Layering Sleeping Bags

MattC · Oct 22, 2005

I'm curious about the cumulative warmth-retaining ability of layered bags. I know there's no easy answer to this since there's so many different factors. But let's just say you were to layer a 20 degree and 40 degree bag together. Generally speaking, to what "rating" would that be about equivalent? Should one layer the lighter bag inside the heavier bag, or vice versa? If one is using both down and synthetic bags, which should be layered on the inside? Anyone have any ideas or experience w/ this stuff?

Matt

Nadine · Oct 22, 2005

Matt Typed:

But let's just say you were to layer a 20 degree and 40 degree bag together. Generally speaking, to what "rating" would that be about equivalent?

Ummm, a really heavy 10 degree

Would be pretty bulky though, not sure if it would fit inside a pack very well.

paul ron · Oct 22, 2005

Interesting question...

I once did some research into this but that was many years ago. I do remember something about loft playing a big part in the equation. "Thermodynamics of insulators" is what you might want to look up to answer your question.

I don't know what the temp rates of the layered bags will be but I do remember something about using the natural down inside and synthetic on the outside if you are to layer them. The reasons for natural inside, the moisture will tend to freeze in the outter bag so it is better to have the synthetic move the moisture, it's designed to do that, and let the down be more efficent at trapping the warm air inside which is what down does best.

DougPaul · Oct 22, 2005

The combined loft is the best indicator of the warmth of the combination.

Typ lofts of cold weather mummy bags:
0F: 6-7 in
-20F: 9-10 in
-40F: 12 in

Note that real winter bags have more than half the loft above the sleeper (differential loft), draft collars, and hoods which are designed to close down to just a breathing hole. And of course, you will need a good insulating pad underneath you.

Down inside synthetic--keeps the down drier. (I have used such a system--worked but was rather heavy.)

Doug

MattC · Oct 22, 2005

Thanks for the replies, guys. I can see the point about the extra features on real winter bags being an important factor.

Matt

Gris · Oct 23, 2005

Always been interested in this as well. Agree loft is biggest factor. Marmot makes a hybrid bag w/ synthetic layer OVER down, on account of waterproofing i guess... I question that strategy as seems synthetic layer would compress down and keep it from fully lofting - thus leading to inefficiency. For that reason i'd personally use the down on outside unless was wet weather...

DougPaul · Oct 23, 2005

Gris said:
Always been interested in this as well. Agree loft is biggest factor. Marmot makes a hybrid bag w/ synthetic layer OVER down, on account of waterproofing i guess... I question that strategy as seems synthetic layer would compress down and keep it from fully lofting - thus leading to inefficiency. For that reason i'd personally use the down on outside unless was wet weather...

Not a good idea.

As moisture from your body (and any wet clothing that you are drying in your bag) diffuses out through the sleeping bag, the temp drops. As the temp drops, the amount of water that the air can carry drops (ie the relative humidity increases). When the water vapor reaches the point where the relative humidity is 100%, water begins to condense out of the air and into the bag. Thus the water is deposited (as water or frost) in the _outer_ portions of the bag system. (Inner portions of the bag system are dried by your body heat.) Since polyester is much less affected by the moisture and frost, it is better on the outside.

If I want to use a down + polyester insulation system, I'd prefer to use two bags rather than have the manufacturer do it for me. Personal preference. More control and easier to maintain.

Doug

BrentD22 · Oct 23, 2005

bag liner

I've been using a silk inner bag liner. It really helped a lot last year. I also have a heavier duty fleece bag that I can use as a liner (although it take up a lot of pack space).

Gris · Oct 23, 2005

the water is deposited (as water or frost) in the _outer_ portions of the bag system. (Inner portions of the bag system are dried by your body heat.) Since polyester is much less affected by the moisture and frost, it is better on the outside

Well, based on that logic shouldn't everyone opt for synthetic over down all the time? (i know i won't...)

DougPaul · Oct 23, 2005

Gris said:
Well, based on that logic shouldn't everyone opt for synthetic over down all the time? (i know i won't...)

Weight, stuffed size, and durability.

And even a polyester bag (or outer) isn't good enough in some conditions. Tens of pounds of ice can collect in a bag with extended use in polar conditions. Pretty much have to use a vapor barrier liner here.

Doug

percious · Oct 24, 2005

To get back to the original topic. I use a 15 inside a 20 for my cold winter trips. At -15 its too hot to have the outer bag closed up. At -30 I am comfortable with everything closed. Keep in mind, it depends on the bag. If the inner bag's down is compressed it wont do anything for you. My inner bag is a Marmot Sawteeth, which is a narrow bag. The outer bag is a Kelty Light Year, which is a very wide bag. I think you are going to experiment on this one matt. There is no absolute way to calculate the ratings of 2 bags.

-percious

Mad Townie · Oct 24, 2005

For years I used a down 20 degree inside an EMS "Polar Outer", synthetic and wide, designed specifically for doubling. Worked pretty well above zero.

DougPaul · Oct 24, 2005

percious said:
If the inner bag's down is compressed it wont do anything for you.

One thing that happens is that a down inner bag (because it is constrained by the amount of space inside the outer bag) lofts inward toward you and fills up the free space inside the outer bag. This is good because it prevents the pumping of air from inside the bag caused by the user's breathing. (This was an issue with some of the older stiffer polyester-fill bags. Perhaps less of an issue with some of the newer softer-polyster fill bags.) The downside is, of course, that you may not get full loft from the inner bag.

Doug

jfb · Oct 24, 2005

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.

Oldsmores · Oct 25, 2005

Didn't someone (Slumberjack?) market a two bag "system"? I remember thinking it was a great concept. It was a lighter outer bag and a heavier inner bag. You could use either one alone, or both for cold temps. Can't remember the relative ratings of the two bags.

hikerfast · Oct 25, 2005

stephenson's warmlite has a triple bag system. the bag is good to 40 degrees.you zip the additional layer on and its good to -10, then the 3rd layer if its colder, up to -60. the bag also comes with a down filled air mattess built in. its about 750 bucks. the guys point is you get 3 bags and a down filled built in mattress for the same price and dont have to bring a sleeping pad. he is a big vapor barrier guy, it comes sewn into the bag.

post'r boy · Oct 25, 2005

Oldsmores said:
Didn't someone (Slumberjack?) market a two bag "system"? I remember thinking it was a great concept. It was a lighter outer bag and a heavier inner bag. You could use either one alone, or both for cold temps. Can't remember the relative ratings of the two bags.

not sure but the feathered friends website has a bag layering system,with explanations
http://www.featheredfriends.com/productlist.aspx?SubCatId=4&CatId=1&selection=4

Pete_Hickey · Oct 25, 2005

jfb said:
For about 20 years, I used two down bags that were designed to nest together./QUOTE]

If they are DESIGNED to fit together, the outside bag would be larger to minimize the compression of the loft of the inner bag.

If I HAD to do that with my bags, I would probably decide which is inside/outside based on its size, rather than the material of constructions. I would also think of simply draping the outer one over, like a blanket, so as not to compress the loft.

Lawn Sale · Oct 25, 2005

I bought a few expanders that will enlarge a regular bag. This makes any bag it zips into larger and it works well. My older brother, who is much larger than I, uses one so he has more room when he sleeps. They're pretty cheap, I think new you can find them around $15 (mine were $2 at Goodwill, 2 Polarguard Delta and 1 was 775 Down).

Layering Sleeping Bags

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