The Best Down Sleeping Bags
Down is still the best insulation for a sleeping bag by weight and packed size, and it still lasts decades if looked after. Three things about buying one have changed.
Read the standardised rating, not the name. Bags now carry EN or ISO figures with two numbers: comfort and limit. Comfort is the temperature at which a cold sleeper is comfortable, and the test assumes a colder sleeper for that figure; limit is for a warm sleeper. A bag called a "20 degree bag" tells you nothing on its own. This standardisation is the biggest improvement in the category since this post.
Understand fill power properly. 800-fill down gives the same warmth as 600-fill for less weight and bulk. It does not make the bag warmer; the rating does that. Paying for high fill power buys you a lighter, smaller bag at the same warmth, which is worth it if you carry it far and not if you car camp.
Hydrophobic down changed the down-versus-synthetic argument. Treated down resists moisture far better than untreated, which removed most of the old case for synthetic. Synthetic is still cheaper and still right if the bag will be permanently damp.
Ask where the down came from. Not a question in 2009, and a straightforward one now: the Responsible Down Standard and similar schemes certify that birds were not live-plucked or force-fed. Most reputable brands use certified down and it costs nothing extra to choose.
The care that decides whether you own it for twenty years or five: never store it compressed. Hang it, or keep it in the big cotton sack it came with. Wash it rarely, in a front-loader, with down-specific soap, and dry it with a couple of tennis balls to break the clumps.
Part of the guide to sleeping bags and pads.