Sleeping Bag Temperature Ratings Explained

The number on a sleeping bag is one of the most misunderstood numbers in camping. Buy a "30-degree bag," take it out on a 30-degree night, and a lot of people spend the night shivering. That's not because the bag lied. It's because of how the number is measured.

01The standard ratings

Most major brands test bags using a standard lab method (ISO 23537, which replaced the older EN 13537). It uses a heated mannequin in a cold chamber and gives up to three numbers:

• Comfort: the temperature at which a typical cold sleeper (the standard uses an average woman) stays comfortable in a relaxed position.
• Limit (or Lower Limit): the temperature at which a typical warm sleeper (an average man) can sleep for about eight hours curled up, without waking from the cold.
• Extreme: a survival rating. The lowest temperature at which a person can survive for about six hours without hypothermia. It is not a temperature you'll sleep at. Ignore it when you're buying.

When a bag is sold as a "20-degree bag," the 20 is usually the limit rating. If you sleep cold, look at the comfort rating instead, which is often 8 to 12 degrees warmer.

02Why you might be colder than the rating

The lab test assumes:

• A sleeping pad under you with decent insulation.
• A base layer and a hat.
• The bag being dry and fluffed up.

In real life, a lot of people are sleeping on a cheap air mattress with no insulation, in cotton clothes, after a cold, damp day. The ground pulls heat out of you through the bottom of the bag, because your body weight crushes the insulation underneath you. That's why your pad matters as much as your bag. See sleeping pad R-values explained.

03How to pick a bag for fall camping

• Pick by the comfort rating for the coldest nights you'll camp in.
• Add a margin. If nights will be in the 30s, a bag with a comfort rating around 30 or a bit lower is a good bet.
• Down or synthetic? Down is lighter and packs smaller for the same warmth. Synthetic is cheaper, handles dampness better, and dries faster. For car camping, synthetic is a fine value.
• Shape: mummy bags are warmer (less air to heat) but tighter. Rectangular bags are roomier and colder. A semi-rectangular bag is a good middle ground.
• Hood and draft collar make a big difference in cold weather.

04How to sleep warmer in the bag you have

1. Insulate from the ground. A foam pad under an air pad, or a higher R-value pad, is the biggest single upgrade.
2. Wear dry layers to bed: wool or synthetic base layers, warm socks and a hat. Not the cotton you wore all day.
3. Eat a snack before bed. Your body makes heat digesting it.
4. Go to bed warm. Do a few minutes of easy exercise before crawling in, but don't sweat.
5. Use a liner. A fleece or insulated liner adds several degrees.
6. Put a warm water bottle in the bag. Fill a sturdy bottle with hot water, close it tight, and put it at your feet or between your thighs.
7. Close the hood and cinch it around your face.
8. Pee before bed. Your body wastes heat keeping that extra fluid warm.

05Kids and sleeping bags

Kids lose heat faster and are more likely to wiggle out of a bag. Use a kid-sized bag (an adult bag has too much empty space to heat) and check on them through the night in cold weather.

06Storing your bag

Don't store a bag in its stuff sack; compressing it for months breaks down the insulation. Keep it loose in a big storage sack or hanging in a closet.

For the full cold-weather setup, see fall camping: staying warm when nights drop below 40.

Photo by StockSnap on Pixabay.

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