sleeping bag sizing

[Dennis Park]

A quick scan of the articles section didn't pan out so can anybody give tips on the proper way of sizing the girth of a sleeping bag to one's self or is it purely an issue of personal comfort?

[Dewey Riesterer]

With a soft dressmakers tape, have someone measure you around your shoulders, with your arms held at your sides. Whatever this measurement is plus 8-10 ins. is usually a good fit, I am 59" and find any bag under about 66" girth to be too restrictive.

A bag that is too tight for you will cause you to be restless and lose sleep and will also be COLDER as the insulation is compressed from within. So, a lot of the bafflegab about winter bags being too "roomy" is just BS, you NEED a certain amount of wiggle room and a good bag drapes properly to seal in warmed air, even if it is roomy.

A manufacturer who inveighs against collars, for example, demonstrates an ignorance of both efficient bag design and human catabolic heat transfer, something to consider when buying a bag.

[Diplomatic Mike]

Hi Dennis,
The most thermaly efficient bag is one that is close fitting, so there is no 'dead air' to heat up.
Personal comfort dictates how close fitting you find comfortable. Montbell uses elastic to gently hug you, whilst giving you room to move, and a quality bag from other top manufacturers should drape well, so empty spaces are limited. Cheaper bags often use stiffer fabrics that create pockets of dead air.

[Roger Caffin]

> so there is no 'dead air' to heat up.
Old urban myth. Air cost almost nothing to warm up.

What costs is drafts in and out of the bag.

Cheers

[David Ure]

nm

[Diplomatic Mike]

You learn something every day. I never new that was untrue about space being easy to warm up. So a concert hall is as easy to heat up as a bedroom?

I never mentioned the word tight. Close fitting may be uncomfortable, especially for someone who moves around a lot(me), but some folk never move all night.
I prefer a quilt anyway. :)

[Tom Clark]

Roger,
I don't agree with your statement that air costs almost nothing to heat and the implication that a loose bag is as warm as a snug one. On what basis do you make that statement?
Tom

[Roger Caffin]

Hi Mike

> So a concert hall is as easy to heat up as a bedroom?
You may be taking things to a slight extreme here?

Even so, I will stand by the statement when read correctly: the air in the concert hall is almost as easy to warm as the air in a bedroom. But the walls of the concert hall – ah, that's another matter. Huge thermal mass in the walls.

Cheers

[Roger Caffin]

Hi Tom

> I don't agree with your statement that air costs almost nothing to heat and the implication that a loose bag is as warm as a snug one. On what basis do you make that statement?
I make that statement on two things.

The first is the thermal mass which the air represents: it is very small. That's because the mass of air in a bag is so microscopic. That's physics.

The second is the fact that so many people (including my wife and I) use quilts in the snow, and a quilt simply does not wrap around the sleeper the way an elasticated bag might.

You can test both of these very simply. Pull a quilt over your head and take about 6 deep breaths under the quilt. See how warm the air under the quilt feels. I do this every night in the snow – warms me up wonderfully. If it is really cold, my head may not emerge for quite some time – and I don't suffocate either.

Cheers

[Roman Ryder]

I slept warm all night in the 30's a couple weeks back. I woke up a little early to use the bathroom. I wasn't able to get the bag warmed up again.

[Richard Nisley]

Heating Air

I half agree with both Roger and Tom… how is that for being diplomatic?

I graphed the output of a mathematical model that calculates the convection heat loss between your skin and a bag/quilt. The model uses 34C (93.2F) as an average skin temperature and 20C (68F) as an average temp on the inside lining of your sleeping bag. I varied the gap from 2mm (.08 inches) to 50mm (1.97 inches).

The model shows that Roger was correct when he said, “…air costs almost nothing to heat…”.

The fact that the US Patent office granted the Montbell super stretch patent indicates that Tom was correct when he said, “…I disagree with the implication that a loose bag is as warm as a snug one…” The super stretch patent concept was “When the user turns in sleep, some of the air in the gap is forced out of the sleeping bag device to dissipate heat with the result that the warmth in the sleeping bag device cannot be efficiently retained. This problem is particularly critical where the size of the user's body is small relative to the size of the sleeping bag device.”

[Roger Caffin]

Hi Richard

> The fact that the US Patent office granted the Montbell super stretch patent indicates that Tom was correct when he said,

Forgive my for my mirth, but the fact that a patent was granted does NOT mean that the idea has any validity at all. The USPTO won't patent perpetual energy machines, but they are happy to grant a patent on anything else you like to write up (and pay for). It certainly does NOT have to be true or to work. That is NOT a requirement these days.

In fact, one could argue that it is easier to get a patent on something which does not really work: the competition from other vested interests is much less.

> “When the user turns in sleep, some of the air in the gap is forced out of the sleeping bag device
One could conceivably use this as an argument for the loft to be essentially zero. That way there would be no air forced out of the sleeping bag when you turn over. Yes, this is a stupid argument, but it is almost the same.

