Ernest's Question:
I am looking for a four season down sleeping bag. And I would like to know if down is cool in summer and warm in winter. Is that the way it works in down sleeping bags? What fill, what down, is the best?

—Ernest

Karen's Answer:

	Karen Berger Meet Karen Previous Answers Hiking Forums

Dear Ernest —

Unfortunately, that's not the way down works. There really isn't such a thing as a sleeping bag that will keep you comfortable in all four seasons. Down works by trapping air. Your body heat warms the trapped air, and that's what insulates you. The bigger and fluffier the bag, the more air it traps, and the warmer you stay.

There are three reasons why some bags are warmer than others:

The amount of down in the bag, because more down traps more air.
The quality of the down. Ounce for ounce, higher quality down traps more air than lower quality down. This means that three ounces, say, of high-quality goose down will trap more air than three ounces of lower quality down or down mixed with feathers. The type of down will be described on the tag.
Design. Good-quality winter mummy bags are tight-fitting so that your body doesn't have to heat too much air. They also have hoods, to keep heat in, baffles designed to keep the down in place, and channels over the zippers, which prevents drafty air from sneaking in.

The bottom line is that a bag that's designed to keep you warm when it's 40 below zero is going to make you hot and sweaty when it's 40 above. All down bags are assigned temperature ratings by the manufacturer. These ratings are reasonably consistent (but not foolproof) within the industry. Also, different people react differently to cold; some of us need warmer bags than others. I've found that temperature ratings are a bit optimistic for me: If it's really and truly 0 degrees out there, I'll be a lot more comfortable in a minus 10 bag.

Sleeping bags are comfortable for a range of temperatures. It differs from person to person, but I find that if a bag is rated to 20 degrees, it will usually be comfortable up to about 50 degrees. If it gets warmer than that, I'll unzip the bag and use it like a blanket. There's nothing wrong with carrying a sleeping bag that's a little too warm, except that it's more heavy than it needs to be (And guess who has to carry it?). But at some point, even if it's unzipped, a heavy winter bag will simply be too warm for summer conditions.

Most Appalachian Trail thru-hikers (who travel in conditions ranging from late winter to summer) use two different bags on the journey.

Karen