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View Full Version : Using 2 sleeping bags, how can I get the effective tempature?



Starchild
04-24-2014, 11:33
That is if they match together well, not with one constricting the other.

I am thinking of combining a 40F synthetic semi-rect bag with a 35F down 'alpine blanket', basically what one may consider a semi-rect 'quilt'. In the configuration of sleeping mat, me in the semi-rect bag and the quilt on top with the quilt footbox over the bag. What type of temps can I expect from that?

(Yes I left my 18F bag at another person's house)

Mobius
04-24-2014, 11:44
I've done it with a 35 deg down quilt and a 45 down quilt (lightest one on top so as not to compress the lower one). I'd guess it was good for 20-25 deg (based on it getting down to about 25-30 that night and I slept very well.) I was on an XTherm pad at the time so had a pretty good base. Given that all these temp ratings are so subjective it's a tough call.

Starchild
04-24-2014, 11:58
Thanks that puts my mine a bit to ease, will report back how well it works.

Old Hiker
04-24-2014, 12:10
I'm sure your friend appreciates the late (early?) Christmas present. Feel free to drop by MY house and leave gear behind !!

I've only done this at home, in bed, in North Dakota, with a sleeping bag and blankets, but like the previous poster the heavy one went next to us (wife and me) with the lighter ones on top. Seemed to work well, down to about 25-30* inside the house. Heat was off in base housing for a night.

slbirdnerd
04-24-2014, 12:17
I recently bought probably the same 35 degree down blanket for the same reason--but to put over my 23 degree down bag in winter. I froze my butt off in January with just the bag when another hiker who was along had a down quilt over her bag and was okay. People do it, I assume it works, I won't know till next winter though.

Franco
04-24-2014, 18:06
At Backpackinglight someone posted a "scientific" way of working that out but I would just add the loft of the two and compare that to the given rating from a reputable manufacturer.For example a 5" loft gives you a 20f rating


http://www.westernmountaineering.com/index.cfm?section=products&page=Sleeping%20Bags&ContentId=27

by loft manufacturers mean topp and bottom (total loft)

Don H
04-24-2014, 18:54
Single Layer Down Loft and Estimated Temperature Rating
Temp (F) Loft (in)
50 1.2
40 1.5
30 1.8
20 2.2
10 2.6
0.0 3.0
-10 3.5
-20 4.0

I've used a 15* Montbell and a 32* bag, the lowest rated bag as a sleeping bag with the other bag as a quilt.

Meriadoc
04-24-2014, 19:21
Thanks for that chart Don H. So, like I thought and felt, my setup is pretty warm! 20 degree quilt on top of a 15 degree bag.

Regarding the 'total loft' idea of adding the top and bottom - I wouldn't. The bottom is compressed and next to useless when lying flat. Don's chart would be the way to go.

Starchild
04-24-2014, 21:10
Well I'm here. So far with oneof
The two but temps are dropping fast and getting the other ready now. I can't see this not working but fingers are getting very cold for smartphone use.

Franco
04-24-2014, 21:40
"Regarding the 'total loft' idea of adding the top and bottom - I wouldn't"
I was just pointing out that manufacturers use that way so that there is no confusion a about the two ways...

Do also keep in mind that some use a 40/60 mix, so a 10" loft on a manufacturer's site could mean 5 or 6" on top depending on the bag.

Franco
04-24-2014, 21:54
Just in case it isn't all that clear still. the WM 5" (2.5 above and 2.5 below) loft for a 20f rating is close to the 2.2" loft (above) in the Don chart...

Venchka
04-24-2014, 22:51
Thanks for the chart. I have been pondering a 40 degree quilt for summer use. Now I wonder if a 1.5" quilt over my 3.5" might not make a better combination than a big down parka inside a mummy bag. Hmmmmm.

Wayne


Sent from somewhere around here.

Venchka
04-24-2014, 22:52
Should be: 3.5" top layer of my


Sent from somewhere around here.

Venchka
04-24-2014, 22:53
Sleeping bag. Dumb fingers.


Sent from somewhere around here.

shakey_snake
04-25-2014, 02:30
I've heard that a synthetic over bag/quilt can help push the dew point out of the down underbag in below freezing temperatures. So, something to think about.

bigcranky
04-25-2014, 07:05
Take the two ratings and add them together. In your case, the 45 plus the 30 gives the combo a rating of 75 degrees F. :)


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Venchka
04-25-2014, 20:51
I've heard that a synthetic over bag/quilt can help push the dew point out of the down underbag in below freezing temperatures. So, something to think about.

