Big Agnes Sidewinder SL 35 Sleeping Bag Review

Big Agnes Sidewinder SL 35 Sleeping Bag review

The Big Agnes Sidewinder SL 35 is a new type of sleeping bag designed for side sleepers and so-called rotisserie sleepers that roll around and change positions when they sleep. Instead of a mummy hood and draft collar, the Sidewinder has a jacket-style hood so you can sleep with your head turned to the right or left side or both sides if you roll back and forth during the night. Gone too is the notion of a right-hand or left-hand zipper: now the zipper runs down the center of the bag so you don’t have to choose. But is the Sidewinder really the”Award Winner” that Backpacker and Outside Magazine claim, or is it just ho-hum? Let’s take a closer look and you can decide for yourself.

Specs at a Glance

  • Temperature Rating: 35 degrees
  • Gender: Men’s (a women’s version is also available)
  • Size: Regular (a long size is also available)
  • Fits: Up to 6′ / 183 cm
  • Girth: 61″- 155 cm/ 55″ – 140 cm /  36″ – 91 cm  (shoulders / hips / feet)
  • Insulation: 650 fill power DownTek and Fireline ECO Synthetic Insulation
  • Weight: 2 lb / 907 g
  • Fill Weight: 14.5 oz / 411 g
  • Compressed Size: Regular: 6″ x 7″ / 15 x 18 cm
The Sidewinder has a jacket-style hood and a center zipper.
The Sidewinder has a jacket-style hood and a center zipper.

Jacket-Style Hood

The Sidewinder SL has a jacket-style hood like one you’d find on a down parka or jacket. Technically speaking it is still a mummy hood, but it’s oriented so that the zipper runs up the center of the sleeping bag instead of on the right-hand or left-hand side, ending just below your chin.

The jacket-style hood is quite deep to make room for an inflatable pillow, which can be tucked into a pillow pocket at the back of the hood. If you don’t use a pillow or don’t want it inside the sleeping bag hood, or you simply have a small head, there will be a large gap between your face and the hood where your body heat will escape. If you do use a pillow, the gap will still be there, just smaller.

The central zipper and jacket-style hood mean you can roll onto your side and breath freeline without having to exhale back into the hood like ypu wood with a conventional mummy hood.
The central zipper and jacket-style hood mean you can roll onto your side and breath freely without having to exhale back into the hood like you would with a conventional mummy hood.

This is exacerbated by the fact that the Sidewinder doesn’t have a draft collar, a tube of insulation that wraps around your neck, upper chest, and neck found on other sleeping bags that prevents hot air warmed by the rest of your body from leaking out around the hood. It’s a pretty fundamental omission in a sleeping bag designed to keep you warm in freezing temperatures.

Big Agnes contends that you can seal out drafts by cinching the insulated tube along the upper hood opening, but I found that has little effect at closing the gap between your neck, face, upper chest, and the hood. Most quality sleeping bags and backpacking quilts have draft collars inside the bag to prevent this sort of thing.

A pillow placed inside the hood, can, depending on the size of your head and the size of the pillow, position your head more forward in the hood. But even then, I found that a big gap between my chin and the rest of the sleeping bag where warm air would escape whenever I moved around. You can prevent this from happening by wrapping a sweater or down jacket around your neck like a “poor mans” draft collar, but this isn’t rocket science.

Center Zipper

The Sidewinder SL 35 comes with a full-length, two-way zipper, so you can unzip the bottom part of the bag to vent your feet. However, the top half of the zipper, which you pull up to close the bag over your body and chest, snags constantly. I’ve closed the zipper on this bag a hundred times and the shell fabric gets caught in the zipper every time without fail. It’s really annoying.

The zipper snags every time you try to pull it up - without fail.
The zipper snags every time you try to pull it up – without fail.

That said, the zipper is backed with a beefy insulated draft tube that folds over the zipper to prevent cold drafts from leaking in and works well.

Insulation

The Sidewinder SL bags (35 and 20 degrees) are insulated with a combination of down and synthetic insulation. Water-resistant 650 fill power DownTek down is used throughout the bag but is augmented with synthetic insulation in the foot box and hip area since these are areas that you’re likely to compress at night, thereby limiting the ability of the down insulation from lofting and holding warm air.

The zipper has a beefy draft tube by there’s no draft collar to prevent warm air from leaking out past your neck.
The zipper has a beefy draft tube by there’s no draft collar to prevent warm air from leaking out past your neck.

The synthetic insulation used in the bag is called Fireline ECO which is 100% post-consumer recycled polyester. It features low denier fibers that are tightly woven and provide high loft, compressibility, high weight-to-warmth ratio, and durability or so they say. Big Agnes isn’t forthcoming on how much down and how much synthetic insulation is used in the bag by weight, however, so it’s hard to assess what the mix is. Still, the combination is a good hedge against dampness from affecting the bag’s insulation value.

But one thing worth noting is that the temperature rating of the Sidewinder bags, both the 20 and 35-degree models, have not been measured using the standard ISO sleeping bag temperature rating standard. I’m a little surprised by this since I’d have thought the bag’s construction qualified for that kind of independent test, and even more so by the fact that a retailer like REI would even carry it without an ISO-tested lower limit and comfort rating. I wonder what the back story is there.

There’s still a huge gap between my neck/face and the neck area even when the hood is fully tightened. This allows warm air heated by your torso to be released through the hood (without your consent).
There’s still a huge gap between my neck/face and the neck area even when the hood is fully tightened. This allows warm air heated by your torso to be released through the hood (without your consent).

Big Agnes Sidewinder SL 35 Sleeping Bag

Warmth
Weight
Comfort
Features
Versatility

Lackluster

Great concept, but poor execution. The hood leaks cold air and the zipper snags all the time. Give this one a pass.

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Assessment

The Big Agnes Sidewinder SL 35 Sleeping Bag is pretty lackluster and hardly deserving of any “industry awards.” The terrible zipper and leaky hood disqualify this bag from serious consideration. If you’re a side sleeper who wants a sleeping bag that’s more comfortable than a conventional mummy bag, I recommend you get yourself a hoodless sleeping bag (see Hoodless Sleeping Bags: Pros and Cons) or even a quilt, and use a down or synthetic insulated jacket to keep your neck, shoulders, upper chest, and head warm instead. Check out my recent review of the Kelty Galatic 30 for a hoodless sleeping bag that is easily paired with an insulated hooded jacket for half the price of the Big Agnes Sidewinder SL 35 sleeping bag and is equally suited for side sleepers.

Disclosure: The author purchased this sleeping bag.

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One comment

  1. Wow, so glad I didn’t buy this recently! I was almost sucked in by the side sleeper marketing, but bought a traditional mummy instead. Turns out it’s plenty roomy for me. Thanks for your review!

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