Cheers

[Jørgen Taxt Walnum]

Not to distract an interesting discussion about size of bags, but I have been wondering about a thing with the montbell bags.
In a normal bag without the stretch system(and I guess, also in these)the inner fabric is cut a bit smaller than the outer fabric to not compress the down (this is just an assumption, correct me if I'm wrong). In the montbell bags, when you stretch on the inner fabric, the outer fabric will not stretch until the inner fabric more or less touches the outer. Will this not compress the down and make it cold on that spot? Or even a larger part of the bag if you for example have a shoulder girth close to the maximal stretch…?
I have not found montbell bags here in Norway to see how this works, so anyone who has a monbell bag, please explain.

[Richard Nisley]

Roger,

We both agree that the initial heat up time is primarily a result of the bag’s thermal mass and not the air in the interior. I think we also agree that larger bag exterior surface will increase its cooling. Let’s attempt to see if we can find common ground on the “bellows” phenomenon.

All of these sources attempt to explain the same “smaller is warmer aka bellows action” phenomenon using different wording. Is there one that you agree with?

1. Assume you have two bellows. One has a large volume air chamber and one has a small volume air chamber. Assume you rollover on them both and then off. The large volume air chamber will expel more air followed by drawing in more air.

Source: Richard Nisley

2. Next, look at the bag's Girth. This will give you an idea of a bags internal volume. A bag with a large internal volume will take longer to heat up and will be subject to more convective heat loss when air moves in and out of the bag as you move inside. This is called the bellows effect. Narrow bags or bags with a small internal volume are warmer.

Source: Western Mountaineering
http://www.westernmountaineering.com/index.cfm?section=Product%20Tips%20and%20Care&page=Tips%20and%20Care&viewpost=2&ContentId=7

3. People choose the size sleeping bag they want according to personal preference, body size, and sleep pattern. The size of a sleeping bag determines the outer surface area for heat loss and the volume of air inside the bag. The ideal size of a sleeping bag should be about two inches larger in length and girth than a sleeper’s body to provide efficient Insulation and comfort. A sleeping bag that is too small will feel tight and uncomfortable. A bag that is too big will not provide thermal efficiency since the extra space inside the bag will need to be heated up by the body and the occupant may pump warm air out of the bag during movement (and increase convection heat loss).

Source: University of Kansas Sleeping Bag Research Paper referencing Palik, B. (1982, July). Sleeping bags: materials and construction determine price and practicality. Protect Yourself, 35-38.

4. Snug-fitting bags are generally warmer than loose-fitting ones, in part because there is less cold air and icy sleeping bag surrounding you when you first climb in, so the bags warm up faster. More importantly, a snug-fitting bag, combined with an effective hood, helps prevent your movements from pumping warm air out of the mouth of the bag, then drawing cold air in. A bag that’s too snug, however, will give you claustrophobia, so be sure to slip inside the bag at the shop before buying.

Source: http://www.netbackpacking.com/sleeping-bag-size-shape-and-fit.html

5. Mummy bags can indeed be confining, particularly if you’re not used to them. But you understand why they exist, I’m sure—the trim cut weighs less than a semi-rectangular or rectangular bag, and the snug interior space conserves heat by reducing the bellows effect when you thrash about.

Source: http://outside.away.com/outside/gear/gearguy/200611/20061103.html

6. …Note also that differences in the size of the bag used by the subjects generally produced very small changes between the insulation measured by the copper manikin, as well as very small differences in mean heat debt measured for this group of subjects. However, the surface area for heat loss in a sleeping bag is a function of the sleeping bag size, and the heat production of a smaller individual is lower than that of a large individual, so that a small individual in a sleeping system which is unnecessarily large, probably will not be as comfortable as when the proper size bag is used. This inference, derived from the physics of heat transfer and the physiology of heat production, appears to be validated by reports from the field…

Source: Biomedical Effects of Military Clothing and Equipment Systems”, 2007”

[Richard Nisley]

Jorgen,

The following is the link to their patent. It contains detailed diagrams and narrative to best explain how the Super Stretch system works.

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=4,888,828.PN.&OS=PN/4,888,828&RS=PN/4,888,828

[Roger Caffin]

Hi Richard

Oh, no worries, I think we all understand the bellows effect. Me, I would pay more attention to the design of the neck muff and the hood, precisely to deal with the bellows effect, than bits of elastic. But there IS (or can be) a bellows effect.

> 3. People choose the size sleeping bag they want according to personal preference, body
> size, and sleep pattern.
Too right. And for some having enough room to move their joints during the night to help reduce the build-up of lactic acid in the muscles and at the joints is far more important.

> 4. Snug-fitting bags are generally warmer than loose-fitting ones, in part because there is
> less cold air and icy sleeping bag surrounding you when you first climb in, so the
> bags warm up faster.
Mainly inexperienced waffle.
If it is that cold outside you won't feel the temperature of the fabric of the bag when you climb in because you will be wearing a warm clothing layer, and the SB fabric will warm up so fast it isn't worth worrying about anyhow. Typical psuedo-expert on the web.

> 6. …Note also that differences in the size of the bag used by the subjects generally produced
> very small changes between the insulation measured by the copper manikin, as well as very
> small differences in mean heat debt measured for this group of subjects.
I agree of course, but I don't think copper manikins create or experience a very large bellows effect :-)

Actually, I don't experience much of a bellows effect when I climb under my quilt either. Usually I am out within a couple of minutes!

Cheers

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