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Topic: What's 35 + 20, Sleeping bag< Next Oldest | Next Newest >
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PostIcon Posted on: Nov. 21 2013, 8:29 pm  Skip to the next post in this topic. Ignore posts   QUOTE

I think I asked this before but I can't remember the answer. Sorry.

I'm planning on using a 35F bag inside a 20F bag, both down. What will the resulting temperature rating be?
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PostIcon Posted on: Nov. 21 2013, 9:02 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

Assuming there is room inside without compressing, the best you could get would be to measure their loft, and extrapolate from there. I'd guess about 10*  

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PostIcon Posted on: Nov. 21 2013, 9:39 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

How many inches of loft will you have? I would be unzipping the 35 degree bag and laying it on top like a quilt. You'd get more warmth from it. That said, I'd rough guess it at 0, if not lower.

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PostIcon Posted on: Nov. 21 2013, 9:59 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

Agree with the above responses.  Also, don't forget to insure you've got plenty of good insulation BELOW you, too!  You'll be losing plenty of heat there.

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PostIcon Posted on: Nov. 22 2013, 11:29 am Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

35 + 20 = 55.   Sorry had to say it.

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PostIcon Posted on: Nov. 22 2013, 11:53 am Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

Assuming they're both correctly sized the bag stuffed inside will be severely restricted in it's loft, which kills the insulating ability.

The better route is likely the above quilt usage unless that 35 was WAY oversized.
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PostIcon Posted on: Nov. 22 2013, 2:19 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(TrailTramper @ Nov. 21 2013, 7:29 pm)
QUOTE
I think I asked this before but I can't remember the answer. Sorry.

I'm planning on using a 35F bag inside a 20F bag, both down. What will the resulting temperature rating be?

I have tried this multiple times. After never really having it work well (because the top bag invariably shifted off of me repeatedly - nylon is slippery), I'm left to ask what anyone here who's ever tried it did to "stabilize" the upper bag/quilt.

Of course, I'm assuming here that the "inner bag" is being used as a zipped sleeping bag, though I suppose that's not absolutely necessary. Using 2 quilts would, I think, make "slippage" even more of a problem. I figure that some variation on tying the edges of the top bag/quilt to your mattress is the answer, but I feel this, or even using the standard (for me) method of tucking under the body (if using 2 quilts, because this wouldn't be reasonable with a zipped bag), could exacerbate the problem of loft compression.

I only ask my question about stabilizing the setup out of pure curiosity, mind you, because I never was able to solve the problem of "slipping" to my satisfaction.

More importantly, after some consideration, I've ruled out this "solution" because it doesn't make much sense outside of some kind of "emergency situation" or as a stopgap to save money in the short run: carrying 2 bags to achieve the same result as 1 bag with a lower temp rating, while it may make sense in an emergency, makes absolutely no sense in terms of weight or convenience, because you are clearly carrying almost twice as much outer shell fabric than you would be with 1 bag.* And, unless someone has an elegant solution for the "slippage problem", you end up fighting the top bag all night.

*"almost" because of the larger increase of surface area for a fixed, closed shape vs. volume.

ETA: Tigger's method of adding clothing inside a bag with adequate volume seems eminently more reasonable.
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PostIcon Posted on: Nov. 22 2013, 4:50 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

"How many inches of loft will you have?"
Just to elaborate a bit on this point from Tigger, at Backpackinglight someone came up with a formula to work this out but there is an easier way.
As Tigger suggested, measure the total loft on top of you.
Now go to the Western Mountaineering site find the suggested temperature with bags having a similar loft.
You need to divide the WM loft by two because they give you the above and below loft.
So for example if you get a 3.5" loft (on top of you...) that corresponds to the 7" loft of the Antelope rated at 5f .
Note that the also 7" Kodiak is rated at 0f (it has 6 oz of extra down) so they are just a suggestion.
Note also that a top quilt may let warm air escape easier than a zipped up bag .
Now on the practical side, using two bags is what I had in mind but using puffy clothing does work better for me because I do move a lot in my sleep (so as Gabby pointed out you may lose the top..) and of course I already use the puffy clothing at camp .
The trick is not to get your down clothing wet or dirty (I use rain jacket and pants over them at camp if needed and don't hike with them on)
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PostIcon Posted on: Nov. 23 2013, 2:59 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(Gabby @ Nov. 22 2013, 2:19 pm)
QUOTE

(TrailTramper @ Nov. 21 2013, 7:29 pm)
QUOTE
I think I asked this before but I can't remember the answer. Sorry.

