Sleep Systems Episode 3: Sleeping Bag Temperature Ratings
Don't Get Caught Cold: Decoding Sleeping Bag Temperature Ratings
Welcome back to the Got Outpost blog! In our "Everything Sleep Systems" series, we're helping you navigate the complexities of outdoor gear. Today, we're tackling a critical topic that often traps new and even experienced campers: sleeping bag temperature ratings. That "15-degree bag" on the label might not actually keep you warm at 15 degrees, and understanding why is paramount for your comfort and safety in the backcountry.
As Jay and Drew learned from personal experience, trusting the advertised temperature rating at face value can lead to miserable, freezing nights. But thankfully, there's a standardized system designed to provide accurate information.
The ISO Rating System: Your Guide to True Warmth
To bring accountability and a level playing field to sleeping bag manufacturers, an international standard called the ISO rating system has been implemented. This system provides three key temperature tiers you need to understand:
Comfort Rating: This is the temperature at which a "cold sleeper" (typically a woman, or someone who naturally feels colder) can expect to sleep comfortably and warmly through the night.
Limit Rating: This is the lowest temperature at which a "warm sleeper" (typically a man, or someone who naturally feels warmer) can expect to sleep comfortably. Below this temperature, a warm sleeper might feel cold.
Extreme Rating: This is a survival temperature only. It indicates the absolute minimum temperature at which the bag might prevent hypothermia for a short period. It is NOT a comfort rating, and you should never plan to rely on this temperature for a comfortable night's sleep.
Real-World Example (Nemo Disco Bags): Drew illustrates this with two Nemo Disco sleeping bags, both marketed as "15-degree bags":
Men's Bag: Often carries a "Limit Rating" of 15°F, meaning a typical male "warm sleeper" might be comfortable at 15°F. However, its Comfort Rating is actually 25°F. The Extreme Rating is -21°F.
Women's Bag: Often carries a "Comfort Rating" of 15°F, meaning a typical female "cold sleeper" can be comfortable at 15°F. Its Limit Rating is 5°F, and the Extreme Rating is -31°F.
This crucial distinction shows that a "15-degree bag" can mean very different things depending on the gender intended and whether the advertised number refers to the comfort or limit rating!
The Danger of Manufacturer Estimates
Many manufacturers do not submit their sleeping bags for ISO testing. Instead, they provide "manufacturer estimated temperatures," which are essentially unchecked claims. Drew shares his own regrettable experience: he bought a "20-degree" bag that left him freezing at 35 degrees on his first backpacking trip because it was an unverified estimate.
How ISO Testing Works
The ISO rating is a rigorous process:
A thermal mannequin, dressed in base layers, is placed inside the sleeping bag.
Sensors on the mannequin measure heat loss and retention in a controlled environment.
The Comfort, Limit, and Extreme temperatures are then scientifically determined based on the mannequin's ability to maintain body temperature.
Why Accurate Temperature Ratings Matter
Choosing a sleeping bag with a verified ISO rating is incredibly important for several reasons:
Guaranteed Comfort: It helps ensure you select a bag that truly matches the conditions you expect, preventing uncomfortable and sleepless nights.
Safety in Unpredictable Conditions: Especially in mountainous regions like Colorado, where a September night can quickly drop from 35°F to 5°F with unexpected snow, having a bag you can trust is a matter of safety, not just comfort. Being under-equipped in cold weather can lead to serious risks like hypothermia.
Value for Money: You invest hard-earned money in your gear. Knowing that your sleeping bag will perform as expected is crucial to avoiding buyer's remorse and having to buy multiple bags to find the right one.
Don't let misleading temperature ratings ruin your next adventure. Always look for ISO-rated sleeping bags and understand the difference between comfort, limit, and extreme ratings.