How To Choose A Sleeping Bag: Temperature Ratings Explained

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Decode ISO ratings to find the perfect sleeping bag temperature rating for your body type, sleep style, and forecast. Learn the differences between comfort, lower limit, and extreme ratings so you stay cosy on your next night under the stars.

 

Choosing the best sleeping bag for camping or hiking isn’t just about the weight, packability, or insulation type (down vs. synthetic). First and foremost, it’s about the temperature rating. 

When you buy sleeping bags online or in-store from top brands like Sea To Summit or Nemo, you’ll notice the specifications list and label include a comfort rating and lower limit rating in degrees Celsius (°C) or Fahrenheit (°F).

Above ISO temperature rating based on the Sea To Summit Spark -9C Down Sleeping Bag.

Today we’ll explain what these temperature ratings for sleeping bags actually mean, and highlight how they influence your real-world comfort during your next night under the stars. 

Whether you’re looking to fight off the goosebumps and stay warm during winter camping, or prevent your sleeping bag from feeling like a sauna when summer rolls in, we have your temperature-related camping gear questions covered. 

Understanding Temperature Ratings Of Sleeping Bags

Temperature Rating Type Best Temperature Guide For: Sleep Expectation

Comfort rating

(ISO comfort | T-Comfort)

Cold sleepers and

‘standard women’

Cosy, undisturbed sleep in a relaxed position

(8 hours)

Lower limit rating

(ISO limit | T-Limit)

Warm sleepers and

‘standard men’

Comfort threshold - just warm enough for undisturbed sleep in a curled-up position

(8 hours)

Extreme Rating

(T-Extreme)

Emergencies only - not to be used as a guide for regular camping.

Based on a ‘standard woman’.

Survival limit - severe cold stress that can only be endured for a limited time

(maximum 6 hours)

Before we explain the three key temperature ratings of sleeping bags in more detail, it’s important to note that ‘comfortable warmth’ is subjective and differs between individuals based on personal preference, sleeping position (e.g. curled-up or stretched out), gender and other factors. 

While laboratory data (more on this later) provides an essential foundation for understanding what sleeping bag is right for you, knowing whether you are a naturally ‘cold sleeper’ or ‘warm sleeper’ is essential to achieving a quality night’s sleep in the great outdoors.

What Is A Cold Sleeper vs A Warm Sleeper?

Above: Sea To Summit Trek Down Sleeping Bag 

As reported by ABC News, variations in metabolic rate, muscle mass, blood flow distribution, and hormones mean gender-based biological differences play a critical role in the degree to which you ‘feel the cold’.

Women are often ‘cold sleepers'. Female campers usually have less heat-generating muscle than men, and therefore require more insulation to stay comfortably warm, which is reflected in the temperature ratings of women’s sleeping bags. With a greater muscle mass and higher metabolic rate, most men are naturally ‘warm sleepers’. 

Cold sleepers:

  • Tend to hoard the blankets
  • Layer up to get cosy in bed 
  • May wake up feeling chilly during the night, even in moderate temperatures

Warm sleepers:

  • Kick off the blankets
  • May wake up feeling overheated or sweaty during the night 

How Are Sleeping Bag Temperature Ratings Tested?

To make temperature ratings of sleeping bags easily comparable, Australian brand Sea To Summit, Nemo, and many other top outdoor brands test their sleeping bags based on ISO 23537 standards - set by the International Organisation for Standardisation. (Previously, the European Union protocol EN 13537 was the standard across the industry). 

The ISO test measures the thermal performance of sleeping bags in certified cold chamber laboratories through the following steps. 

1.      A heated electronic mannequin wearing standardised thermal baselayers is placed inside the sleeping bag on top of an insulated sleeping mat (R-value 5.38). 

2.      The temperature in the cold chamber is lowered.

3.     Approximately 15 – 20 temperature sensors measure the electrical energy needed to maintain the mannequin’s skin temperature at thermal balance. The more energy required to maintain heat, the less insulation the sleeping bag provides.

According to Australian Hiker, the ISO test measurements are based on the definitions below: 

  • ‘Standard man’: 25-year-old male, 1.73m in height and 73kg in weight
  • ‘Standard woman’: 25-year-old female, 1.60m in height and 60kg in weight

The three key outcomes of this test are the comfort rating, lower limit rating, and extreme rating.

