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zoidfu
01-15-2008, 02:35
What do you think the temp rating would be if I put a 40 degree bag inside a 20 degree bag? Are there any drawbacks other than the extra weight?

Jimmers
01-15-2008, 02:42
Probably not all that much better than the 20 deg. bag alone. One of the bags will not be able to loft properly, being compressed within the outer bag. I doubt the extra weight would be worth it. If you're looking for a way to extend a bag's temp rating cheaply, and with minimum expense and weight, you can always add another layer of insulating material on the ground under your bag. That's what I find helps me anyway.
My 25 deg bag has kept me cozy warm down to 0 just by adding an extra closed foam pad. But I'm a warm sleeper too.

Roland
01-15-2008, 02:50
At one time, some companies produced combination bags; inner plus an outer bag. The inner bag could serve as a summer bag. The outer bag was rated for 3-seasons. Combined, they equaled a winter bag. I have such a set-up, purchased from EMS in the mid-70s. It is versatile, but heavy.

I agree with Jim. Unless the bags are cut to work in conjunction with one another, it's likely that the inner bag would not have space to fully loft.

Sly
01-15-2008, 02:52
I've seen (not used) some systems that have an inner and outer bag but were made to fit inside the other and as Jimmers suggests trying this on your own unless one is bigger than the other, most likely won't provide that much more protection, whereas in older weather a thicker pad will

Also, a fleece or silk sleeping bag liner will help some, since they don't need loft.

PS. You could probably open the 40* bag and throw it over the 20 to help more than putting it inside.

River Runner
01-15-2008, 03:40
PS. You could probably open the 40* bag and throw it over the 20 to help more than putting it inside.

That would be my suggestion too. (Or get a quilt to go over the 20 degree bag.)

Grinder
01-15-2008, 20:22
a year ago, on Hammock forum, one of the Jacks are better guys posted a formula for computing the temperature of two nested bags.

Here's a link to the article. There's a few typos at the beginning that are confusing, but the formula towards the end is clear.

HTH
Tom

Grinder
01-15-2008, 20:28
aaaak!

I forgot the link.:eek:


http://www.hammockforums.net/forum/showthread.php?t=297

mountain squid
01-15-2008, 20:34
Are there any drawbacks other than the extra weight?The bulk in your backpack would probably be substantial. Concur with Sly, check out a sleeping bag liner instead.

See you on the trail,
mt squid

shelterbuilder
01-15-2008, 21:00
If the outer bag utilizes what is known as a "space-filler cut" (where the inner and outer fabric layers are cut to the same size, so as to push the insulation IN toward your body and "fill in" all of the "space" around your body), then putting two bags together will not increase your insulation substantially. If the outer bag utilizes what is known as a "differential cut" (where the outer fabric layer is cut larger than the inner layer to allow the insulation to loft up to its full potential), then putting another bag inside of this one MIGHT increase the overall insulation value. It will, however, feel like you are trapped inside of a cocoon (I know, because I've done it several times with a couple of lighter-weight bags).

Will it be worth the extra weight? THAT question can only be answered by the person carrying the extra weight!:-?

dessertrat
01-15-2008, 23:23
The best way to do this might be to use a lightweight rectangular bag outside a heavier mummy bag, or vice versa. That way the loft compression won't be as big a factor.

Also, you might want to investigate vapor barrier systems. A vapor barrier and light bag inside another bag might work great.

Tipi Walter
01-15-2008, 23:42
When I was dirt poor(1970s-1980s-1990s)the two-bag system was the main way I stayed warm in single digit temps or worse. Weight and bulk will keep anyone warm(Indian buffalo robes-warm and heavy), and I can't remember how many times I bedrolled it in the snow under two big bags.

When I had my excellent North Face Ibex down bag(rated 5 degrees), I would regularly augment it with either a heavy Boy Scout flannel bag or a down-feather army bag, both very heavy, since the North Face just didn't cut it in really cold temps. It isn't easy staying warm all night in 10 below zero without an expensive down bag, so the two-bag system works great for those willing to lug it.