Are you saying that condensation may wet the sleeping bag under the quilt? That calls for backyard testing. Thanks.

Wayne


Sent from somewhere around here.

Meriadoc
04-25-2014, 22:12
Are you saying that condensation may wet the sleeping bag under the quilt? That calls for backyard testing. Thanks.

Wayne


Sent from somewhere around here.

The dew point depends on the humidity (absolute). What he is talking about is where the dew point is reached. If the dew point is outside the sleeping bag/quilt then escaping vapor will escape to the atmosphere. But as the temperature changes from ambient to skin, if the dew point temperature should occur within the sleeping bag/quilt then dew will form inside the insulation. The vapor coming off of the human body will condense inside the insulation.

This concept really becomes a big issue in cold weather where water vapor coming off of the body actually freezes inside the insulation.

So placing a synthetic insulation outside where it is cooler may place the dew point temperature inside the synthetic insulation instead of inside the inner down insulation.

I tend to think the biggest issue is compression of the insulation. My quilt can't have anything layered on top, so it becomes the de facto outside insulation.

Venchka
04-25-2014, 22:36
I guess there is no free lunch. Two layers of insulation may not equal a third bag. It would be interesting to do some experimenting with a vapor barrier. Close to a warm bed incase none of works.

Wayne


Sent from somewhere around here.

leaftye
04-25-2014, 23:00
I tend to think the biggest issue is compression of the insulation. My quilt can't have anything layered on top, so it becomes the de facto outside insulation.

Backpackinglight did some testing that indicated compression of down wasn't that big of a deal.

Meriadoc
04-25-2014, 23:52
Backpackinglight did some testing that indicated compression of down wasn't that big of a deal.

I think their results were that compression to 50% of full loft had little effect. I've since used that as my cutoff for too much compression.

rafe
04-26-2014, 07:22
Backpackinglight did some testing that indicated compression of down wasn't that big of a deal.

Then backpacking light must be wrong. It's loft and trapped air that provide insulation. Loft is why you pay big bucks for the best down. It is possible to ruin down, apparently. I'm not sure how but I've seen the end result. I think it comes from storing it in a compressed state for too long.

IMO one of the earlier posts sums it up nicely -- measure the loft of the combined layers, then get the temperature rating from that. I've done what you're propsing, and it can work.

Starchild
04-26-2014, 07:37
Well I made it.

While it does work, and I estimate the temps in the low 20's and a strong wind that my tent really can not totally block, I did experience some manageable issues.

1 - Warmer bags don't really concentrate on a substantial hood, so while the rest of me was comfy warm my head was cold.

2 - the double bag made it difficult to used the little head protection one bag offered. I couldn't get in one totally and then set up the other, perhaps if I switched them.

3 - since the outer bag was really a quilt it occasionally would be partly thrown off.

Some other comments on comments:

1 - I don't beleive compression of the down would have been a problem if used as the inner layer. It may have worked better, the down would have lofted 'inwards' conforming better, and the above mentioned issues may have been mitigated with the quilt inside.

2 - I think I may know a way of estimating how to add 2 bags together, if the temp rating on a bag could be converted to a 'additional' temperature of a sleeping bag liner. But for that one would need to know the starting point temperature and that may vary with the person.


3 - I know these temperature ratings are just a general guideline but it's a starting point, and you got to start somewhere.

Starchild
04-26-2014, 07:40
Backpackinglight did some testing that indicated compression of down wasn't that big of a deal.

Two different takes from your post and need clarification, is it:
1 -Compressing down while storing/hiking didn't effect it much?
or do you mean
2 -Compressing down while sleeping (such as placing it under a heavier layer of insulation) didn't matter much?

I suspect your post was about the first one, while what this post was about was the 2nd.

leaftye
04-26-2014, 08:25
Then backpacking light must be wrong. It's loft and trapped air that provide insulation. Loft is why you pay big bucks for the best down.

Yay, assertions without qualification. Tell me why those big double height uninsulated Coleman mattresses aren't super warm even though they have a couple feet of loft and several cubic meters of trapped air. Or why aerogel is so incredibly warm, yet so incredibly thin. Or better yet, just qualify your statement with at least one reference or show us your own testing.


Two different takes from your post and need clarification, is it:
1 -Compressing down while storing/hiking didn't effect it much?
or do you mean
2 -Compressing down while sleeping (such as placing it under a heavier layer of insulation) didn't matter much?

I suspect your post was about the first one, while what this post was about was the 2nd.

