So, to your first conundrum: down or synthetic? I'm generally a down advocate, based on its lightness, durability, and comfort. The exception would be below-freezing winter camping, when melting snow and moisture can be a problem. But for most cold-weather camping, down is clearly the winner.
As for the temperature rating, I'd say you should aim for at least zero degrees Fahrenheit. Minus ten would be even better. Generally, buy a bag that's rated for the lowest temps you're apt to face. You might see colder weather, of course, but can always add clothes or just tough it out a little.
So what bags? On the down front, you won't go wrong with Western Mountaineering's Puma Super MF ($495; www.westernmountaineering.com), rated to minus ten with inches and inches of loft from its 850-plus fill power, a durable and breathable microfiber shell, and a comfortable hood and draft collar. Sure, it's expensive, but it will last for years. And it weighs only three pounds, ten ounces. For a more budget-minded bag, L.L. Bean's Mount Washington bag, rated to zero degrees, has 775-fill down and an excellent Pertex shell. Only $225a real bargain (www.llbean.com).
On the synthetic front, Integral Designs' North Twin uses Primalofta soft, down-like syntheticto create a bag with a ten-degree rating (good enough for quite a bit of winter use) and a somewhat reasonable weight of three pounds, eight ounces. Cost is $220 (www.integraldesigns.com). The North Face, meanwhile, offers its minus-20-degree Tundra, which is notable for its not-unreasonable weight of four pounds, four ounces and its eminently reasonable price of $239 (www.thenorthface.com).
I'd probably go with the Bean bag. And, get two sleeping pads: a self-inflating pad as well as a closed-cell foam pad such as the Cascade Designs Z-Rest ($36; www.cascadedesigns.com). This setup really makes a difference in cold-weather camping.
Filed To: Snow Sports