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I have never used a Mummy type sleeping bag, rather I have hardly used a sleeping bag. I am afraid, I am more of a carry-mat, sleeping-pad and a blanket guy.

It never occurred to my mind that What must be the advantage of a Mummy type sleeping bag over a Blanket type? over even vice-versa if it is so?

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When you say "blanket", would another term for that be quilt? Aka your blanket might be made of down or synthetic insulation? –  Ryley Apr 9 at 5:11
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@Ryley Indeed this might need clarification. In my understanding a blanket type sleeping bag is sleeping bag that has the form of a blanket that was folded once and then closed with an L-shaped zipper so that it forms a bag with one of the short edges open (cf. yeti.com.pl/en/yeti-wspiera-wydarzenia/…). But maybe I got WedaPashi wrong. –  Benedikt Bauer Apr 9 at 15:30
    
@Ryley: It is supposed to be made for synthetic insulation –  WedaPashi Apr 11 at 4:59
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What you call "blanket type" is usually referred to as a "square sleeping bag" or "rectangular sleeping bag". –  vclaw Apr 14 at 1:27

2 Answers 2

up vote 12 down vote accepted

Basically it boils down to "less weight for the same insulation" in favour of the mummy type for two reasons:

In a blanket type bag you have more or less two insulating sheets, one on top and one below your body. For a mummy type maybe a bit more than the area of one of those sheets would be enough to wrap your whole body. This already reduces weight and pack size.

Above that, a mummy type bag wraps your body quite closely, therefore leaving only a small air volume that has to be warmed up. The blanket type has way more "dead volume" that you have to heat and keep warm with your body's heat. To get the same insulation, a blanket bag needs more loft material, which makes it heavier and bulkier.

Another feature of mummies is a yoke or heat collar and a hood. The yoke closes around your neck and prevents warm air from escaping from the bag and both, the yoke and the hood can be closed by drawstrings, so that everything but the face is snuggly enclosed by the bag. A blanket type bag can in contrary not be closed densely at its open end, which allows the warm air to escape quite easily.

All those factors might be not of great importance in warm climates where you don't need high insulation but only something to cover your body at night. As you get to climates where the sleeping bag is necessary to prevent hypothermia, a blanket type bag that provides the insulation needed will soon be way to heavy and bulky.

Just as a side note: if you go to very cold climates (winter mountaineering, arctic tours), everything is done to prevent heat loss, therefore some of the bags designed for those climates have only a half length zipper or even no zipper at all to prevent cold bridges.

The major drawback of the mummy bag is its tightness. Rather "active" sleepers that move a lot during the night or like to take other sleeping positions than just lying straight (legs spread, fetal curl or similar) might feel a bit less cramped in a blanket type bag. But some manufacturers also have mummy bags that have a more flexible and expansible material in the middle part to allow for leg movement to some extent.

TL;DR: Whenever it gets cold enough that insulation is a critical factor, a mummy has less weight and pack size for the same insulation.

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Great answer, just a couple points to add that mummy bags also have yolks as well as center and hood cinch cords. This really helps to trap warm air inside and keep the cold out on those frigid expeditions! –  AM_Hawk Apr 3 at 13:34
    
@BenediktBauer: I do have a habit of folding my knees when I sleep, does that make an issue in a mummy-type bag? I don't know why that I have this impression that the mummy-types really restrict the movement of legs. (kinda wrap around ?) –  WedaPashi Apr 8 at 10:17
    
@AM_Hawk I'm not a native English speaker and every dictionary entry I found concerning the term "yolk(s)" was totally unrelated to sleeping bags. Could you explain what it means in this context? –  Benedikt Bauer Apr 8 at 10:22
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@WedaPashi Yes and no! As the bag covers your body like a tube, it will just follow the bending of your knees, so that could be fine. If you are more like taking a fetal curl position, this might not work so well in most mummy bags as you won't be able to fold yourself within the bag. However, there are bags that are a bit wider and to some extend expansible in their middle section to allow for some knee movement. I will edit this in as soon as I will have some more time the next days. –  Benedikt Bauer Apr 8 at 10:29
    
@BenediktBauer forgive my spelling error the YOKE is the piece that covers your shoulders and has a groove for your neck, this keeps all the warm air inside the bag and cold air out. The pads rest on your shoulders effectively creating a seal. This is the bag I have, it performs very well. I really like the yoke feature it is quite comfortable, doesn't make you feel trapped. See Also –  AM_Hawk Apr 10 at 16:10

My answer will assume we are talking about a quilt. You can get down quilts that are made of exactly the same material as a mummy bag (Pertex Quantum face fabrics with 800+ fill down). Most quilt users that are going into colder climes will add a down balaclava to keep their head warm. Futhermore, quilts are often cut in the same shape as a mummy bag (on top), specifically to prevent creating areas of dead air inside the bag. Expect any good down quilt to look very similar to a mummy bag until you look under it. They will also have a yolk as @AM_Hawlk mentioned.

Quilt Summary

  • Pro: lighter for the same warmth. A minor primer on how goose down works - it provides insulation by trapping air in the lofted material. So in your average mummy bag, that means that every part you are lying on is not providing much warmth.
  • Pro: hood isn't attached to your sleeping bag, so you can wear it around camp to keep warm
  • Con: you can't move around without causing drafts
  • Con: side sleepers will not be happy, as it will be hard to keep the bag sealed around you (see drafts)

For me, the cons kill quilts - I like zipping into my mummy bag and not having to worry about lying perfectly still :) I think in summer trips, they are a pretty awesome idea... I always end up half out of my mummy bag anyways, no matter how light it is, so why not?

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