A sleeping bag has three temperatures under ISO 23537, and the one on the label is usually the middle one. Rab publishes all three for the Neutrino: comfort 14 °F (−10 °C), limit of comfort 0 °F (−18 °C) and extreme −40 °F (−40 °C), against the ISO 23537-1:2022 rating, and names the bag after the limit. The same maker defines them plainly: comfort is the lowest temperature at which a person sleeps relaxed and gets a good night's sleep, limit is the lowest temperature at which someone curled up and fighting the cold still sleeps through the night, and extreme is an emergency figure for up to six hours in a curled position at which frostbite is very possible. The condition that changes all three is the pad, because the test puts a thermal mannequin on a standard foam mat.
Comfort and limit sit 14 degrees apart
The gap between the two usable numbers is large enough to change what the bag is for. On the Neutrino it is 14 °F, and on Therm-a-Rest's Parsec 0F the published EN comfort is 14 °F (−10 °C) and the EN limit 0 °F (−18 °C), the same 14-degree span. A bag sold as a 0-degree bag is therefore a 14-degree bag for anyone who sleeps cold, and the difference is not a manufacturing variation but the definition of the two figures.
The name on the bag rounds the limit
Rab states that it names its bags by the limit of comfort because that reflects the minimum temperature most users would expect to sleep at. Naming conventions are not uniform: Therm-a-Rest's Parsec 20F publishes an EN limit of 18 °F (−8 °C) and an EN comfort of 29 °F (−1 °C), so the name rounds the limit by two degrees and sits eleven degrees below the comfort figure.
Two bags rated 0 °F, two different fills
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| Bag | Comfort, as stated | Limit, as stated | Fill weight | Total weight | Price |
|---|---|---|---|---|---|
| Rab Neutrino 0°F | 14 °F (−10 °C) | 0 °F (−18 °C) | 28.2 oz (800 g) of 800 fill power | 2 lb 13 oz (1,280 g) | $660.00 |
| Therm-a-Rest Parsec 0F | 14 °F (−10 °C) | 0 °F (−18 °C) | 27 oz (762 g) of 900 fill | 2 lb 7 oz (1,100 g) | $599.95 |
| Therm-a-Rest Parsec 20F | 29 °F (−1 °C) | 18 °F (−8 °C) | 17 oz (490 g) of 900 fill | 1 lb 13 oz (820 g) | $469.95 |
The first two rows hit identical ISO figures by different routes. Rab fills the Neutrino with 800 g of 800 fill power European goose down in a 1,280 g bag, which is 62.5 percent of the bag's weight. Therm-a-Rest fills the Parsec 0F with 762 g of 900 fill duck down in a 1,100 g bag, or 69 percent. Higher fill power buys the same warmth in less weight, and the finished bag is 180 g lighter for it.
Fifteen grams of down per degree
Example, from one maker's two bags. The Parsec 0F carries 762 g of fill and the Parsec 20F carries 490 g, a difference of 272 g of the same 900-fill down in the same shell fabric. The published limits are 0 °F and 18 °F, 18 degrees apart. That works out to about 15 g of down for each degree Fahrenheit of limit, and 280 g of extra bag weight for the pair of numbers.
The price follows the same line: $599.95 against $469.95, or $130 for those 18 degrees, which is about $7.20 a degree. Packed size moves too, from 7 by 10.5 inches to 7.5 by 11 inches. Those are the three costs of a colder rating, and none of them appear in the name on the stuff sack. The volume difference lands directly on the hiking backpack it has to fit into.
The standard stops at −20 °C and leaves quilts out
The rating scheme has an edge. Rab states that its quilts and expedition sleeping bags fall outside the scope of the EN standard, which does not cover quilt constructions and does not measure performance below −20 °C, and that it tests those products by the same methods and publishes an advisory limit of comfort calculated from the results. An advisory figure and a rated figure are not the same claim, and only one of them was produced inside the standard's scope.
Below that boundary the comparison reverts to construction details: baffle type, fill weight, shell denier and shape. The Neutrino uses a tapered mummy cut with a 10-denier Pertex Quantum top and 20-denier base, while the Parsec 0F is 80 inches long and fitted to a user height of 72 inches. A bag that is too long holds air the sleeper has to heat, which is the same reason a camping tent sized for three feels cold with two in it.
Before you buy a sleeping bag
- The comfort figure as well as the limit, since they sit about 14 degrees apart.
- The standard and its year, ISO 23537-1:2022 or the earlier EN 13537, printed beside the temperatures.
- The fill weight in ounces or grams, not just the fill power number.
- Whether the product is inside the standard's scope, since quilts and bags below −20 °C are not.
- The packed dimensions, because the stuff sack sets what pack volume the bag needs.
- The bag's length and the user height it is cut for.
- The care and storage instructions, since down bags are stored uncompressed.