Buying An Ultralight Sleeping Bag: When Ultralight Is Right

Big Agnes Mystic SL 15 - Ultralight Sleeping Bag

The Big Agnes Mystic 15 is on my wish list.

Looking for an ultralight sleeping bag? Me too. I like the idea of a warm, comfortable ultra light sleeping bag that weighs next to nothing and packs down small.

Actually, I already own an ultralight bag: the Kelty Light Year 25. (The latest model is the Light Year 20)

It’s a great bag, at a great price, but it’s not quite warm enough for the fall season in Alaska.

Choosing a new ultralight bag might seem straightforward–just get the lightest bag that you can afford, right? However, there are a few more things you might want to think about.

What type of insulation do you need? Down is the obvious choice for most people, since it offers the greatest warmth for the weight. The higher the “fill power” (volume of loft per ounce), the better–and the more expensive.

However, if you hike in very wet weather, it might make sense to get an ultralight synthetic bag. Unlike down, synthetic insulation retains its loft, and therefore its warmth, when wet.

Synthetic bags are also better for drying clothes inside your bag.

Although down still rules when it comes to weight, some synthetic bags come pretty close. Consider the North Face Aleutian 3S, a 20 degree bag that weighs just over 2 pounds. If synthetic seems like the ticket for you, look for Polarguard, Primaloft or Climashield–those are the lightest types of insulation.

You can also get a down bag with a waterproof, breathable shell like the Marmot Helium. This adds a little to the weight–and a lot to the price. Whichever type you choose, make sure the shell is at least windproof and has a DWR coating to shed light moisture.

Choosing a temperature rating can be tricky. Do you want to be on the safe side and get one that’s 10 degrees warmer than your expected temperature, or do you want to push it as light as possible with a higher rating?

If you opt for the latter, you can stay warm by wearing more clothing to bed or by rising before the coldest part of the night (about 4am — ouch).

Size is another head-scratcher. Of course a smaller cut equals less weight and more warmth, but do you really want your bag to live up to the name “mummy”? A little wiggle room might make a big difference, especially if you’re going to wear several layers of clothing to bed.

A top bag solves this problem by eliminating the bottom insulation. This allows a bag like the Big Agnes Zirkel SL +20 to come in at a mere 1 lb, 14 oz without acting like a puffy straightjacket.

I decided I wanted even more room (and warmth), so I’ve put the rectangular Big Agnes Mystic SL +15 on my wish list.

Another way to get an efficient cut without courting claustrophobia is to get a bag with stretch baffles like the Sierra Designs Nitro 30 (weighing in at 1 pound 10 ounces!) or an innovative fabric cut like the MontBell Super Spiral Down Hugger 15 . They both combine body-hugging warmth with extra freedom of movement.

Ultimately, choosing the best ultralight sleeping bag for your needs comes down to a thoughtful consideration of what’s most important to you — weight, comfort, or price. With so many great ultra light bags available, there’s sure to be one that’s perfect for you.

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