# Thread: Combined bag temperature rating

1. Originally Posted by Peter_pan
Lot depends on how well they go together... if one restricts the other from full loft then you won't get a lot of gain...

First estimate take the send bag reating subtract it from 70, devide that number by two and then subtract it from the first bag rate for a combined value ex

Two 40 degre bags... 70-40 =30 devided by 2 =15 subtracted from other bag 40-15=25 for a first cut estimate of the combined bags... it may actually be better but probably not worse, so this works as a good conservative estimator, IMHO.

Pan
I found this in Articles; it's useful. However I had to read it 3 or 4 times to figure out what was being said because of the typos. I'm used to decoding ordinary posts, but I think articles could benefit from a little editing. It took me a while to recognize that "send" was "second". I think Pan's computer hiccuped and dropped two letters. The other misspelled words are easily recognizable. I think it should be clarified which bag is the first and which is the second. Which is inside the other, and why? If the bags have different ratings, you get different results if you change which one is "first" in the equation.
I'm guessing that with a rule of thumb like this there's much wisdom in Pan's caution in the first sentence.

2. Originally Posted by WV
I found this in Articles; it's useful. However I had to read it 3 or 4 times to figure out what was being said because of the typos. I'm used to decoding ordinary posts, but I think articles could benefit from a little editing. It took me a while to recognize that "send" was "second". I think Pan's computer hiccuped and dropped two letters. The other misspelled words are easily recognizable. I think it should be clarified which bag is the first and which is the second. Which is inside the other, and why? If the bags have different ratings, you get different results if you change which one is "first" in the equation.
I'm guessing that with a rule of thumb like this there's much wisdom in Pan's caution in the first sentence.
As, mods,

This article was originally a reply to a post that was moved to the article section on general merit... My edit button capabilty does not reach back that far... Would one of you please edit this article... Thanks.

Pan

3. Originally Posted by Peter_pan
As, mods,

This article was originally a reply to a post that was moved to the article section on general merit... My edit button capabilty does not reach back that far... Would one of you please edit this article... Thanks.

Pan
Mods,
I didn't see the thread this was taken from until I had composed my response to the article. The thread has lots of useful info, explained in more detail. It might be good to relabel the article's link to include "more info available" or somesuch, in addition to "discuss this article or suggest new entry".

4. ## Combined bags

When winter camping, I nest a down bag inside of a larger synthetic bag with very good results.

The outer syn bag actually works better in the combined arrgt when it does on its own.

inner - rated 15 deg F down bag - a true 15 deg F for me
outer - rated 15 deg F syn bag - but really only good to about 25-30 deg F for me because it is a wide cut with lots of dead air space

I have used the combined set down to -5 deg F and feel I still have a tiny bit margin.

I think the key is that the bags fit together extremely well so that the inner down bag gets full loft and there is very little dead air space between the bags and around me.

Of course, nutrition, hydration and fatigue play huge roles as well since it is the heat from my metabolism that heats the air trapped by the bags

5. I tried your formula, and it doesn't work for my experience.
So I though I should share

25+ years ago I lived in a tent through a couple of winters that had periods of extreme cold eg. -40

I was using two mummy/Dacron hollow fill bags that were rated to -7c
The bags were twins.

-7c or 19.4 degree Fahrenheit
70 - 19.4 = 50.6F/2 = 25.3F
-19.4 - 25.3 = -5.9F

-5.9 degree Fahrenheit = -21.0555556 degree Celsius

So the combo of the two bags is good to -21c according to the formula.
I guess the question is to what degree of comfort is that -21c.

Here is my experience:
In my tent I slept in the two bags, on a single mattress off the ground.
I slept in boxer shorts and Tee shirt only . . .
(When it got colder I should have worn long johns. Duh.)
. . . Down to -34c/-29.2F I was just OK
-36c/-32.8F and I was not wanting to even wiggle my toes as they would feel the extreme cold of the bag.
-38c/-36.4F and I would spend 1/4 of the night, I guess closer to morn, shivering.
-40c/-40F I just happened to be far away from home, and in a motel room
- I was checking out a wood lot to horse log.
I never got a chance to do it, but that is where I was.

