of insulation: synthetic or goose down. There are several brands of synthetic fill. The least expensive (like Hollofil and Quallofil) are better for car-camping because they weigh more and take up space, while the lightest and most compact fills are preferred for backcountry bags but are more expensive; in the latter category, Polarguard 3D is most common. In general, compared with similarly rated down bags, synthetic bags are less expensive and dry more quickly, but are also heavier and bulkier. Synthetic fills also keep you warm (if not incredibly comfortable) when wet, while a wet down bag is useless.

While they are more expensive than synthetic, down bags are lighter and more compact. Given proper care, they last longer. Down fill is rated with a number that represents the volume of space, in cubic inches, an ounce of that down fills. An ounce of 700-fill down will puff up to occupy 700 cubic inches of space. Higher fill ratings correlate with a lighter and more compact bag as well as higher cost. You can expect to pay $70-270 for a three-season synthetic bag and $125-400 for a three-season down bag.

So, which should you buy? For starters, while big-wall climbers and polar explorers use synthetic because their bags could get really wet, most recreational backpackers sleeping in a good tent probably don't need to worry about that. Many frequent backcountry travelers I know prefer down because it's so much lighter and more compact. But if you sleep in the backcountry only occasionally or don't want to spend too much, you'll probably be perfectly happy with a synthetic bag for the several years of the bag's life. Mummy bags, which are tapered to human dimensions and usually have a hood, are the most common design for backcountry camping because the shape is most efficient at keeping you warm. Semi-rectangular bags are roomier but also more cumbersome and less efficient at trapping heat. Most bag models come in two or three lengths.

You Better Shop Around
When it comes time to choose your cocoon, remember: The best cursory measure of a bag's warmth is its loft, or thickness. In the store, lay out a few bags and compare their fatness.

Before buying, crawl inside the bag in the store and close it up. See whether it has enough space for you, and whether you like the feel and position of the zipper, hood, and drawcords. Consider how much clothing you want to be able to wear inside the bag and whether it has the space for that. Don't get a bag that's roomier than you need, because having a lot of extra space to heat up can leave your body feeling colder.

Women tend to get cold more easily than men because women often have less body mass relative to surface area, making them lose heat more quickly. Women's sleeping bag models are typically a little shorter and cut to conform to a woman's body (narrower in the shoulders, wider in the hips) than standard unisex (read: men's) bags, making them more efficient at keeping a woman warm at night. Some also have a little more insulation in places like the foot to keep those extremities toasty.

Papa's Got a Brand-New Bag
Some recent innovations in backcountry bedding include:

• Down fill ratings up to 900, thanks to continuing improvements in the quality of down feathers and cleaning techniques. Due to the commercial opening of Eastern Europe, older geese, with their finer, smaller "plummules," are now more available on the market- and so is higher loft down. What's the difference, you ask? Just take two 20-degree down bags, one containing 900-fill, the other 600-fill. While they're probably equally warm and fat, the former could be half the weight and bulk of the latter.
  
  
• Bag shells that are water-resistant, highly windproof, and breathable, most notably those made with either DryLoft or Epic by Nextec. These add cost to the bag and make it less breathable than the more common microfiber shells that aren't water-resistant, but they're a good choice for anyone who sleeps in extremely wet conditions or without a tent.
  
  
• Bags that pull double duty as campsite parkas, like the Nunatak Raku Alpine Bag ($497 to $581, depending on size and options; 866-686-2825, www.nunatakusa.com), which has sleeves and a drawstring closure at the feet allowing you to pull it up to your knees or hips to walk around; and the Exped Wallcreeper ($199, 888-609-7187, www.exped.com), which has zippered arm openings, exterior hand-warmer pockets, and a cinch cord at the foot opening.
  
  
• Bag accessories that let you get a little closer to your sweetie, like the Sweetie Pie Bag Doubler, from Functional Design ($99, 206-323-0061, www.functionaldesign.net). Available in models rated 10 and 40 degrees, the Bag Doubler is a wedge filled with Polarguard 3D that zips into many mummy bags, turning a bag for one into a bag for two. It's lighter, less bulky, and less expensive than a second sleeping bag, and also zips up by itself into a bag the right size for a child.

With the right bag, a blissful night in the backcountry doesn't have to be something to die for.

—Michael Lanza is author of The Ultimate Guide to Backcountry Travel, from AMC books.