Sleeping Bag Ratings ? - moved from Q&A

[SLB]

Sleeping Bag Ratings ?

Last weekend, I used two sleeping bags to fend off the cold. I put my 20F bag inside my 40F bag, but I was still cold.

Does anyone know how to calculate the rating ? I asked my algebra teacher, but I think he was sniffing glue. He came up with 800F. Then he passed the glue to me, and I wasn't cold any more, in fact I was feeling no pain.

I'm still curious about the sleeping bag rating, though.

40F + 20F = 60F ?
40F - 20F = 20F ?
40F x 20F = 800F ?

 

[Chip]

DougPaul, I think, has a loft index. It's all about the loft and trapped, dead air. If the 20 bag is down, you may have turned it into a bag liner by compressing all the down inside the 40 bag. Cold weather warmth also has alot to do with what's under you, 2 good pads seems defacto. Then there's the "how hot do you sleep ?" thing. I carry the 1 bag I think I'll need. I always have another clothing layer and a VBL available if it's (much) colder than anticipated.

 

[dug]

Chip is dead-on. By compressing the loft, you are actually decreasing the rating of the bag (that is if it was compressed). You might do better to to just lay one bag over the top, like a comforter, so it won't compress as much.

Other factors: How cold was it at night? If it was, say 40 degrees, and you had that much bag, you could actually sweat and the moisture will make you cold.

How old are the bags? Bags lose their loft over time and therefore lose their insulation value

What did you have under you? Cold comes from the bottom as well as the top

What was on your head? A warm head goes a long way to warming your body.

What type of bag do you have? Down or something synthetic?

Also, every manufacturer stretches the comfort ratings of their bags. I figure they all lie by 10-15 degrees (based on absolutely no scientific facts whatsoever).

 

[DougPaul]

Remember there are multiple ways of losing heat. How good is your pad? Is the bag a mummy (a bare or minimally covered head will lose a lot of heat)? Is your bag "pumping" (your breathing can cause the bag to act as a bellows and pump warm air out and cold air in)?

If you have a roomy outer bag (polyester or down), putting a down bag inside can work very well--the inner bag fills all the excess space in the outer bag and prevents pumping. But if the outer bag is tight, you will just compress the inner bag and get only a small benifit.

The insulation of a sleeping bag is primarily related to its thickness. (Don't forget that, in practice, you also have to block all those other avenues for heat loss.) Once again, here is the insulation table:

US Army Quartermaster insulation table
temp ... sleeping ....... light work ..... heavy work
40F .........1.5" ................. .8"................. .20"
20F .........2.0" .............. 1.0" ................. .27"
. 0F .........2.5" .............. 1.3" ................. .35"
-20F ....... 3.0" .............. 1.6" .................. .40"
-40F ....... 3.5" .............. 1.9" .................. .48"
-60F ....... 4.0" .............. 2.1" .................. .52"

Remember, for a sleeping bag, half is above you and half below, so the total loft should be at least twice the above.

Some total (top+bottom) lofts from real bags:
20F ... 5"
.. 0F ... 6"
-20F ... 9"
-40F .. 12"
If your bag doesn't have a draft collar, you can use a fleece or down jacket as one by draping it around your shoulders.

Different people have different metabolisms so they may require rather different amounts of insulation for the same temps. Women frequently sleep colder than men.

And finally, one's metabolism varies. How tired were you? Did you have a good dinner in you? What you ate is also important (fats at dinnertime keep you warm in the wee hours...). Eating a snack before hitting the sack can help, too.

I suggest to compare the loft of the combined bags to the table of lofts from real bags to estimate the rating of the combo. (Total loft is approximately the thickness of the bag after it is laid out on a flat surface and given ample time to expand.)

Other relevant threads (search and ye shall find...): http://www.vftt.org/forums/showthread.php?t=8069
http://www.vftt.org/forums/showthread.php?t=6164

Doug

 

[DougPaul]

I'm still curious about the sleeping bag rating, though.

40F + 20F = 60F ?
40F - 20F = 20F ?
40F x 20F = 800F ?

Should have asked your physics teacher...
The basic math would be based upon temp differences across the insulating layer.

Thus a 40F bag would have about 60F (=100-40) of insulation and the 20F bag would have about 80F (=100-20) of insulation. The two combined would predict 60+80 of insulation or a rating of -40F (=100-(60+80)). However, the combo is not 100% efficient and there are other heat losses which cannot be prevented with a sleeping bag (eg breathing), so I wouldn't expect the combo to actually keep you warm at -40F.

(Note: I'm using 100F as body temp because it simplifies the math and is close enough. One could also argue that I should have used 80F as the reference temp because that is about the temp at which one starts requiring insulation to stay warm while sleeping. That would predict a combined rating of 0F.)

Doug

 

[Gris]

Actually, the proper equation is as follows:


L x C x DB x %BF x TWF = Real World Temp Rating


Where:


L = loft

C = compression

DB = Number of Double-Baggers consumed

%BF = Percentage Body Fat

TWF = The Wuss Factor



Seriously, there is a lesson here. I will never go back to a double-sided bag after switching to a top loft only bag. My 1.5 down bag has 4" of loft and is toasty - and I'm a way cold sleeper!