- Nov 22, 2017 at 2:06 am #3503404
I need a deep winter system for a forthcoming trip and I am buying an outer bag to use with my existing sleeping bag
However I have two very similar sleeping bags, both are the same design and Expedition cut bags; one has 900FP down and the other has 650FP down
There is a 200 gram difference between the bags; one is 1050 grams using the 900FP the other using the 650FP is 1210
Both bags are the same age and about the same usage and have suffered about the same 10% loss of loft. I am taking one in to have it cleaned and topped up with an additional 100Grams of down this W/E
Intuition is telling me to get the bag with the lower FP down topped up with some new stuff and use that as the inner but I am having second thoughts
Is my intuition wrong?Nov 22, 2017 at 3:56 pm #3503488
Jason BBPL Member
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 Granted I got this info back in the 1980’s so it may be outdated or flat out wrong.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.Nov 22, 2017 at 7:36 pm #3503533
@iagoLocale: Boston & Galicia, Spain
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.Nov 24, 2017 at 12:29 am #3503738
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 ownNov 24, 2017 at 5:26 am #3503774
@ryanLocale: Rocky Mountains
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.Nov 24, 2017 at 6:27 am #3503784
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 sectionNov 24, 2017 at 1:29 pm #3503807
Rachel PBPL Member
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.Nov 24, 2017 at 3:41 pm #3503819
Tipi WalterBPL Member
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.Nov 25, 2017 at 7:34 am #3503986
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 questionsNov 25, 2017 at 7:48 am #3503987
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 rightNov 25, 2017 at 3:16 pm #3504012
Rachel PBPL Member
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)Nov 25, 2017 at 3:40 pm #3504015
Jerry AdamsBPL Member
@retiredjerryLocale: Oregon and Washington
“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.Nov 25, 2017 at 7:54 pm #3504046
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 temperatureNov 25, 2017 at 8:37 pm #3504047
Jerry AdamsBPL Member
@retiredjerryLocale: Oregon and Washington
650 FP bag for $500? wow!
(from the perspective of a lightweight backpacking site)Nov 27, 2017 at 5:13 am #3504260
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
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