One thing is for sure, in the morning, minus the wind,
temp inside and out of the tent are equil.

one corner on the ground and the other three some what elevated,
it was built on a small slope.
The tent walls were a light industrial tarping, no insulation value.
The wood cook stove with oven, heated well enough to sponge bathe late in the evening.
Lighting was an old coal oil table lamp.
Shortly after bedding down the fire was out.
In the dead of winter the sun was down at 16:30,
and I would only spend an hour or two with supper and a night cap, journal entry if the pen worked.

I don't know if that helps, but it is just as it happened to me.

6. Originally Posted by tjm
When winter camping, I nest a down bag inside of a larger synthetic bag with very good results.

The outer syn bag actually works better in the combined arrgt when it does on its own.

inner - rated 15 deg F down bag - a true 15 deg F for me
outer - rated 15 deg F syn bag - but really only good to about 25-30 deg F for me because it is a wide cut with lots of dead air space

I have used the combined set down to -5 deg F and feel I still have a tiny bit margin.

I think the key is that the bags fit together extremely well so that the inner down bag gets full loft and there is very little dead air space between the bags and around me.

Of course, nutrition, hydration and fatigue play huge roles as well since it is the heat from my metabolism that heats the air trapped by the bags
I'm on board with Tim

The difference in ratings from mfg are all over the board so the bottom line is the true loft of the insulation

Not allowing for varence in metabolisem the bottom line it all has to do with LOFT !!

The more loft = more insulation

Comprised insulation , less loft reduction in insulation= colder

The other factor is your pad or under insulation , its a third of your bag or more.

So when in a hammock the heat lose from below is a major factor.

When using two bags always make sure the over bag is larger then the inner
and insulation is not compressed.

A good wool sweater will add 5-10 degrees or more to a bag.

If you have cold feet its better to have a down foot sack that one can place both feet to gether then down booties.

To me there is down and there is every thing else !!

fourdog

7. I live in Kansas City Missouri, I have sleeped in a ENO hammock with no tarp and I only had a 20 degree sleeping bag and a sleeping bag liner or sheet plus a Fleece zip up blanket. I was hot and the outside was 18 degrees It could have gone down to 8 degrees and I would of been good as long as it don't rain. I plan on camping out tue with a Tarp and space blanket and sleeping bag and under that so hot rocks. I done this before and it was great. I was 12 inches above it (the hot rocks).The space blanket reflected the heat and with space sliding under the area it spread the heat. When I did this it was -10 outside and I was sleeping in long johns and sleeping bag and fleece blanket.

8. ## good question...

Originally Posted by bobcat13
Is this a recommended way to increase warmth? I don't hear much about people doing this, but I have thought about it as an option.
I haven't tried it yet. Probably most helpful in deep winter condition and not shoulder season. I'd like to hear more how helpful this technique has been.

9. Geeking out here,

I decided I wanted to put Peter_Pan's formula into a shell script since my wife and I often find we combine bags/quilts in the winter.

It's a very basic script that requires bash and "bc"(basic calculator). Both are usually installed on most modern Unix variants. My Android phone doesn't seem to have "bc" installed:-(

This should run from the command line on most modern Unix based computers, Linux, MacOSX, BSD,... except Android and any other computer that doesn't come with "bc", unless you install it.

It will not work on Microsoft computers, unless Unix tools are installed.
Anyone want to write one that will run on MS Windows?

I call it "tempcalc"

Be sure to make it executable. Example:
chmod 755 tempcalc.

############################
#!/bin/sh

# tempcalc - estimates combined temp rating of two sleeping bag/quilts

# usage:
# tempcalc <bag#1> <bag#2>

totaltemp=`echo "\$1 - ((70 - \$2)/2)" | bc`

echo ""
echo ""
echo ""
echo "Temperature rating of item one = \$1"
echo "Temperature rating of item two = \$2"
echo "Combined Temperature rating = \$totaltemp"
echo ""
#######################

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