Montbell Seamless Down Hugger WR 900 #3

United States Patent No. 7,900,301, filed on December 19, 2008, is notably vague in its abstract. It describes “a woven fabric product made of woven fabric pieces cut out of woven fabric,” positioned in a particular way. The diagram adorning the patent’s first page depicts three stacked rectangles. The outermost two are characterized by diagonal lines and the one in the center by pointillist dots; it might be an aerial view of a scenic parkway or a cross-section of a grilled cheese sandwich.

It’s neither. The patent is for Montbell’s Spiral Stretch System, a sleeping bag innovation the brand swears by to this day. Its inventor-speak language describes an elasticized construction that allows a bag to embrace its occupant while stretching to accommodate any tossing and turning. It’s cozy, non-restricting and, because it eliminates extra space between bag and body, heat efficient, too.

“Spiral Stretch” refers to the “Down Hugger” portion of the Seamless Down Hugger WR’S wordy moniker, but there’s another innovation, patent pending, that gets you the “Seamless” bit. It’s called Spider Yarn, and it enables Montbell to abandon the traditional method of building sleeping bags with baffles, the interior fabric separators typically belied by a bag’s exterior rows of stitching. Baffles exist to create tubular channels, which prevent the down filling they hold from shifting into uneven concentrations.

Spider Yarn effortlessly assumes that function. Strands of it, arranged like a web, trap clusters of down in place, creating a sheet of evenly distributed fluff. Without baffles, the down is left to loft unrestricted, increasing overall warmth and minimizing cold spots. Bonus: the stitch-free design also makes it look as sleek and cool as a sleeping bag can.

Specs

Temperature Limit: 30°F

Insulation: 900-fill Power EX Down

Shell: Weather-resistant Gore-tex Infinium Windstopper $589