Temperature Ratings: Comfort vs Lower Limit vs Extreme 

ISO temperature ratings help you buy a sleeping bag based on the outdoor conditions you expect to face, and whether you are a ‘warm sleeper’ or ‘cold sleeper’. 

Comfort Rating

The comfort rating (ISO comfort temperature) refers to the lowest external air temperature at which a ‘standard woman’ can experience a cosy and comfortable 8-hour sleep in a relaxed sleeping position (e.g. on your back), without having to curl up to retain warmth.

  • The comfort rating is the key temperature rating for women’s sleeping bags
  • The comfort rating is always a higher temperature than the lower limit rating
  • When choosing a sleeping bag, shop based on the comfort rating if you are a ‘cold sleeper’(this is applicable to most women and to men that are prone to feeling the chill).

Lower Limit Rating 

The lower limit rating (ISO limit temperature) refers to the lowest external air temperature at which a ‘standard man’ can sleep for 8 hours without being disturbed by the cold in a curled-up sleeping position to retain warmth. It represents the threshold of being ‘just comfortable enough’ without feeling noticeably cosy.

  • The lower limit rating is the key temperature rating used on men’s sleeping bags and unisex sleeping bags
  • The lower limit rating is always a lower temperature than the comfort rating
  • When choosing a sleeping bag, shop based on the lower limit rating if you are a ‘warm sleeper’ (or if you prefer to customise the warmth as needed with additional layers, thermal clothing, sleeping bag liner or higher R-value camping sleeping mat)

Extreme Rating 

The extreme rating refers specifically to the coldest external air temperature the sleeping bag is hypothetically capable of enabling an occupant to survive for 6 hours. Under these dangerous conditions, you’d be under high cold stress and at serious risk of frostbite and hypothermia.  

  • Please note: Avoid the mistake of buying a sleeping bag based on the extreme rating. The extreme rating is a survival limit only – relevant to emergencies, not regular camping.

How Reliable Are ISO Test Temperature Ratings? 

Above: Sea To Summit Spark Down -1C Sleeping Bag

Every time you camp outdoors is a unique experience. Laboratory tests simply can’t replicate all the variables to consider while offering a consistent, objective result across all sleeping bags. Your sleep movement patterns, the weather conditions (other than temperature), and your larger sleep system can all affect whether the comfort rating and lower limit rating are an accurate representation for you in the real world. Therefore, temperature ratings on sleeping bags should be considered as a guide, not a promise of warmth. 

The following factors may affect whether your nighttime camping experience accurately reflects the temperature ratings on the label of your sleeping bag: 

Your Sleep Movement Patterns:

Many of us naturally roll or turn during the night, unlike the stationary mannequin used in the ISO test. Changes in your sleep position can result in cold spots in your sleeping bag where the insulation has been compressed as you move your knees or hips, or unintentionally shift the placement of internal draft tubes, allowing cold air to sneak in. 

Real-world Weather & Conditions: 

The ISO test does not account for real-world conditions like humidity levels, condensation, wind, altitude, ground surface (e.g., wet ground will pull heat away quicker than dry leaf litter, which can be naturally insulating). For additional warmth, consider sharing a camping tent with a buddy, rather than camping solo. 

R-Value Of Your Sleeping Mat: 

ISO testing standards for sleeping bags are based on them being paired with a sleeping mat with an R-value of 5.38 to provide protection against cold ground. This means if you are using a sleeping mat with a lower R-value, your sleeping bag may not live up to its listed temperature rating. 

Your Camping Sleepwear: 

Alongside your sleep system, your camping clothing - whether you’re rugging up in thermal clothing and multiple layers or keeping it minimalist - can influence whether you stay comfortably warm at night according to the temperature ratings of your sleeping bag. The ISO test is based on the mannequin wearing a standardised set of baselayers including a baselayer top, baselayer leggings, socks and a beanie. 

How To Shop For A Sleeping Bag Based On Temperature Ratings

Outdoor brands tend to use the comfort rating as the key value of women’s sleeping bags, and the lower limit rating as the key value of men’s and unisex sleeping bags. 

