The Therm-a-Rest Mira takes advantage of the fact that virtually every backpacker carries a sleeping pad as well as a bag. By relying on the pad’s insulation, Therm-a-Rest designers cut weight by eliminating most of the insulation in the bottom panel of the Mira. The result is a somewhat roomy down bag that proved to be among the warmest we tested, while also being the lightest.
Compressibility Because the Mira (and men’s Antares) eliminates a significant portion of the insulation needed in the bag–the bottom panel holds very light down–the bag can be stuffed into a miniscule 7-inches by 10-inches tube (fully compressed). The Antares cinched down to 7-inches by 12-inches.
Warmth to Weight Properly attaching the bag to a sleeping pad is easy to accomplish thanks to a pair of broad, stretchy straps. Just slip the pad under those straps and your bed is made. The Mira kept even our coldest-sleeping tester warm and comfortable during a blustery night of snow flurries on Mount Rainier when temperatures bottomed out at 18ºF. The inclusion of an extra foot pocket, dubbed the “Toe-asis Foot Warmer” within the well-contoured toe box meant our chronically cold tester had happily warm toes throughout the night–something that’s usually impossible to achieve! That foot pocket acts like a secondary two-footed mitten within the bag, helping hold heat around your feet. The men’s bag lacks this innovation, but our male testers felt it wasn’t necessary.
Comfort The reliance on a sleeping pad to provide insulation meant that the thin bottom of the bag had to remain on the pad. That was not a problem for back sleepers, nor for most side sleepers. But some of our team twist and turn, rolling from front-to-back throughout the night. Usually, they just roll the entire bag, but because the Mira requires strict adherence to its proper orientation, these active sleepers found they had to move solely within the bag rather than moving the bag with them. That also meant the hood couldn’t move with them on a full rollover, so they’d end up face down into the back of hood. Still, for the average sleeper, the bag was roomy enough to wiggle and shift a reasonable amount.
Durability The 20-denier polyester shell showed no signs of wear or tear. The placement of the zipper and its backing effectively prevented fabric snagging in the zipper’s teeth. In short, we didn’t experience any problems or see any potential for problems.
Value Though it may not be the best option for everyone, the lightweight design and incredible warmth more than offset the price of the Mira/Antares.