I'm planning on using a 35F bag inside a 20F bag, both down. What will the resulting temperature rating be?

I have tried this multiple times. After never really having it work well (because the top bag invariably shifted off of me repeatedly - nylon is slippery), I'm left to ask what anyone here who's ever tried it did to "stabilize" the upper bag/quilt.

Of course, I'm assuming here that the "inner bag" is being used as a zipped sleeping bag, though I suppose that's not absolutely necessary. Using 2 quilts would, I think, make "slippage" even more of a problem. I figure that some variation on tying the edges of the top bag/quilt to your mattress is the answer, but I feel this, or even using the standard (for me) method of tucking under the body (if using 2 quilts, because this wouldn't be reasonable with a zipped bag), could exacerbate the problem of loft compression.

I only ask my question about stabilizing the setup out of pure curiosity, mind you, because I never was able to solve the problem of "slipping" to my satisfaction.

More importantly, after some consideration, I've ruled out this "solution" because it doesn't make much sense outside of some kind of "emergency situation" or as a stopgap to save money in the short run: carrying 2 bags to achieve the same result as 1 bag with a lower temp rating, while it may make sense in an emergency, makes absolutely no sense in terms of weight or convenience, because you are clearly carrying almost twice as much outer shell fabric than you would be with 1 bag.* And, unless someone has an elegant solution for the "slippage problem", you end up fighting the top bag all night.

*"almost" because of the larger increase of surface area for a fixed, closed shape vs. volume.

ETA: Tigger's method of adding clothing inside a bag with adequate volume seems eminently more reasonable.

I don't have a solution, but didn't find it as problematic as I expected actually. The only thing I'd do differently next time is swap which bag went in which. My larger Marmot helium over my slightly narrower REI Mojave.

The main problem I had was condensation on the top of the bag form my breath compromising insulation. It was ok after a couple of days, but any longer would have been an issue.
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PostIcon Posted on: Nov. 23 2013, 3:23 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

I have not zipped a bag in years, I use them like a quilt and now that I use a hammock most of the time, I alway use a quilt.  I have used 2 bags together and to solve the slipping, I sewed 7" pieces of flat bootlace in matching places on each bag and then tied the laces to keep the bag still.  If you zip one bag and not the other you could put your feet into the footbox of the outer bag and have the top tied together.
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PostIcon Posted on: Nov. 23 2013, 4:49 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(Gabby @ Nov. 22 2013, 1:19 pm)
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...makes absolutely no sense in terms of weight or convenience, because you are clearly carrying almost twice as much outer shell fabric than you would be with 1 bag.* And, unless someone has an elegant solution for the "slippage problem", you end up fighting the top bag all night.

*"almost" because of the larger increase of surface area for a fixed, closed shape vs. volume.
Well, no.

I can't believe I made this silly mathematical error, and I can't believe no one caught it right away, though it has been clear for some time that hardly anyone actually looks at the absolute gibberish I post.

I left it because I actually thought someone might be paying attention - and because I feel silly about it – and admitting it.

Why did we laugh so hard at all those 50s monster movies?

Well, we all knew that an ant or amoeba could never grow to hundreds of times its size without radical changes to either 1) its environment or 2) its respiratory structure.

If you remember your elementary biology and math, you know that the volume of a three dimensional object increases much more rapidly than the surface area. It's right there in the calculation: you figure volume in 3 dimensions (cube) vs area in 2 (square).

