Layering Sleeping bags

I think your math is off. Per the formula I've seen around, you subtract 35 from 70, giving 35, then divide that by two, giving 17-1/2. Subtract 17.5 from the helium's rating, resulting in a combined rating of -2.5 degrees farenheit.

I'm no expert on the matter though, so anyone who understands it better please jump in…

(edited 'cause I got the formula wrong…)

To start with I need to say that I have SOME experience combining sleeping gear for cold weather but my couple dozen nights experience is not enough to give you a confident answer.

This graph by Richard Nisley provides a starting point for answering your question. Other Nisley postings make it clear that for full coverage gear like sleeping bags, CLO values ARE additive. IF the model used to make the graph (ISO TR 11079) is good, then the graph suggests that a Highlite provides about 5.75CLO and the Helium provides about 7.75CLO. The graph ends at 0F but the fact that it is linear over it’s full range suggests that you may be able to extrapolate … and doing so places 5.75+7.75=13.5CLO at about -30F.

BUT! extrapolation is always suspect and this is a very big extrapolation. I’d want to test this in a situation where I had a solid plan B (e.g. a 100% reliable bailout option).

Regarding a silk liner … I’m sure it helps some but I’m a skeptic concerning 10 degrees (but have no experience so …)

Regarding pads, Richard’s postings pretty emphatically say that you need R5 insulation value for below freezing when sleeping on ground or snow regardless of the actual air temp. He backs that up with a graph of dozens (hundreds??) of soil temp vs air temp measurements showing that soil temp is pretty constant at about 30F regardless of how far the air gets below freezing. So use pad(s) providing R5 or better and you’re good on that count.

There are practical considerations:

* It is tricky keeping the breathing holes aligned when using nested sleeping bags … and you REALLY want to be breathing to the outside if you are gonna be out more than one night.

* While a fixed amount of down apparently maintains thermal efficiency over some range of densities, you would still want the outer bag to be large enough to allow the inner to achieve much of it’s normal loft. Most UL bags get some of their light weight from reduced girth so that may not be the case in your scenario.

* Dealing with the zippers when nesting bags is a pain in the backside.

Most of my recent experience has been using a bag and a quilt that is attached to the pad or attached inside the top/sides of a bivy. The quilt extends up to about chin level. I use a variety of balaclavas to beef up the head area. It’s been more manageable than nested bags IMO.

Please DO let us know how this works out.

So if I understand correctly, I take the value of the highest temperature rating, divide it by the number of layers, and subtract that answer from the lowest value sleeping bag?

The part I like is:

Marmot Lithium 0 degree rated 3 pound bag is 479.00. I can't comfortably use this bag durring the summer

Marmot Helium 15 degree rated 2 pound bag is 389.00. I can't comfortably use this durring the summer either.

Western Mountaineering Highlite 35 degree rated 1 pound bag is 269.95. I can't comfortably use this bag durring the winter.

I can do four seasons for 658.90 or roughly two bags for 325.00 each. If I got the Lithium I'd need a summer bag regardless of what I did. For the additional cost I "go to" -2.5 degrees (instead of 0) and a system that adapts to the weather so I only carry the weight I need to. I don't have to buy the whole system at once and break the bank. Anyone else know if this is an accurate portrail of sleep system temperature rating?

If you layer bags, you'll compress the down and you wont get what you think out of it. Look at these brands information on loft of their -20 bags and you'll see combining 2 isn't the same amount. Also in winter your sweat will freeze inside of them, making it less effective.

Feathered freinds -25 bag has 44oz of 800 fill down. that's almost 3lbs of just down.

why don't you look at a top bag/quilt. You could get one custom made to fit over top of your current bag.

I don't think you can just "add" 10 degrees to a sleep system adding foam pads or a silk liner(gimmick for travel wont add warmth.)

Look at Vapor barriers for winter to add warmth, and increase efficiency in winter.

Down compression….

As per a few posts by Richard Niesley…

I can't recall the specific numbers, but you can compress down to a specific point before it starts losing its insulating capabilities. I *think* its around 2-3x compression. So, again, you may want to find a more reliable source for this, but if you use an overbag, I don't think it will negatively affect your warmth too much.

My New Math response was posted while an intelligent answer came from Jim (thank you by the way).

So my non-scientific test will be this:

I have a Helium. I'm buying the Highlight. I'll put one in the other and test it (inside my Salathe bivy) durring this winter. I love just south of Denver in Colorado. I'll post hourly nightly temperature, and hourly bag temps with me in the set. It'll also indicate any problems I have with the setup, and if I have to bail (too cold) what I do to fix it (including abandonig the experiment, and going inside to my warm bed, and likely mad wife for being cold :).

Edited because I might have made it sound like I was saying something insulting because I didn't think it through…

Re the silk liner–I tried one and it added basically no warmth whatsoever to my 30* sleeping bag. I also got thoroughly tangled up in it during the night.

As far as compression of insulation goes, that would definitely be an issue when trying to put one sleeping bag inside another, unless the outer bag is quite a bit larger in girth than the inner bag. And both would have to be larger in girth than the person inside.

A warm sleeping pad can make a LOT of difference, as I discovered last fall when I was trying to get by with a NeoAir and a thin CCF pad at 18*. The sleeping bag was warm, but underneath I was shivering! A later trip with a warmer pad and equally cold temps was far more comfortable!

Personally I have had great success layering Western Mountaineering's Badger 15 deg with the Mitylite 40 deg. Both these bags have a roomy cut which helps in minimizing any compression of the down that would occur during layering of these two gems. The Mitylite is a hoodless bag and therefore eliminates the problem cited by another poster having to keep the hoods lined up. Both bags that I am layering also feature continuous baffling which allows me to shift the down either above me or below me helping me to further regulate my temperature. I have used this arrangement down to zero degrees with a pad having an R-Value of 3.2 and having the down in the Mitylite shifted on top of me via the continuous baffling. I expect that I could push these two bags to minus 10 or minus 15 if I step up the R-Value of my ground pad to 5.0 and wear a down jacket to bed. And yes the cut on these bags is generous enough to allow for the use of heavy down jackets and pants too. Heck if I ever had to wear the down jacket and the down pants I could probably go down to minus 20 no problem. I just love the flexibility afforded by layered bag systems. I rarely see temps below zero in the winter when I am out but I love knowing that I can push these two bags if needed.

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=16189&disable_pagination=1

This thread has it, about 3/4 of the way down the page. You can compress down by a factor of 2 and not affect the warmth.

Travis that is brilliant! Thanks for finding that thread. I knew layering was good for both cakes and sleeping bags.

Jeffrey,
There was also another thread where this is discussed, but not in so much detail. Basically, IIRC, it was s a light "argument" going on about down compression, and there were some people who were trying to explain that yes, you can compress down to a certain degree with no ill effects.

Also, I'm well aware of layering cakes….I'm planning a wedding…

Marriage is like Democracy. It works if all persons involved would sooner die than have it succeed for self.

My idea about layering sleeping bags turned out like this:

I've got a Long Marmot Helium (6'6"). I'm getting a Katabatic gear 6' Palisade Quilt. I'll use the Palisade with my Z-Rest, or the Helium with it, or I'll put the Palisade in the Helium and expect to sleep at 0F (may need to use a different sleeping mat for this).

Anyway, I don't have to worry as much about loft of one bag in another (got the sizes I did intentionally to avoid this), I won't have to worry about getting the mummy hoods lined up (the palisade doesn't have one), Weight is acceptable at 3 pounds for both bags. Just where I ended up, for anyone interested.