To help take the guesswork out of shopping for this core part of your sleep system, at Wildfire Sports & Trek, the naming strategies below are used for Sea To Summit and Nemo sleeping bags:

Sea To Summit Sleeping Bags

Above: Sea To Summit Spark Down -9C Women’s Sleeping Bag Regular

Sea To Summit Men’s Sleeping Bags / Unisex Sleeping Bags: 

  • Lower limit rating located in the product name in degrees celcius (°C)
  • Both comfort / lower limit rating located in subtitle (°C) directly underneath product name

Sea To Summit Women’s Sleeping Bags: 

  • Comfort rating located in the product name in degrees Celsius (°C)
  • Please note: The above applies to sleeping bags with “Womens” in the product name only
  • Both comfort / lower limit rating located in subtitle (°C) directly underneath product name

E.g. As a guide, the Sea To Summit Spark Down -9C Sleeping Bag (unisex) allows: 

  • Most men/warm sleepers: To sleep adequately in a curled-up position when the external temperature is -9°C (lower limit rating) 
  • Most women/cold sleepers: To sleep comfortably in a relaxed position when the external temperature is -2°C (comfort rating). 

As a guide, the Sea To Summit Spark Down -9C Womens Sleeping Bag allows: 

  • Most women/cold sleepers: To sleep comfortably in a relaxed position when the external temperature is -9°C (comfort rating)
  • Most men/warm sleepers: To sleep adequately in a curled-up position when the external temperature is -15°C (lower limit rating). 

Catering to cold sleepers, the women’s Sea To Summit Spark -9°C is designed to provide more warmth at the same external temperature than the unisex Sea To Summit Spark -9°C, which caters to warm sleepers. The same is usually true of all women’s vs unisex sleeping bags.

Fun camping tip: Some Sea To Summit sleeping bags can be zipped together to form a duo sleeping arrangement, providing more adaptability than using a double sleeping bag. 

Nemo Sleeping Bags

Above: Nemo Sonic Down Mummy Sleeping Bag

Nemo Men’s Sleeping Bags / Unisex Sleeping Bags:

  • The approximate lower limit rating and comfort rating are located in the product name in degrees Fahrenheit, rounded to 5°F intervals in respective order (ISO limit first, ISO comfort second)
  • Check the subtitle or specifications list for accurate °C temperature values for comfort rating and lower limit rating

Nemo Women’s Sleeping Bags: 

  • The approximate comfort rating is located in the product name in degrees Fahrenheit, rounded to 5 °F intervals
  • Check the subtitle or specifications list for accurate °C temperature values for comfort rating and lower limit rating

Key Factors To Consider 

Women’s sleeping bags may be strategically shaped to comfortably fit the female anatomy

Narrower at the shoulders, wider at the hips, shorter length, and featuring body-mapped insulation (e.g., increased insulation at the foot box to keep your feet toasty warm).

Women’s and unisex sleeping bags of the same model are not interchangeable

Even if a men’s sleeping bag and a women’s sleeping bag have an identical temperature value in the product name, it doesn’t mean they provide equivalent warmth. 

The temperature ratings for comfort and lower limit are different between women’s and unisex sleeping bags. A women’s sleeping bag assumes the buyer is a ‘cold sleeper’, and a unisex sleeping bag assumes the buyer is a ‘warm sleeper’. 

If you’re a woman/cold sleeper buying a unisex sleeping bag, you may find yourself shivering on your next camping trip if you don’t take the comfort temperature rating into account. 

Please note: Sleeping bags without a gender identifier in the product name are usually unisex.

Quick Guide: How To Choose The Right Sleeping Bag Temperature Rating For You

Experienced campers know that nature isn’t always forthcoming when it comes to following the weather forecast, and that controlled conditions in a lab can’t account for every real-world variable. Stay prepared for the unexpected by using the temperature recommendations below as a guide to choosing your sleeping bag: 

  • ‘Cold sleepers’: Choose a sleeping bag with a comfort rating that is at least 5°C lower than the coldest anticipated external air temperature. 
  • ‘Warm sleepers’: Choose a sleeping bag with a lower limit rating of about 2-5°C lower than the coldest anticipated external air temperature. 

Usually it’s better to be overprepared for the cold, as in warmer conditions, a sleeping bag can be unzipped, or layers can be shed to help find that ‘just-right’ sleeping temperature. This may mean you have to carry additional weight, and of course it’s important not to overdo it. However, it is better to be safe than shivering. 

When buying a sleeping bag, remember: 

  • Sleeping bags with a lower temperature rating can provide more warmth than those with a higher temperature rating on the label. Put simply, the colder the temperature rating, the warmer the insulation. 

If you have any more questions, please contact our camping gear experts online or in-store. We are passionate about ensuring you’re well-equipped for your next adventure. 

Happy camping!