The latter. They've actually found this to be the case in two types of tests. That of apparel and also of air mattresses. It really helps with shopping. Where some manufacturers might try to trick customers by making their baffles bigger, wiser shoppers know that fill weight and fill power are far more important.

rafe
04-26-2014, 08:48
Yay, assertions without qualification. Tell me why those big double height uninsulated Coleman mattresses aren't super warm even though they have a couple feet of loft and several cubic meters of trapped air. Or why aerogel is so incredibly warm, yet so incredibly thin. Or better yet, just qualify your statement with at least one reference or show us your own testing.


Because, in those big Coleman mattresses, air is free to circulate. Whereas in foam or fiberglass or down, it is not. Next question?

Aerogel is unfamiliar to me. It looks space age and very pricey and I'll bet you're not saying it's "incredibly warm" from personal experience.

I'm certain the laws of thermodynamics have not been violated. The mechanisms of heat transfer (and its prevention, which we call "insulation") are well known. Everything else is mere technology.

leaftye
04-26-2014, 10:53
Because, in those big Coleman mattresses, air is free to circulate. Whereas in foam or fiberglass or down, it is not. Next question?

Aerogel is unfamiliar to me. It looks space age and very pricey and I'll bet you're not saying it's "incredibly warm" from personal experience.

I'm certain the laws of thermodynamics have not been violated. The mechanisms of heat transfer (and its prevention, which we call "insulation") are well known. Everything else is mere technology.

Whoa, you're certain laws of thermodynamics haven't been violated! Of course it hasn't. You clearly have very little idea why these materials keep you warm, but at least you know to use good materials. Leave the discussion on loft to those of us willing to back up alleged knowledge of thermodynamics by looking at the data from experiments.

rafe
04-26-2014, 12:54
I have down bags that are older than you are, leaftye. I answered your question and now you're sore. I'm a man of science, so show me your experiments. Apologies to Inigo Montoya, I do not think that experiment means what you think it means.

Meriadoc
04-26-2014, 13:45
Permit me to defuse this.
Rafe, Leaftye was asking a rhetorical question to illustrate how insulation works.

Leaftye, Rafe was answering your rhetorical device in good spirit.

It's kind for folks to share knowledge. It's not that kind to use knowledge as a gateway.

So let's continue to keep it civil. Using esoteric examples from materials engineering is probably not the best way to enlighten the public. And this is a public forum.

Whew. I climbed on a soap box near the end there.

rafe
04-26-2014, 15:12
For what it's worth, the following URL (from the educational division of Argonne National Laboratory) is a pretty concise summary of how thermal insulators work:

http://www.newton.dep.anl.gov/askasci/gen99/gen99832.htm


If one were to melt glass, and then spin it into very fine fibers, and then fluff and tangle the fibers together to form a blanket, one would have essentially made a batt of fiberglass insulation such as might be used to insulate the walls of a home.

The material works as an insulator because glass fibers are poor conductors of heat AND, when fluffed into a blanket, they trap air (also a rather poor conductor of heat) so that convection currents cannot be easily established within the mass of material. The effect is to greatly retard heat flow by either conduction of convection.

Franco
04-26-2014, 18:50
Loft and warmthHere is a comment from Richard Nisley who tests this kind of stuff for a living :
Another way of looking at this issue is to compare a Montbell SS sleeping bag with a Western Mountaineering bag using the same approximate amount of fill. The Western Mountaineering bag will loft more and LOOK warmer but the bags will thermal test the same. That is because the MB SS bags stretch elastic compresses the down fill in the linear 2.5x range.



And :
You can probably find hundreds, if not thousands, of Internet posts which make the statement that the loft of a down garment or bag determines its warmth. My contention is that this is true only if they are using the same fill power and density. You can make a down bag or clothing warmer by increasing its density even if the loft is less.



The second comment from :
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=12505&startat=26

(loads of charts and formulae there for the gear nerds)

The way I understand this is that indeed if you compress a sleeping bag or a jacket up to (maybe 50% not sure...) because the density of the fill increases you retain most if not all of the insulation you get when the item is fully puffed up.
I think many will relate to the converse too.
That is when you have a very high buffle but with empty spots therefore losing warm air through them.

BTW, this is also why my "measure the total loft " bit is a rough rule and why for example the WM 2.5 " loft I posted is close but not the same as the 2.2" from the other chart.

LIhikers
04-28-2014, 00:27
If you search Whiteblaze long enough, and go far enough back in time you'll find a simple math formula that was supposed to give you the temperature rating of using one sleeping bag inside another. That might be of some help to the OP.