Doubling up/layering sleeping bag question

I was told to use your best, lowest temp bag as the inner one when doubling up. Granted I got this info back in the 1980’s so it may be outdated or flat out wrong. Hopefully someone with more experience/knowledge will post before your trip.

I have been layering a 40F down quilt over my 20F down bag. I prefer to layer this way, as some wetness ends up on the quilt, which being thinner, dries easier/faster (when I get home–most of my 0F outings are one or two nights). Both are 800 fill, so I am not sure what I would do if both bags were the same temperature rating but different fills. Also, size is my very first consideration when layering. When I purchased the 40F quilt, I oversized it in length and width, so it wouldn’t compress the bag under it. In your case, I would figure out which one has more interior volume.

Back to fill, I think I would place the lower fill on top, in your case the 650, even though that bag would be heavier and cause some compression of the lower bag as opposed to doing it the other way around. The reason stems from pure ignorance. I fear that freezing down degrades it. If I am right, I rather replace the 650 bag than the 900 bag over time. I also read somewhere, I can’t recall how scientific the source was, that 900 fill is more affected by low levels of moisture than 650 fill. This was discussed at some point on BLP, but I can’t find the thread or recall the communal consensus…

Again, I want to be clear that I have no scientific basis for either of those statements on the last paragraph and that my experience with layering is limited to handful of happy, warm nights. I would actually love to hear from folks with actual facts about whether freezing down affects its life and whether 900+ fills are more affected by moisture than 650 fills.

Even the makers of my sleeping bags can’t answer this question. they make double bags for Antarctica but from a totally different premise [ robustness is the priority there sort of like the military] and they have never been asked this question before. The outer bag would be HUGE and relatively heavy even with UL fabrics and materials simply because of the size

If I got an XL/XW Hispar O’Bag from PHD for instance it would be about 1300grams and only good to -6C on its own

https://www.phdesigns.co.uk/hispar-down-overbag

Consider a synthetic fill outer quilt for your outer bag, and go light / thin on it. This layer will capture the condensation and lose less loft.

I use something like this. Wide and long(er) so it doesn’t compress loft on your inner bag, and thin.

Thanx Ryan

I took the advice of the forum and had one made by Nunatak to fit my existing bag, unfortunately it simply isn’t warm enough for an Alaskan winter going by the comments in the winter hiking section

Your 900 FP bag is close to a -25 rating, right?

And maybe I calculated wrong but it seems like your 900 FP bag is warmer than the 650 FP bag.  (900*1050/650 = 1453), I think your 650 FP bag would need 1453 grams of fill to be as lofty as your 900 FP bag.

I prefer to use one single overkill down bag for my “deep winter” i.e. subzero trips—because when you use two bags for this purpose you have two disadvantages—

#1  The down loft in a double bag system tends to get compressed by the 2 bags constricting against each other—which means the single bag loft warmth is lost to a degree with two bags.  It’s like wearing a beefy down parka inside your bag—the parka mashes down the geese from the inside-out.

#2  The cocooning “mummification” effect is terrible unless you really like getting constricted by two bags with two separate zippers.  I used such a system many times to augment my sleeping system and here’s what happens:  You get cocooned and zipped up properly (on a -15F night) and all is well.  You toss and turn of course thru the night.  At 3am you wake up in a panic and need to get out quickly but dangit one bag’s zipper is right below your throat and the other bag’s zipper is right behind your neck.

And you can’t get your arms out easily to unzip and have a hard time pulling down the throat zip and can never reach the back zip.  What fun.

Solution?  A single overkill -25F or better down bag.

I’m familiar with that zipper problem but using 2 bags is a very cost effective solution and the inner bag is one with no zip at all, and it’s one I can escape from in seconds if I ever need too. The compression issue is why I asked which of the two versions of the bag I should use, the unit with 650FP down or the one with 900FP down and why I was thinking of using the lower fill power down and slightly overfilling the shell when it gets the rejuvenation at the factory The factory rejuvenation is $140- with 100grams of 700 FP down included, the shell could easily take another 100 grams This would take the bag weight to 1400grams but it would then probably be much warmer also.

