A travel sleeping bag's temperature rating identifies t […]
A travel sleeping bag's temperature rating identifies the lowest temperature at which a bag was designed to keep an “average sleeper” warm. For starters, you want to select a sleeping bag with a temperature rating that’s lower than the lowest temperature you expect to encounter. When in doubt, choosing a bag with a lower temperature rating is wise because you can always open up a bag to cool down when conditions are warmer.
Having a spec for easy comparison is useful, but it’s important to understand more about temperature ratings and the terms attached to those ratings. The topic can get complex—here are the essentials:
An “ISO” or “EN” temperature rating indicates you can reliably compare any two backpacking sleeping bags. These standardized tests mean you can truly compare temperature ratings between brands. (ISO and EN ratings are comparable. EN was the old standard and ISO is the new standard; the ISO version improves consistency of test results across labs.)
With ISO/EN testing, a bag is assigned two temperature ratings: comfort and limit ratings. “Comfort” rating is the lowest temperature at which the bag will keep the average “cold sleeper” comfortable, and is generally the temperature assigned to women’s bags. “Limit” rating is the lowest temperature at which the bag will keep a “warm sleeper” comfortable, and is generally the temperature assigned to men’s or unisex bags. If a temperature rating omits the term “comfort” or “limit,” then it's likely a brand’s estimate, not an ISO or EN test result.
A temperature rating is not a guarantee of warmth for any bag. The rating is helpful in that all brands test bags the same way, so you can compare bags from different brands. A more complete assessment of your warmth in varying conditions, though, is provided by sleep system data.