Gear Guy

Cross Mountain Sleeping Bag     Photo: courtesy, Big Agnes

Q:

Can two three-season sleeping bags equal one four-season bag?

I’m curious if layering sleeping bags is an effective way to turn a three-season bag into a four-season bag. I currently own a Marmot Arroyo, and I wonder if tucking a Feathered Friends Hummingbird or Lark inside of it would do the trick for snow camping, paired with a four-season tent. I know you have both the Marmot Arroyo and a FF Swallow…care to head out into the Cascades and let me know? Nicholas Seattle, Washington

A:Yeah, glad I didn’t head out there this past weekend—blizzard conditions, followed by a near-hurricane, followed by torrential rains. Sweet!

In principle, you’re on the right track. Insulation is insulation, and how you get there doesn’t really matter. Several manufacturers sell either liners or “overbags" designed specifically to add degrees to another bag, or to stand alone as a lightweight bag. The Big Agnes Cross Mountain ($128; bigagnes.com), for example, is a Polarguard Delta-insulated bag that adds 25 degrees, more or less, to the comfort rating of a bag stuck inside of it.

Where it gets a bit tricky is how two different bags fit together. For maximum warmth, a sleeping bag needs to fit its occupant reasonably snugly, so that you don’t have too much air space to warm up. But you also don’t want to be pushing against the insulation, as that can squeeze it between the bag liner and bag shell and reduce its effectiveness. So stuffing one sleeping bag inside another may not yield the desired result if simply smashed inside another. Similarly, because the Arroyo and the Hummingbird/Lark are fairly trim mummies, you might find yourself a little squeezed. The Arroyo is a little narrower than the Hummingbird, and because it’s rated to a bit higher temperature has thinner insulation. So I’d start by using the Arroyo as the inner bag.

What you add temperature-rating-wise is beyond me, as there are too many variables. But you’d surely have a bag that would take you close to zero Fahrenheit, if not lower.

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