Everyone knows layers of clothing are the way to go when heading out for any backcountry trip. It would be unfathomable to go backpacking and only bring a t-shirt and a down jacket. Yet, when it comes to sleeping bags, that’s exactly what most people do. They have a single three-season sleeping bag, and come heat, humidity, or frigid temps, that’s their bag and they’re sticking to it. The MontBell Ultra Light Spiral Down Hugger Thermal Sheet is a good alternative to the one-size-fits-all approach to sleeping bags. I used this 13.8-ounce featherweight as a liner in conjunction with my 20-degree bag; it kept me warm on St. Mary’s Glacier in Colorado on a below freezing night when a blizzard ripped my tent down. But it also works great as a standalone warm-weather bag (it’s rated to 50°F), as I discovered while camping in Virginia springtime.
The Thermal Sheet includes features that are normally found on top-shelf sleeping bags, like 4.5 ounces of lofty 800 fill-power down, a superlight 12-denier shell fabric, and a special spiral baffle construction that lets the bag stretch and move with you. MontBell achieves the stretch through a combination of features. The first is that the fabric itself is cut on a diagonal instead of “with the grain”, which gives a little flex. Then the baffles (the tubes filled with down) are wrapped in a spiral around you instead of horizontally stacked like in a regular bag. The benefit to this design is that when you need more space and push against the shell, you’re pushing against the stretchy fabric rather than being constricted by a tight seam. This lets the bag move with you when you need space, and decreases the amount of air you have to heat up. While the stretch isn’t as expansive as bags with elastic in the baffles, it’s still considerably more than a standard bag shell--I found it to be about as stretchy as a cotton t-shirt. That was important to me because I’m a side sleeper who rolls around a lot, and my knees tend to get trapped in many narrow-cut bags.
Although the stretch is a key aspect of the Thermal Sheet, versatility is where it really shines. The Thermal Sheet is a rectangular, hoodless bag with an around-the-foot zipper, which lets you use it as a quilt for yourself or to share with a friend if you don’t mind getting cozy. Or you can slide it inside your three-season bag and increase your warmth rating considerably; I estimate that it adds about 15 degrees of warmth to an existing bag. The shell has a Polkatex DWR coating that repelled small puddles of water when snow blew under the edge of my floorless tent while snow camping in Colorado. It packs down to the size of a grapefruit and weighs so little that I’ve been using it as my emergency bag on mountaineering trips, including summit day on Mt. Rainier. Thankfully I haven’t had to pull it from the stuff sack in an emergency yet, but I like the added security knowing that it’s in my pack.
If you’ve already invested in a nice three-season bag but find yourself wishing for more warmth and don’t want to buy a dedicated winter bag, try this solution. It will far extend the useful range of your existing bag, and you’ll have a summer and emergency setup, as well.