So...the surface area (shell) of a larger, more lofty sleeping bag is at the least proportionally smaller in relation to its volume than that of a smaller, less lofty bag, though the direct difference in overall surface (shell) between a single bag and two other smaller ones is equalized somewhat by other factors (the greater loft means more surface somewhere).

Suffice it to say, my explanation of "larger increase of surface area for a fixed, closed shape vs. volume" is completely wrong, and you would be carrying, at the very least, twice as much shell material for the same warmth with two bags, if not slightly more – not less.

Can I get a "who cares, already?"? :^)

These things are only important to me, apparently.

(oldnolder @ Nov. 23 2013, 2:23 pm)
QUOTE
…to solve the slipping, I sewed 7" pieces of flat bootlace in matching places on each bag and then tied the laces to keep the bag still.  If you zip one bag and not the other you could put your feet into the footbox of the outer bag and have the top tied together.
Oh my. This is much farther than I think I’d ever go, considering my other comments. Not only does this add additional weight, if you’re going to go this far because you’re consistently using the bags this way, why not save the extra shell weight and the troubles involved with carrying two bags and just get an appropriately warm system in the first place? I could see it if you are going to be out so long that entire seasons go by, but otherwise, it makes little sense to me. Even with severe changes in environment, I’d rather just arrange to switch out the systems or carry the warmer bag. (Just IMHO.) Question: Why not velcro instead of shoelaces? Lighter (I think), and less trouble (maybe).

Final note: I understand the impulse to "try something different and unique" that you yourself created, and I understand the desire to "cut costs", but this seems like a good candidate for realizing that there are reasons why there are sleeping systems with different purposes. Adaptation using clothing still seems like a better idea to me. I tried silk liners and came to the same conclusion about them for much the same reasons: convenience and multiple use. ...now, if you could use that lighter bag as a parka in camp...
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PostIcon Posted on: Nov. 23 2013, 5:00 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

Well, if you trip on over to warmlite.com, where they design bags to be nested together, you can see the tactics employed to avoid things like slippage and loft loss. And a nekkid girl in a sleeping bag.

(Sorry, service hosting the warmlite page....)


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PostIcon Posted on: Nov. 23 2013, 5:38 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

BPL published a loft vs temp table a while ago.  Ray Jardine also has a formula.

Using the BPL table, if the cumulative loft of a 35deg bag (1.65") and a 20deg bag (2.2") is 3.85", which should be good to about -15degF.

Using Jardine's formula:
Temp = 100 - (40 * loft)
You'd expect the 35deg bag to have a loft of 1.625" and the 20deg bag to have a loft of 2".  Plugging the sum (3.625") back into the equation calculates to a temperature rating of -45degF.  I'm a little dubious about that number.

On the Western Mountaineering spec table, a 3.5" loft would indicate a 0-5deg bag.

If you extrapolate by fill amounts, the result puts you in the 0-5deg range as well.


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PostIcon Posted on: Nov. 23 2013, 9:41 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

Is there some reason you just don't get a bag rated for around 0°-10° instead?

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PostIcon Posted on: Nov. 23 2013, 10:32 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(Montanalonewolf @ Nov. 23 2013, 9:41 pm)
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Is there some reason you just don't get a bag rated for around 0°-10° instead?

I'm just guessing, based on my own experience, but good bags are expensive, especially the lower rated ones.
I add a 20 degree down quilt to my 0 bag when I expect temps lower than 10 and it has served me (us) well.
It gives me more options without breaking the bank.


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PostIcon Posted on: Nov. 23 2013, 10:54 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

Sorry, I should have given more information. The 35 fits perfectly inside the 20 with no compression due to the bags being different sizes and shapes: the 20 is rectangular and the 35 is semi-rectangular, and the 20 is wider. I want to use these for winter camping with a sled, so the weight isn't a problem. I don't know the total loft, but there is 40 oz of 650 down altogether.

Both bags will be zipped. I tested this and the two bags fit together very well when zipped (with me inside).

