Mummy bag at a glance
- Cut
- Wide at the shoulders, narrow at the feet
- Rated under
- ISO 23537-1:2022
- The test is defined around this shape
- Standard covers
- Limit ratings of -20 C and warmer
- Costs you
- Room to move at the knees and shoulders
What does the taper actually do?
Every inch of empty space inside a sleeping bag is air your body has to warm up and then keep warm. A mummy bag deletes most of it: the cut narrows from the shoulders to the feet, and the hood closes the opening at the top rather than leaving your head out in the cold.
That is the whole trade. Less empty volume means less air to heat, and it also means less fabric and less fill for the same rating, which is why the shape dominates any bag you have to carry.
Why the rating on the label assumes this shape
Sleeping bag temperature numbers come from ISO 23537-1, currently the 2022 edition, which replaced the 2016 one, which replaced EN 13537. Clause 5.1.1 is unusually direct about what the test fits: the heated manikin method is suitable for mummy-shaped bags that fit the manikin without being tight.
So the method every bag is compared under was defined around this cut. A maker can scale a tested mummy bag up or down in size and carry the rating across, as long as the materials and the construction are the same. A bag with a lot of extra room inside is being described by a test that was not built for it.
What the shape costs you
Room. A mummy bag holds your legs together and your arms in, and a side sleeper who likes to draw their knees up ends up rolling the whole bag over instead of turning inside it. Plenty of people never notice. Others find it close enough to sleep badly in, and a roomier cut or a quilt suits them better even at the weight penalty.
The hood is the other adjustment. It is doing real work on a cold night, but it takes a few nights to stop waking up with the drawcord across your face. Cinch it so the opening sits around your nose and mouth rather than dragging the whole hood over your head.
Getting the fit right
Length first. A bag that runs long leaves a pocket of cold air past your feet, and one that runs short pushes your toes into the end and flattens the fill exactly where it is hardest to stay warm. Most makers publish a regular and a long, and some add a short.
Then girth, which is the measurement people skip. Shoulder and hip girth decide whether the bag seals against you or leaves space to heat, and they vary far more between brands than length does. The target is a bag that closes at the shoulders and still lets you bend a knee.
What this means when you're choosing
- Shoulder and hip girth published, not just a length, since the taper is where a bag fits or does not
- A hood with its own drawcord, separate from the draft collar's
- The comfort figure rather than the limit number the bag is usually named after
- Extra fill at the footbox, which is the coldest end of any bag
Mummy bag vs Backpacking quilt
| Mummy bag | Backpacking quilt | |
|---|---|---|
| Insulation underneath | Present, and crushed flat by your weight | None. The pad does that job |
| Hood | Built in, and closes the biggest opening | None. You sleep in a warm hat |
| Drafts | Sealed, especially with a draft collar | Possible if the straps or the pad are wrong |
| Rating you can compare | ISO 23537-1, measured on this shape | The maker's own number, since the test needs a closed bag |
| Weight and bulk | Heavier for the same warmth on top | Several ounces lighter, packs smaller |
Which to pick: A mummy bag if you move in your sleep, sleep cold, or want a number you can hold against another brand's. A quilt if you already own a warm pad and are chasing the last few ounces.
Mummy bag vs Rectangular bag
| Mummy bag | Rectangular bag | |
|---|---|---|
| Cut | Tapered to the feet, with a hood | The same width top to bottom, no hood |
| Warmth for weight | The best of the bag shapes, because there is less air to heat | Poor. A lot of volume and an open top |
| Room to move | Restricted at the knees and shoulders | Generous, and it usually unzips flat into a blanket |
| Where it belongs | Anything you carry, and any cold night | Car camping in summer |
Which to pick: Mummy for anything that goes in a pack. Rectangular for a campsite you drive to in July, where the weight does not matter and the room does.
The common mistake
Sizing up for room
Extra girth is extra air for your body to heat, and the taper is where the warmth for the weight comes from. A bag you swim in sleeps colder than one that fits, which is also what the rating assumes: the ISO manikin test is defined on a bag that fits without being tight.
Common questions
Is a mummy bag warmer than a rectangular bag?
For the same amount of insulation, yes, and by a wide margin. A rectangular bag holds far more air for you to heat and has no hood, so a lot of what you produce leaves through the opening at the top.
Are mummy bags uncomfortable to sleep in?
Some people find them close, side sleepers most of all. The fix is usually fit rather than shape: a bag with enough shoulder and hip girth turns with you instead of holding you in one position.
What size mummy bag should I buy?
Long enough that your feet are not pressing the fill flat, and no longer, since spare length is more air to heat. Check shoulder and hip girth as well as length, because those vary more between brands and they are what the fit turns on.
Where this is defined
- ISO 23537-1:2022, requirements for sleeping bags, part 1, International Organization for Standardization
- ISO 23537-1:2022 preview, clause 5.1.1 test principle, iTeh Standards