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Gear Reviews

Gear School: Synthetic Sleeping Bags

The best new sacks are tough on cold and packed with technology.



>> Synthetic versus down Say goodbye to the days of heavy, bulky synthetic bags. Advances in fiber structure and sack construction mean “it’s almost possible to create a synthetic bag with the same weight, warmth, and compressibility as down,” says Wade Woodfill, Marmot’s sleeping bag guru. One remaining drawback is durability. Down lasts up to 30 years versus 10 years for well-maintained synthetics. The advantages? Synthetic bags can be made from recycled material, are hypoallergenic, cost about 30 percent less, and shine in moist climates because of hydrophobic fibers that dry fast and retain insulating power when wet.

>> Types of fill Continuous filament insulation consists of long fibers, while short-staple insulation contains short fiber segments densely packed together. In general, continuous filaments, like Climashield, reduce drifting, but some are stiffer and less compressible. Fine, short-staple insulation, like PrimaLoft One, is generally more packable and softer to the touch. Manufacturers define a new bag’s performance characteristics by matching an insulation type to a specific construction technique. For example, a winter expedition bag might use high-quality continuous filament insulation and an offset-quilted construction to maximize warmth.

>> Filament technology Fiber choices always dictate price and performance, but three new advances have increased the thermal efficiency (and cost) of bag insulations: Weaving together fibers of different thicknesses (thick fibers prevent convective heat loss, while thin ones trap radiating heat); crimping fibers to add loft; and creating hollow-core fibers that are more stable, lighter, and have larger interior cavities to trap more warm air.

>> Temperature rating This metric is the lowest temperature at which a bag will allow an average person to sleep comfortably. Look for an EN13537 certification—a third-party-verified standard generated by using a heated copper dummy to test efficiency. The EN13537 test attempts to replicate real-life conditions (the dummy wears baselayers and uses a sleeping pad), but remember, overall comfort is a mix of many factors, so check fit and features before considering two 20°F-certified bags equal.

>> Shape/fit Cut determines heat-trapping efficiency: Tapered mummy cuts are warmest; rectangular bags have the most roll-around room and space to heat; and semi-rectangular sacks split the difference.

>> Weight Synthetic bags in the 15°F to 20°F range weigh between 2.5 and 4.5 pounds and heft generally increases along with bag warmth; expect synthetic bags to weigh as much as 30 percent more than comparably rated down.


>> Unpack early Maximize loft (and warmth) by fluffing and laying out your bag at least 30 minutes before hitting the sack.

>> Manage moisture Body vapor and condensation encourage mildew. Air out your bag in the shade every morning, or when it’s wet.

>> Wear baselayers Body oils damage lightweight liner fabrics and fills.


>> Clean Grime decreases loft. Wash spot stains by pulling the shell away from the insulation with your hands, scrubbing mild powder detergent and water in with a toothbrush, rinsing, and air-drying. Watch a demo on eradicating all-over grunge at

>> Store After trips, air-dry and stow your bag in a large cotton sack.

>> Retire Expect about 20 washes or up to 10 years out of your bag before it loses its loft. Without loft, the insulation won’t keep you warm.

Text by Kristin Bjornsen; Illustration by Don Foley

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