Mattress: Exped Synmat 7 (1 degree F).
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PostIcon Posted on: Nov. 23 2013, 11:12 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(Montanalonewolf @ Nov. 23 2013, 9:41 pm)
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Is there some reason you just don't get a bag rated for around 0°-10° instead?


I had a zero bag and found I wasn't using it very often. I felt I had too much money invested in a bag I only used a couple of times a year. So I traded it in for the 20, thinking I would use the 20 more often and combine the 20 and the 35 for the rare occasions when I would need that much warmth. The 20 and 35 have worked very well from early spring through late fall. The 20 is compact enough to use for kayak camping but the zero was not.
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PostIcon Posted on: Nov. 23 2013, 11:42 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

I'll stick to my 0, if not a little lower that I mentioned above, not knowing your true loft.

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PostIcon Posted on: Nov. 24 2013, 6:46 am Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(toesnorth @ Nov. 23 2013, 8:32 pm)
QUOTE

(Montanalonewolf @ Nov. 23 2013, 9:41 pm)
QUOTE
Is there some reason you just don't get a bag rated for around 0°-10° instead?

I'm just guessing, based on my own experience, but good bags are expensive, especially the lower rated ones.

True enough.
I have 7 that vary from about +50 down to -20. Granted they take up space and some rarely get used but personally I'd rather have a bag for the temps I expect although doing so isn't for everyone. The -20 went for about $350 IIRC yet only gets used once or twice every 2-3years.
Being single is a big factor too.


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PostIcon Posted on: Nov. 24 2013, 1:57 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

I found some anecdotal stories in forums that support a rating of around zero. Examples:

"Ive put a 40 degree inside a 20 degree, both synthetic, and was toasty at about -5."

"For about 20 years, I used two down bags that were designed to nest together. The inner bag was sewn-through construction about 3 in. of loft, rated to 40 degrees F and weighed 2 1/2 lbs. The outer bag was slant baffle construction about 6 in. loft, rated for 15 degrees and weighed 3 1/2 lbs. The combination was rated to -20 F and weighed about 6 lbs. Unfortunately, both bags were rated optimistically for both loft and temperature but the combination was still good to below zero. "
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PostIcon Posted on: Nov. 27 2013, 8:42 am Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

I do this. I put my 35 degree bag inside my 15 degree bag.  The 35 degree bag is a slimmer cut that the 15, so it fits inside without significant compression (which probably means the 15 is too large for me). I don't know what the exact temperature rating would be, but I'm a cold sleeper and have been comfortable down into the single digits.  A single 0 or -10 degree bag would be lighter and take up less room in the pack, but good ones are pretty expensive. I'm only out in those conditions a few times a year and both bags are among the lightest in their temperature class, so the overall weight isn't too bad.  It isn't a perfect solution, but it has worked reasonably well for me.

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PostIcon Posted on: Nov. 27 2013, 8:40 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

I do this as well.

With two bags I cover all four seasons.
Summer bag gets used in, you guessed it, summer.
Three season bag is used in spring, fall and higher elevations, which is much of my Sierra backpacking.

For the one or two yearly winter trips that I do, I bring both sleeping bags.

Yes, it is heavier than a dedicated winter bag by maybe a full pound. But lets be realistic, in winter I bring an additional 10 pounds of gear anyway, so it is no big deal.
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PostIcon Posted on: Nov. 29 2013, 9:24 am Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

I came down on this idea pretty hard, but then I'm a gear hog. I relent: it might be a good idea if you're only going to use the setup a few times and you don't care about weight at all. After some consideration, I decided that this might even make a fair amount of sense for my 2 JRB quilts, since they're the same shape, size - and all that would really be necessary to stabilize them in place would be to insert one footbox into the other and tie a simple overhand in the ends of the drawstrings at the top. I might even try that just for grins the next time I'm out in weather in the teens or lower. Just for grins.
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PostIcon Posted on: Nov. 29 2013, 6:49 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(Gabby @ Nov. 29 2013, 9:24 am)
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it might be a good idea if you're only going to use the setup a few times and you don't care about weight at all.

