Gear Explained: Sleeping Bag Shapes, Sizes, Fits

8:54 a.m. on March 15, 2011 (EDT)
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This thread is for comments on the article "Gear Explained: Sleeping Bag Shapes, Sizes, Fits"

Choosing a sleeping bag requires understanding the different shapes, sizes, and versions available. Shapes: mummy, rectangular, semi-rectangular Sizes and fits: length, double wide, women's, children's Specialty bags: half bags, over bags, parka b...

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1:36 p.m. on March 15, 2011 (EDT)
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Fit is mentioned. but I think it should be added that a mummy bag needs to fit tightly in order to hit its tempature range.

4:27 p.m. on March 15, 2011 (EDT)
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Fit is mentioned. but I think it should be added that a mummy bag needs to fit tightly in order to hit its tempature range.

I respectfully disagree. 

The rating is specified based on the rate heat will dissipate through the insulation layer.  The nature of what lies on each side of the insulation layer is only important as it relates to the temperature differential, not the material nature of what occupies these spaces.  The bag protects dead air inside the bag just as effectiviely as it protects a body.  And dead air is dead air, regardless it is trapped within the down layer or trapped inside the bag.  While moving around may displace excess air volume inside a bag, this happens regardless if the bag is snug or oversized.  If you think a bag is ineffective at protecting air space inside of it, try draping your bag over your tent in the hot midday sun; you will realize it is capable of providing the same insulating qualities to the tent interior – which is mostly air – only in this case it keeps the hot sun from heating the tent interior.

The reason for the perception that an oversized bag is less warm has to do with intimate contact with the bag.  Just like the blankets in your bed at home, the layers that touch you become warmed to skin temperature, but as you move you contact different surfaces that are warmed to the ambient temperature under your covers, and obviously are cooler than your skin.  These quickly warm, however, just like at home.  Wearing long johns and socks to bed will almost completely eliminate this temporary discomfort, in the backcountry as well as at home.  Thus a -40 bag is a -40 bag, regardless of its size and that of the occupant.


8:48 p.m. on March 15, 2011 (EDT)
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If a bag is too tight, it can make the insulation less effective by streching it, having the same effect as crushing it. A hood or draft tube are the best ways of retaining warmeth in my experience. If a bag is loose fitting your body will have to warm the dead air (which it will), but if the bag can not hold the heat, it will not be warm. One the coldest nights I ever had was when I let my wife (who moves constantly in her sleep) talk me into zipping our sleeping bags together. This created a gap at the hoods the let the warm air out everytime she moved. Did it once. Never again.

June 20, 2018
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