We felt it was warmer than most of the other bags rated similarly, and could probably push it into the 20°F range if you are feeling lucky. Although you feel the ultralight shape a bit during the night, we think that the warmth-to-weight ratio is excellent, and enough to deserve the Editor's Choice for a Mummy Sleeping Bag.
The Summerlite kept us warm and dry on a rainy overnight in the Canaan Mountain Wilderness.
On a spring climbing trip to the Utah Hills, when nights were around freezing, the Summerlite kept our lead tester warm and toasty without supplementing much other clothing, something other ultralight bags and quilts don't often provide. The full zip and hood allow the full burrito option the way any traditional mummy bag would, which will always be at least slightly warmer than a quilt, especially for those who tend to toss and turn at night. Except on the coldest, windiest nights in this bag, we had to keep it at least slightly unzipped to vent the heat. Although Western Mountaineering doesn't do EN ratings, we think that the 32-degree Fahrenheit rating is more accurate than some of the other bags in the review.
Part of the reason the Summerlite is so warm is because of the continuous baffles. Unlike the Western Mountaineering Highlite, there's an unbroken four-inch layer of down separating the two nylon shell fabrics. This eliminates cold spots and allows the ten ounces of down to work together, minimizing the need for more fill. The tighter cut also eliminates dead air space in the bag and allows the Summerlite to do more with the ten ounces of 850+ down fill. Perhaps the only bag warmer (proportionally for its rating) in the review is the Zpacks 20 Degree.
The Summerlite fluffs up to a warm 4-inch fill and is one of the toastiest bags we tested.
For a full zip, hooded mummy bag, the Summerlite does pretty well in this department. It's not the absolute lightest mummy we tried; the Highlite and the Marmot Phase 30 are lighter, but it isn't much heavier than either while delivering more warmth. In fact, for an extra .07 ounces, we felt that the Summerlite was a good deal warmer than the Phase 30.
The Summerlite weighs within half an ounce of the Feathered Friends 40 UL and the Katabatic Gear Palisade 30. We think that for the same weight, the Summerlite delivers a bit more on warmth. Still, there are lighter bags in the review if weight is the primary consideration.
The Summerlite doesn't pack down as well as some of the other bags, and at 19 oz it's about middle of the road for weight.
In the ultralight sleeping bag game, there are three main categories: warmth, weight, and comfort. You get typically two of those categories at the cost of the third. For the Summerlite, Western Mountaineering decided to sacrifice a roomy fit to achieve something with a high warmth to weight ratio, as opposed to removing zippers and hoods like some other bags. The space around the shoulders and head was roomy enough for our 5'8" tester, but the footbox felt especially tight up to the knees. For those, like our main tester, who tend to spread out and roll a lot while sleeping, this wasn't as comfortable. The Summerlite might not be an issue for more still sleepers, but it is a consideration if you plan on keeping things like water bottles or spare clothing in your bag at night.
We do like the fabric, and there isn't anything that was poorly placed, like the drawcords or velcro. This sleeping bag feels the way sleeping bags should; like a big warm down hug. Although the hood wasn't quite as deep as we liked, the oval shape allowed for side sleepers to roll and not end up trying to breathe through nylon all night.
The WM Summerlite saves weight by shaping the bag slim in the legs and feet.
Like any full zip mummy, the Summerlite is really versatile. You can unzip it when it is hot, or burrow down into it when the temps drop. The Marmot Phase also has a full-length zipper, which, along with the Summerlite, allow for better venting than the Patagonia 850 and Highlite.
The warmth allows this bag to be used early spring or late autumn and could work for an entire through hike such as the PCT, especially if you incorporate a clothing system into the mix. Because it vents to well, our testers are happy to use this bag on warm summer nights as well.
The Summerlite did pretty well with repelling condensation, and even a deluge from a water bottle without the fabric wetting out much.
The features of this Western Mountaineering bag are pretty well thought out. We like the full-length, burly #5 YKK zipper, and the small strip of stiffer fabric next to it to avoid being sucked into the teeth, which was nice when we were adjusting it in the middle of the night. We like the offset drawcord of the hood that didn't get in the way, but we did think that the hood was a bit shallow, and didn't allow for as much coverage as some hoods.
The stuff sack is adequate, but it isn't a compression sack or drybag, which are excellent additions other bags provide. However, we acknowledge that many thru-hikers will leave the included sack, whether it's a stuff or compression type, at home to save weight, preferring to stuff an unwrapped sleeping bag into their packs. Overall, with some small design tweaks, this could have scored a bit higher.
While we liked most things with the Summerlite, we wished the hood was a little deeper.
This bag could be great for those who value weight and warmth over a roomy cut, like smaller bodied thru-hikers. We would happily use this bag on summer alpine climbing trips or long through hikes. Less restless sleepers or smaller folks will get more use out of this bag than larger folks, who might look at the similar but larger Western Mountaineering Megalite.
Western Mountaineering thoughtfully integrates a stiff strip of fabric to avoid zipper snags.
The Summerlite isn't the most expensive bag on the list, but at $410, it's pretty close. Still, you're paying for quality, and the Summerlite delivers. We like this bag, and other mummy bags in the review are almost as expensive, but if you're looking to save money, a quilt might be a better option.
The Western Mountaineering Summerlite does many things well. It keeps you really warm, packs down fairly well, and doesn't add too much to your base weight. To keep the warmth and weight down, the cut is fairly slim, which might impact larger folks or restless sleepers. Still, the Summerlite performed at the top echelon of bags we tested, and if you're looking for a traditional sleeping bag for an ultralight setup, we think the Summerlite is a great option, enough to win the Editors' Choice Award.