Having a 40 degree bag with a 20 degree would probably work down to about, oh, I'd say near 15 degrees realistically, maybe lower if these bags are heavy. But the bulk issue is rough(where to pack it?)and the total weight would have to be someone around 7 or 8 pounds if synthetics.

dessertrat
01-15-2008, 23:48
When I was dirt poor(1970s-1980s-1990s)the two-bag system was the main way I stayed warm in single digit temps or worse. Weight and bulk will keep anyone warm(Indian buffalo robes-warm and heavy), and I can't remember how many times I bedrolled it in the snow under two big bags.

Having a 40 degree bag with a 20 degree would probably work down to about, oh, I'd say near 15 degrees realistically, maybe lower if these bags are heavy. But the bulk issue is rough(where to pack it?)and the total weight would have to be someone around 7 or 8 pounds if synthetics.

I'm a bit more optimistic on the benefits, but agree that one heavier bag might work better. My LL Bean 0 degree bag, which seems appropriately rated, weighs just under 4 pounds (but is extremely bulky). A twenty and a forty would probably weight quite a bit more.

Nightwalker
01-16-2008, 01:56
What do you think the temp rating would be if I put a 40 degree bag inside a 20 degree bag? Are there any drawbacks other than the extra weight?

20+40=60, right? Makes sense to me!

zoidfu
01-16-2008, 03:10
Wow..... I never thought it would be such a bad idea:D

fiddlehead
01-16-2008, 04:31
It's not necessarily a bad idea. Depends on your bag. (s)

My 20 deg bag fits me like a glove and is very small (Feathered Friend's Hummingbird)
My 0 deg bag is too big for my body.
The two work fine together.

they don't compress each other's loft because no. 1 bag A (hummingbird) has been used so many times (over 1,000 nights now) that is doesn't have much loft and is probably really a 30 deg bag and #2, my bigger bag is very inefficient simply BECAUSE it is too big and has a lot of dead space inside that i have to heat up for what reason (simply cause i bought a bag that didn't fit)

So, all that being said, there are circumstances where it is advisable.
and my system probably adds another 10 deg. to my 0 deg bag.(or subtracts really)

i'm a cold sleeper so have tried lots of things
The hot water bottle trick works best though. good luck.

deadhorsejoe
01-16-2008, 08:50
Big Agnes makes use of the over bag concept by combining their 15 degree lost ranger (to me the lost ranger is closer to a 25 degree bag) with the 40 degree cross mountain or 50 degree lost dog. They say that the two bags together add another 10 or 15 degrees to the comfort range of the lost ranger (i.e. lost ranger plus cross mountain gives you a warm sleep down to 0 degrees). The cross mountain is cut large however so as not to compress the inner bag according to them. I have both bags but have not used them together.

SlowLightTrek
01-16-2008, 16:28
I have slept in -35 with a 20 degree bag with with all my winter clothing. This was an old bag 1980's walmart, which weighed 10 lbs. I believe the way the bag manufactures rate their bags, put people in danger.

zoidfu
01-16-2008, 16:31
The reason I asked was because of the slumberjack system that they make for the military(it's probably more than one manufacturer actually) is a 3 bag system.

shelterbuilder
01-16-2008, 18:43
The reason I asked was because of the slumberjack system that they make for the military(it's probably more than one manufacturer actually) is a 3 bag system.

If the set-up is designed AS A SYSTEM, then hopefully the designers took into account the loft of each bag and sized them so that the bags will not compress each other's insulation. In that case, a bag in a bag should work without any trouble at all.

greentick
01-20-2008, 23:19
The reason I asked was because of the slumberjack system that they make for the military(it's probably more than one manufacturer actually) is a 3 bag system.

I have used the army's 3-bag system (inner->outer) 10* bag, 30*bag and Goretex bivy down to -20*. Worked fine except that I was a long soldier trying to fit into a regular bag. The first time I got into a long sleeping bag it was like "where have you been all my life!"

I have seen those systems for $130, might not be a bad deal for a restrained budget.

JAK
01-20-2008, 23:30
I saw one of those in town here for $99. Very attractive. Very bulky if you packed it all, but definitely worth a look for year round versatility. There is a lot more to surviving -20F or worse than an adequate sleeping system, but its a good start, if you can carry it.

What's with all the cotton on military clothing systems?
Some of the wool stuff and ponchos and mukluks are great, but some of its totally whacked.

