Using two sleeping bags

down inside synth so that the synth outer takes the moisture that travels outwards

as to an exact rating, i leave that to the experts … i generally assume an additional 20F or so from adding a summer bag to another bag …

The rating will depend on compression and loft also. Check the fit of the two bags and weather or not one compresses the other. I also agree. The down handles the sweat moisture much better than the synthetic.

Maybe this will help you estimate how much warmer it will be – http://www.bigagnes.com/Product/Overbags

BM

Down inside the synthetic.
Water vapor will travel from your body outward till it reaches a temperature where it will condense into water. It is best if this point is inside the synthetic bag. If you use a vapor barrier liner, it will matter less, but probably still better to have the down bag on the inside. How well it works depends on how much you're compressing the inner bags loft and on air leaks (winter bags are better sealed, baffles,etc.).

I considered this approach but ended up getting an EMS -20F bag on sale; this is a pretty good bag. It's worth checking EMS online regularly for sales on these bags.

Maybe 20 years ago EMS used to sell a lightweight down liner bag/summer bag rated at 45F and a coordinated down three-season bag rated at 20F that were designed to work together as a winter bag. EMS rated the pair for -30F. The summer bag was cut tight and the three-season bag was cut large so that when used together the three-season bag wouldn't compress the inner bag. I used it to -38F. I sold it to an acquaintance seven years ago and he was still using it on a winter trip we did earlier this year. Together the bags weighed around 6lbs.

I currently have two MB bags, 15F #1L and 40F #3. I am hoping and expecting to use them together down to -20F at a combined weight of 3.4, though I may need to use my down top and bottom.

I can tell you 15 + 35 = 0

I use my 15* long bag, (I'm 5' 11") with my 15 ounce quilt and can be comfurtable down to 0*

It's not just loft, it's also being able to fill in all the voids that a single bag can not. Having the bag in a bag fills in those voids very well.

I plan to layer my summer quilt with my Marmot Helium if I ever decide to go out in (expected) temps less than 10 F. I am hoping that by draping the quilt over my Helium I should be fine to temps around 0, but I haven't got to test this out yet…unfortunately our daily temps are now over 100 F so it may be a little while… This combo should be right at 3 pounds.

However, if I ever start going more frequently into temps lower than this I plan to get a WM Kodiak…

I did this but I was surprisingly a little cold. It was with a usgi patrol bag and a lamina 35 bag. I have a feeling this was due to compression.
A lot of military sleeping bag systems use the bag in a bag system, but they are made and properly sized for that. You would have to experiment with the bags to see if they compress or not. You could always try unzipping one bag and laying it over you like a quilt.

I don't necessarily agree with putting the synthetic bag on top. Most likely which ever bag is on top will stay dry. In temps as cold as -20C your body temp shouldn't be able to push vabor through 3 fabric layers before it condenses. Also the new down bag is lighter so most likely won't compress the heavier sythetic bag as much allowing you to capitalize on more loft. The most important thing is loft so take the two bags and open them all the way up and layer them together and see which layering gives you the most cm of loft. My guess from when I did it with a 3 season down bag and a summer synthetic bag, is that your down bag will be on top because it is lighter. When you go to test it you want to fully zip up the inside bag and use the outside bag as a quilt overtop.

The most important thing however is to test it in your "back yard". I don't really think you can do any amount of math to come up with how cold the 2 bag system can take you. Too many variables, your body, loft, compression, vapor… Just test it on single overnights until you hit either as cold as you want to go or as cold as the double bag will take you. Eat a big meal too!

Also you can try a vbl (emergency foil bivy) but they are hit or miss. I really don't like them at all. Don't underestimate the power of the sun it may be -10C out side but in the sun with a little breeze a bag will dry out enough to retain its loft for several days of a trip.

Last winter I tried laying a MB Thermal Sheet (fully unzipped) over a WM Summerlite. Seemed like a good idea in my living room but I'm too restless a sleeper – the MB "quilt" just slipped off the bag. The slippery fabrics didn't provide enough friction to keep the upper layer in place when I moved around.

there has been at least a thread or two here where someone used down over synth and ended up with a moist bag

as to water vapor traveling outwards in cold env and condensing …. it happens wherever the dew point is in yr system … most overbags manuf wll tell you explicitely to use the synth overbag on the outside … also when dealing with loose snow and condensation in the tent synth on the outside protects the inner down …

compression is a potential issue when layering any down system … clothes or bags … size appropriately

So is it a bad idea to layer a down quilt over a down bag?

yr outer down layer may well get a tad moist … if you can deal with that its up to you

big agnes pimping some of their down bags for overbag use aside … you dont see that many down overbags these days, or i dont

i suspect theres a reason for that …

if its that cold you can avoid the issue entirely with a cheap VBL

Good feedback here, I am going to experiment in my backyard before going out next winter. But something else came to mind. I also have a cotton liner that I use at times to up the temp on my syth 3 season, but should I use it with my down bag? Or with the down bag/summer syth bag combo in the winter?

If I do use it, where would be best? As my first layer, over the first bag, or over the 2nd bag? Or should I not use it at all?

I am leaning towards having the summer syth bag on the inside because it is tighter and I like the idea of having more loft in the down bag.

Thanks for all the insights.

I did this over the winter using two bags with a total of 19 oz of 750 fp down. I was more than comfortable to temps as low as 12 degrees (inside a double-walled tent).

I would skip the cotton liner. My big concern is that it would get moist at night from vapor and then freeze during the day. Once frozen you wouldn't be able to thaw and dry it until the trip was over. The general rule of thumb about cotton kills is especially true in winter, where it sucks the heat out of your body and never dries. Your best bet for adding a light weight temperature bump is an emergency bivy vbl which I don't like using but many people do (claimed up to 10 F in temperature gain), vbl clothing which I hope to try next winter, or a silk bag liner on the inside. I think the claim on those is that you get a few degrees of warmth added, but I have never used one so I can't say for sure.

Query so I am asking here rather than starting a new thread.

Does the same formula apply when using three layers in a system?

I have a cold weather sleeping bag in an expedition cut that was designed to be used with a down suit for HA mountaineering, A simple zipperless bag conservatively rated to -4C /25F on its own.

I just acquired a Brooks Range half bag on sale, also rated the same and the BR bag comes up to my armpits and fits inside the big bag with no compression of either layer and still room to wear any reasonable parka inside. When I do the arithmetic I get a rating for the combination of -20F/-30C.

Calculating another layer of synthetic insulation in an overbag or quilt is it enough to use that simple formula or is there an allowance made for the air trapped between each layer?

Other reading here at BPL says that a 50F overqulit adds 10 degrees according to that formula; I want to know if the formula is conservative or optimistic as I’d like to keep the weight of the overquilt to the minimum