Both apply. I would be using two sleeping bags once a year with a sled and a couple of times car camping. For all nonwinter trips either the 35 or the 20 alone is sufficient. The 20 covers everything from late March through mid November in the Northeast. (Although this fall is unusually cold.)
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(Gabby @ Nov. 29 2013, 9:24 am)
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it might be a good idea if you're only going to use the setup a few times and you don't care about weight at all.

Both apply. I would be using two sleeping bags once a year with a sled and a couple of times car camping. For all nonwinter trips either the 35 or the 20 alone is sufficient. The 20 covers everything from late March through mid November in the Northeast. (Although this fall is unusually cold.)
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PostIcon Posted on: Dec. 31 2013, 2:13 am Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

I have an answer. I tried this on my porch last night.

Conditions:
No tent!
Temperature 5 degrees.
Clothing: base layer, fleece jacket, hat. Started out with heavy socks and mittens.
Did not use hood.
Nalgene bottle filled with hot water. Maybe this was "cheating" but I do this while camping if at all possible.

Results: In all honesty I was too hot. I took off the socks and mittens and unzipped the 35-degree bag down to my waist. Obviously I could easily have gone another 10 degrees, probably 15 degrees with a tent and using the hood.

So the answer is that a 35F inside a 20F will be comfortable to about -10 degrees.
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PostIcon Posted on: Dec. 31 2013, 11:27 am Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE


(TrailTramper @ Dec. 30 2013, 11:13 pm)
QUOTE
I have an answer. I tried this on my porch last night.

Conditions:
No tent!
Temperature 5 degrees.
Clothing: base layer, fleece jacket, hat. Started out with heavy socks and mittens.
Did not use hood.
Nalgene bottle filled with hot water. Maybe this was "cheating" but I do this while camping if at all possible.

Results: In all honesty I was too hot. I took off the socks and mittens and unzipped the 35-degree bag down to my waist. Obviously I could easily have gone another 10 degrees, probably 15 degrees with a tent and using the hood.

So the answer is that a 35F inside a 20F will be comfortable to about -10 degrees.

I will clarify - That is for your particular bag setup.

I have done similar experiments with a synthetic bag that was more dense over the outside of a down bag (and switched it the next night). It was part of an experiment I was doing to determine where moisture would be trapped within the layers from within the sleeping bags and if moisture would lay on the outside vs. being trapped inside the outer layer of the bags, if overheating affected them, etc.

There was drastic comfort differences between the setups as the synthetic bag collapsed the down bag.

Other things to consider - You could have even gotten a warmer setup had you put the inner bag on top of you inside the other bag, adding even more loft vs. sleeping inside both bags. I've done this method inside my bivy using only my +40 bag with success.


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(Tigger @ Dec. 31 2013, 11:27 am)
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You could have even gotten a warmer setup had you put the inner bag on top of you inside the other bag, adding even more loft vs. sleeping inside both bags.

That's interesting. I think you're saying that the second layer of down is not needed underneath, so you could pile it around you on the sides and top. That could be true, as long as there are no cold spots along the sides. And it would be important that the outer bag be the larger and warmer one, as is my case (20 rectangular outer and smaller 35 semi-rectangular inner). I also found it more comfortable to unzip the inner bag quilt style (less constricting).

Anyway, the experiment was a success!
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PostIcon Posted on: Dec. 31 2013, 8:20 pm Skip to the previous post in this topic. Skip to the next post in this topic. Ignore posts   QUOTE

I have been playing with this for many years. (Why I got into BA bags in the first place.) But now I use a quilt as the over-bag (and sometimes the base too). I just did a couple reviews discussing it.

http://www.backpackgeartest.org/reviews....strella

http://www.backpackgeartest.org/reviews....strella


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PostIcon Posted on: Dec. 31 2013, 8:41 pm Skip to the previous post in this topic.  Ignore posts   QUOTE

Ray, thanks for the links to your reviews. Can you address how you decide which bag should go on the outside?

Wouldn't a zipped outer bag be warmer than unzipped? If the outer is unzipped, isn't that drafty around the edges?
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