I have always rated this bag at the EU female comfort temperature of -4C because I am a very cautious solo walker but nobody has any idea of what the original factory rating was, the designer and business owner seems to have  passed away and all records went missing in a huge fire 20 years ago. With a free loft of 155mm I guess most people would rate it at being a 0F/-18C bag and perhaps it is in a dry environment, I have felt cool in it at -5C here, a lot of that has to do with the very high humidity usually around 90% at those temperatures in our snow fields

I have never felt that wearing my parka or down jacket inside this bag has caused any of those compression problems, isn’t that the whole point of using an Expedition cut sleeping bag?

Sleeping bag has a girth [ internal] of 62 inches As I’m 57 inches in girth I thought that left room for a reasonable down parka

But again this is why I am asking all these questions

Your 900 FP bag is close to a -25 rating, right?And maybe I calculated wrong but it seems like your 900 FP bag is warmer than the 650 FP bag. (900*1050/650 = 1453), I think your 650 FP bag would need 1453 grams of fill to be as lofty as your 900 FP bag.

I’m interested in how that calculation works Rachel P

Both bags appear to have the same free loft and 1250 grams is 200 grams more fill or approximately 30% ??

FP being Fill Power I guess you use Loft Power ? So about 550 grams of 900 down in the lighter bag

I have had this bag for so long I have lost the original order documents but 550 grams sounds about right

If they have the same loftiness then I guess it doesn’t matter what the fill weight is! Have you decided if you are you going to layer bags or get a warmer bag? Where are you going anyway that gets that cold? (I’m just curious, I’m assuming Alaska)

“900*1050/650 = 1453”

Intuitively obvious to the most casual observer : )

assumes that 1 inch of loft of 650 FP has the same warmth as 1 inch of 900 FP down

Of course that’s incorrect bit it gets you close.  Plus, the weight of the fabric and other components.  Plus the construction techniques.

Since this is a lightweight backpacking website, I wouldn’t waste anything on the 650 FP bag.  Use it for car camping or give it to a homeless person : )

A lot of expert people advocate getting a synthetic flat quilt on the outside.  Then it won’t compress the sleeping bag underneath so much.  Water vapor will tend to accumulate in the synthetic.

Maybe you could add 900 FP down to that bag and it will be warm enough itself.  That would be the most lightweight solution.

Still trying to work things out Rachel

@Jerry I’m not giving a $500- sleeping bag away if I can use it effectively, if I can’t use either of the bags as part of a system I’ll get a new one rated to a lower temperature

650 FP bag for $500?  wow!

(from the perspective of a lightweight backpacking site)

AHHH A lightbulb moment

In the USA you use a different FP measuring method, this Australian 650 is closer to 760/800 loft down if measured over there; also “Yes”, because it was a semi custom run of specialised bags for an Everest attempt which was not sponsored and in Australian dollars so knock off 35% to get the US Values

I have just returned from a consultation with that bags manufacturer, they are going to clean it at the factory and top it up with new DWR treated down, they no longer use this spec down so they will be using between 100 and 150 grams of 700FP down, this is probably the equal of US 900 down and because they are an EN factory they will not rate it officially  simply because the test is so expensive but ballpark ~-25C when renewed

The recommendation to use this bag as the inner came after trying both versions of the bag inside the proposed outer bag, the heavier bag had the greatest combined loft; which makes perfect sense 800FP down is ethereal stuff

Combined free loft using the old bag was 325mm combined but I intend to have the new outer bag overfilled by 10% or more and with the rejuvenated liner bag I think I may have just a little overkill

As a bonus there would be room inside to wear the medium weight down clothing at need up to 40mm thick

It is a work-around to get to the temperature rating needed but it isn’t that great a weight penalty all things considered