Please let me know the potential flaws in my thinking:

I need a -20F sleeping bag for a winter ultra marathon event. I am reluctant to buy a -20 bag which will probably get no other use besides the race.

I currently own a WM Highlite. ((8oz of down rated 35 deg) If I combine this with something like the WM Antelope (26oz down rated 5 deg) I get a total of 34oz of down.
The Wm Puma has 36oz of down and is rated to -25F.

So, would the combination of these 2 much more versatile bags (Highlite + Antelope) get me relatively close in warmth to the larger Puma?

I understand the highlite may get compressed a bit, but then there may also be a bit of additional air trapped between the 2 bags to help compensate perhaps.

Thoughts / Experiences anyone?

i'm pretty sure some dude at www.hammockforums.net figured out some formula to figure this info out. i'd head over there and do some digging...

There was this thread here :
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=10415
but I think that we had more than that...
Some info here
http://www.warmlite.com/bags.htm
Franco

John Tier knows the answer
i used a highlite inside an ultralite that came out to 7° using Jack's equation. turns out the low was actually 7°, and i was plenty warm on a ba insul air core with a 1/4 ccf on top of that (light clothing, no fancy base layers)

ok, this is my remark copied from a different forum thread - I said: "Your statement reminds me that I was reading about combined (bag) temp ratings and found an article regarding a formula used to calculate your "system" and what rating one could expect from the use of two bags together. The formula looks something like this: If you have (for example only) a 20° and a 35° bag together, you would subtract the liner bag's rating from a value of 70. Then, halve that number (we arrive at 35/2 = 17, roughly) and subtract from the outer bag. So, 20 degree bag/quilt with a liner bag of 35 degrees, gives you a 3 degree rating. So far this seems reasonably correct, I have used a combo of WM Ultralite/highlite down to 7 degrees without supplimental clothing. Does this sound reasonable?"

so i was off by 4 degrees, but close.

Edited by mikeinfhaz on 09/19/2009 23:57:30 MDT.

I have a Mountain Hardware -20 F. bag of Polarguard Delta fill. Should I ever need to go below that temp. rating (God forbid!) the MH bag has a gore zipper above and parallel to the main zipper that opens the bag up another 4 inches.

This opened insulated gore gives me room to insert my Western Mountaineering Megalite 30F. bag without unduly compressing it.
I've tried this combo briefly one 10 F. night and, while cozy but not tight, it was WAAAAY too warm, of course.

My guesstimate says this combo of summer and winter bags is likely good to at least -30 F. I'd never say -40 F., having experienced that temperature as a Nordic ski patroller on the last day of the 1979 Pre- Olympics in Lake Placid, NY. I was warm that day but only because I had a well insulated hooded parka and bib OVER my complete Nordic uniform, plus boiled wool Dachstein mittens as liners beneath leather and nylon mitten shells. If you spit at -40 F. (same temp in Celcius) it freezes before it hits the ground. IMHO -40 is dangerous weather, period.

Eric

Edited by Danepacker on 09/20/2009 00:48:38 MDT.

Mike
Interesting comments because I have those two bags and my initial idea was to use them coupled in winter .
I can fit inside them without much squashing. Never done that as I have not been below the Ultralite comfort range (- 7c for me is about right)

Eric
My cousin (Adriano Darioli) competed at those Olympics in three events (biathlon...) finishing 25/19/9th
Franco

My wife and I have some light summer quilts which we push well into the off-seasons. They are definitely not good enough to be reliable for winter in the mountains. Our solution is to still take them and use them, but to throw a second quilt over the top of us both for serious winter snow conditions. This works very well.

There is more to this than meets the eye. When you layer quilts you almost always have the dew point somewhere in the outer or top quilt. This means the lower quilt will not get any significant amount of condensation inside it. Your primary layer will stay pretty dry - and warm. Yes, we have tested this in the field.

So - go for it.

Cheers
PS: no brands - all MYOG.

Thanks everyone for the useful info. The formula (if it indeed works) seems to support the down weights and temperature ratings - at least per the WM website. I probably need to be a bit more conservative then and go with the 0 deg rated Kodiak combined with the 35 deg Highlite to get a "true" minus 20 degree rating. Still a much better option than buying the single minus 20 bag

cheers

doug

In Britain, PHD sleeping bags offers precisely such combinations of summer bags and winter outer bags. The crucial thing is that the outside bag needs to be large enough to give the inside bag enough room to loft properly. In a similar though less arctic inspiration I have just bought a wider than normal ultralight Minimus summer bag from them, to be supplemented by my down vest for three season or high altitude use.

Edited by willem on 09/20/2009 12:18:35 MDT.

Yet another solution is to take along a Heatsheets emergency bivy. At only 3.4 oz and about $10.00, it surely beats carrying another sleeping bag. It's also waterproof. For those of you (like me) who are sleeping in a single-walled shelter in winter, you'll stay dry even if a bit of snow blows into the foot of your shelter.

Why Heatsheets? It's not made of mylar. Instead, it's a softer, stretchier, and quieter material that won't break so easily as mylar but is still coated with aluminum to reflect your body heat back in with far greater efficiency than a second sleeping bag.

And the thing slips into its own little stuff sack and packs down to a startlingly small 3 x 3 inch package.

Slip yourself into your bag, the Heatsheets over the bag, and sleep the sleep of the righteous.

One proviso: Any emergency bivy is going to be totally impermeable to moisture. If you leave the bivy open at the top, you won't be swimming in your own sweat at daybreak.

Stargazer

The current U.S military Polarguard filled sleeping bags use the inner/outer, summer/fall bag system. I'm sure the Nautick military testing center has a lot of data to support the system's effectiveness. It's been in use for over a decade.

Now it's "likely" Special Forces units, for weight reasons, use down bags, such as in mountain observation posts in Afghanistan. But they are "probably", again for reasons of weight savings, using their pile ECWCS clothing insulation layers for additional warmth in the down bags. Don't ask me how I know this or I'd have to kill you if I told you.

Eric

> Slip yourself into your bag, the Heatsheets over the bag,

Been there, done that, got soaked. NO THANKS!

Cheers

Wiggy's sleeping bags are (or were) the basis for the US military bag system. They claim that their overbag rated for 35F by itself extends the temperature ratings of their other bags from
+20F bag to -20F
0F bag to -40F
-20F bag to -60F
-60F bag to -80F
Their bags are a proprietary synthetic insulation that has many magic powers and is probably the result of technology transfer from Area 51 aliens :)
http://wiggys.com/category.cfm?category=6

I'm thinking of using this approach-- MYOG synthetic overbag over my 10-20F down bag. I'll aim for a total rating of somewhere in the -20F to -40F range. I'll use either Primaloft or Climashield, probably Climashield.

Edited by Snowleopard on 09/20/2009 15:29:38 MDT.

I agree with Walter. Synthetic overquilt on top.

A Montbell super stretch makes a great overbag without collapsing the inner bag insulation.

Their synthetic bags are a bit heavy though.
Down UL would be a good one and light, like maybe a #1. Not sure of that would take you down that far.

Edited by tammons on 09/20/2009 15:44:47 MDT.

Not sure how many nights you'll be spending at -20, but you should consider taking a vapor barrier liner to be used inside the bag/s. Helps to keep your down dry and adds some needed warmth. Chuck