Bob S
01-21-2008, 00:57
Its kinda cold outside right now throughout most of the USA (like 10 here), put them together and sleep on your porch tonight to test them. Better to know now then on the trail.

gaga
01-21-2008, 21:55
chose your bag---> http://www.whiteblaze.net/forum/vbg/showimage.php?i=911&catid=popimages&orderby=posts

mikeinFHAZ
01-22-2008, 01:12
the formula for combining bags seems to be a non-proven point for me, but I did test a combined bag(s) on a night where temps dropped to 7 F.
I used a WM Ultralight, with a WM Highlite inside. 20 plus 35­ in theory would have been 70-35/2= 17 from 20= 3 degrees? I was pretty toasty and Im a usual cold sleeper. I remember having to remove my socks and slept in just nylon hiking pants/shirt. No shelter to speak of on a BA insul air core pad. I did not notice any compression of down in either bag, but im pretty thin. (reason for being a cold sleeper)
So, im in an unscientific agreement with the formula. :)

pittmad
02-07-2008, 17:40
has anyone ever used one of those emergency blankets (metallic looking) inside their sleeping bag to help keep warm? For instance would a 35 degree bag with one of those lined inside be comfortable for a cold sleeper in 10 degree weather?

zoidfu
02-07-2008, 17:47
has anyone ever used one of those emergency blankets (metallic looking) inside their sleeping bag to help keep warm? For instance would a 35 degree bag with one of those lined inside be comfortable for a cold sleeper in 10 degree weather?

Good question. I've always wondered about them. I've also wondered if it would work if I cut cut one to size and use them under the insoles in my winter hikers.

pittmad
02-08-2008, 15:19
bueller?

Blissful
02-08-2008, 16:12
has anyone ever used one of those emergency blankets (metallic looking) inside their sleeping bag to help keep warm? For instance would a 35 degree bag with one of those lined inside be comfortable for a cold sleeper in 10 degree weather?


I'd hate to think what would happen sound-wise every time you roll over. :confused:

For my son a silk liner worked very well. Esp in a 20 degree bag (NF Cat's Meow) in 10 degree weather.

Webs
02-08-2008, 16:22
I've heard the emergency bivies aren't great because of condensation--since water evaporating from your body can't escape, it gets nice and wet inside! I have one, but now I don't really ever want to use it; I just carry it for actual emergencies...

warraghiyagey
02-08-2008, 20:56
60 degrees

shelterbuilder
02-08-2008, 22:50
has anyone ever used one of those emergency blankets (metallic looking) inside their sleeping bag to help keep warm? For instance would a 35 degree bag with one of those lined inside be comfortable for a cold sleeper in 10 degree weather?

As Webs pointed out, condensation could be a BIG problem, unless you went to the trouble of perforating the blanket - a lot - so that it would reflect most of your body's radiant heat, but allow most of your sweat to pass through. That's a lot of work for what might be a small return on your investment. There used to be a material called "texolite" that worked on this principle - and it did work, but you did still tend to get a little damp. That type of liner added another 10* - 15* to your bag's rating.

10-K
07-02-2012, 05:34
This thread is 4 years old and was brought to the top courtesy of spam but I disagree that combining to bags to get a bag rated to a lower degree won't work. I do it every winter with my Montbell #2 and #7 bags and it definitely does work.

Velvet Gooch
07-02-2012, 07:34
A cumbersome 20F bag

perrymk
07-02-2012, 16:47
According to “Lightweight Camping Equipment and How to Make It” by Cunningham (1959, p.117) a 40 degree rating is achieved with 1.5 inches of insulation (measured from skin out), a 20 degree rating is achieved with 2 inches of insulation, and a -40 degree rating is achieved with 3.5 inches (the sum of the 20 and 40 degree thicknesses) of insulation.

According to “Beyond Backpacking” by Ray Jardine (2000, p.97) the formula for sleeping bag insulation is ETR = 100 – (40*T) where ETR is est. temp rating and T is thickness. If we plug in the above numbers we get ETR = 100 – (40*1.5) = 40 degrees, ETR=100-(40*2) = 20 degrees and ETR = 100 – (40*3.5)= -40 degrees.

So the theoretical numbers from two different sources are in agreement. The question will be, are your real-world sleeping bags the same thickness as the theoretical thickness rated sleeping bags? It seems like a stretch to get a -40 rating and I'd be sure to test this at home before staking my life on it. But that's just how I roll.