Down filler bags – full body, or half-body pied d’éléphant?

[Terran Terran]

Why not just buy a smaller bag with more loft? Other than cost. With an inner bag, there’s 2 extra layers of fabric that add very little to warmth. I would argue that there’s less trapped air than with a pair of down trousers, plus if you use them while setting up camp, you’re warmer going into the bag. There’s no heat inside the bag until you crawl in. It can actually be colder. Even with thermals, there is often heat lost through conduction when you lay down.

[George H]

It will be very constricting, to the point of being unusable to size the outer, main bag for a fit close enough to eliminate the majority of dead air space. This is simply not an option, imo.

Which leaves the filler bag, or deal with the air.

I recently tried the filler option on a quickly arising trip opportunity where my winter bag was unavailable. It was cozy and warm and softly snug with room to move, ie pretty much ideal regarding comfort. Quite surprised, actually!

The low outside the tent was 10°F with 12-15 most of the night. Heavy snow added insulation to the tent! Wearing base layers I had a 30°F outer bag and an inner bag of identical size with 7 ounces of down, which the maker calls 45°F, but I’ve used it in colder weather just fine.

This combo was almost too warm, even with just a 3.2R-value pad plus Thinlite on packed snow. I feel like 5°F outside the tent would have been fine.

[Jerry Adams]

yeah, exactly, two extra layers of fabric

if a sleeping bag is 65 inches x 72 inches, that would be 3.6 square yards

7.2 square yards for 2 layers

5 ounces of 0.7 ozyd2 fabric

you wouldn’t notice a pack that weighs 5 ounces less.  I think maybe 1 pound is the minimum noticeable amount.  If you could save 5 ounces here, and 11 ounces elsewhere, you could reduce your pack weight a noticeable amount.

[jscott]

Are folks sure that the “get the lightest bag for conditions…and then bring added fillers of various sorts to keep warm in real terms”…is the best approach?

Down remains the most weight efficient insulation. How about bringing a heavier bag and leaving all the fillers behind? beyond what we’d wear around camp.

i’m suggesting that a single, well filled bag is simpler and ultimately comes in lighter than all the alternatives involving adding yet more layers to compensate for the initial insufficiency of a light bag.

there’s another thread going about the usefulness of vests. Some advocate using a vest and then adding some variety of arm warmers to compensate. In the end, it all adds up to more hassle and weight than simply bringing a good down jacket.

–a vest weighs less than a down jacket! great!!

–but my arms and hands are cold

–add in down arm warmers!!

…really? isn’t that just a jacket, in three parts? Isn’t a bag plus various fillers just a heavier bag than one single, more generous down filled bag, that’s without the hassle of layers?

[Jon Solomon]

@Jerry To wrap over me inside a bag, the filler liner definitely wouldn’t need to be 65 x 72 inches. It could be narrower than the narrowest quilt.

@George H “Which leaves the filler bag, or deal with the air.” I agree. That’s why the idea of a single bag simply can’t do what a filler liner can do.

Sounds like your experience in the field validates the idea and calls for more testing.

[Jerry Adams]

Ahhh… then it would add even less than 5 ounces.  Small enough if it was accomplishing something useful.

[jscott]

“Ahhh… then it would add even less than 5 ounces.  Small enough if it was accomplishing something useful.’

wouldn’t  adding five ounces of down to the single bag accomplish something even more useful? Or 3 ounces? while eliminating the fiddle factor of several layers?

I understand that folks are trying to solve convection issues through so called empty spaces in a single bag. In that case, maybe order a small bag?

Are we sure that those “empty spaces” don’t serve a purpose? for example, allowing the transmission of moisture outwards? or for that matter, O2 inwards? really, I don’t know the answers. I don’t hike in consistent, well below freezing temps that fall even farther at night, so yes, my experience is limited in this regard. I tend to trust simpler solutions to reach the same aim as more complicated designs.

[Terran Terran]

Your air space would need to be pretty evenly distributed or you’d have compression in some areas and gaps in others. Still more feasible then a bag inside a bag.

[George H]

jscott, I agree. My experience w a filler situation happened because my winter bag was unavailable. Would never have thought about using an INNER-bag otherwise. I typically layer something other than down on the outside to deal with frost and dew and spilling coffee

The two bags weighed a little more than a WM VersaLite, with a little less fill – so in theory less efficient had I been just as comfy in a VersaLite. Will never know, as these unique scenarios can’t be replicated at will.

Other considerations: If moisture build-up occurs on a long trip and a drying opportunity presents itself surely it would be faster to dry two thinner bags.

And say one layer had horizontal baffles and the other vertical, maybe potential shifting down mass would behave different in each layer and not create as dramatic cold bridges as in one poorly designed bag.

What about the unavoidable air space between the two bags, however little? Adds warmth or no?

 

 

[Terran Terran]

I don’t use sleeves with my vest. It’s there to add extra insulation to my torso while wearing a light synthetic jacket. The jacket can take a light rain and protects the down. If I rip my jacket, which I tend to do, I don’t lose insulation. I just do what works for me. Not necessarily always the lightest. I also tend to sit around camp more than in the past. To me, adding body insulation makes sense. Living in a cool climate, I get more use out of it. Finances aren’t unlimited. A new heavier quilt is a luxury. While waiting to buy one, I was gifted one. Not what I would have bought, but still a nice quilt. I got a new lawnmower instead.

[Geoff Caplan]

This is becoming interesting!

It seems that George’s experience validates what PHD are claiming – that a filler bag offers excellent warmth for weight, despite the additional non-insulating fabric.

Let’s crunch the numbers.

Their filler weighs 240g and they claim it adds 10C of capacity to their sleep system. Serious reviewers like Chris Townsend and Andy Kirkpatrick regularly confirm PHD’s performance claims and Andy raves about the filler bag, so let’s assume that this is true.

Using the PHD bag configurator for their non-technical hiking bag with 900FP (EU) down:

3C comfort rating: 489g.

-7C comfort rating: 728g.

So to achieve the same 10C improvement, you’re looking at almost exactly the same weight as the filler.

But the filler gives you a more modular system that can be adapted to a wider range of temperatures. The inner bag can be used in alpine huts. Two thinner bags will air out more quickly than one thick bag, as others have pointed out.

That’s quite a range of benefits for the same weight of sleep system. I’m beginning like this idea!

[Author]

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