Thermodynamics with down

RS, no offense intended, but it appears you need more actual experience in the field with different products in a wide variety of conditions.

The article (which I read) offers nothing new. The hydrophobic treatment does give the down the desirable synthetic-like quality of being less moisture absorbent.

But take note that the DriDown bag did retain quite a bit of moisture relative to the regular down bag, 4 oz vs 6 oz. Where was that moisture in the DriDown bag? It was in the outer shell, which was described as 'wet'.

In the right conditions – sustained (say 3 days, or more, in a row) humidity or very cold (below zero Farenheit) – ALL sleeping bags, regardless of insulation type, will see a build-up of moisture/ice. The only exception to this is when full-body vapor barrier liner or clothing (VBL) is worn inside the bag, effectively eliminating water vapor from the body getting into the bag's insulation. Absent VBL, however, over the course of 2-3 nights moisture will freeze in the outer shell first (see previous paragraph) and then the moisture collects in the insulation and freezes. Synthetic insulation and (presumably) hydrophobic-treated down will delay this process, but it will surely happen. Each night you will feel colder and colder and ….

If you don't believe this, take the bag on a trip with you to the White Mountains of New Hampshire in late January.

Bob,
What was it I said that made you think I disagreed with anything you just said?

Would appreciate if we could focus on the real point of the article, which was that specifically down does a better job of transporting moisture away from the body and to the outside than does synthetic insulation, and that in turn makes us feel more comfortable (i.e. warm to point of equilibrium). DriDown apparently enhances this effect, as well as making the insulation more usable over a greater range of conditions.

I'm out…

Let me back up here…

Heat loss causes us to get cold, and the reduction of heat loss insulates us from the cold, preserving our body temp. Have I got that one correct?

What role in the reduction of heat loss does moisture management play? Let's leave VBL out of the discussion for the moment. Talking its 30 degrees in your 20 degree bag, and whatever level of clothing you're good with.

Does the temperature of the air trapped by down have a role in reducing heat loss? In other words, "hot" trapped air that is close to our body temp as opposed to "less warm" air in the down, as you might have if you're wearing bulky clothing.

Let's start this again from there, shall we, for anyone still around. I'm trying to get to why the authors of this article would say that there is a "feeling to down that's hard to quantify".

http://www.easternslopes.com/2013/05/02/gear-snapshot-dridown-technology-is-it-a-game-changer/

Not my words…

"you have to heat up the bag. Getting in hot is the best way. Wearing tons of clothes stops the heat getting into the down. You may not be cold as your clothes will provide some insulation, but you’re not making the most of the potential warmth of the down. Heat will seep out into the down slowly if you are wearing all your clothes and you may wake up warm in the night, but it’s better to get warm from the start."

Again, not my words, but this is exactly what I was getting at earlier with radiating heat coming back to the body from the heated trapped air in the down…

"Absorption…
Using a material that will absorb and hold the radiant energy transferred from the body will gradually reduce the amount of heat transfer that occurs and, potentially, radiate heat back to your body in the process. Insulation materials with absorptive qualities also increase the rate at which the stable air layer will reach heat balance with your body."

Another outside comment on moisture moving through and out the sleeping bag…

"The downside for using bivies to keep outside condensation off your bag is that many of them will form condensation inside of the bivy sack. As you sleep, warm moisture will work its way through your bag. Upon escaping its shell it will condense on the underside of the colder bivy. This kind of negates the whole purpose of the bivy."

So, the question is, how does this movement of moisture through the down insulation work for or against or neutral to heat loss?

> what I'm hearing is contradictory to what is espoused in the article about down
> keeping us dry, and therefore more comfortable…

Well, if a lot of very experienced people here are flatly rejecting what the article said, perhaps you should reconsider the quality of the article. The bits you have quoted generally seem to clash with the laws of physics to some degree.

Cheers

Roger,

First of all, thank you for still being here and responding.

So far, its you and Bob responding to the article. If Bob read it, then of course the idea of moisture moving through the down is nothing new. Right now, I don't see a lot of very experienced people flatly rejecting what's been said, at least not directly refuting the article. What I see outside of here is a lot of very experienced people espousing similar misinformation as the article – if that's what it is.

You say that is clashes with physics "to some degree" – can you be specific about that? I am truly interested in the detail. My only rejection of what you've said so far was in you basically telling me NO, but without anything to necessarily dispute what I'm hearing from the outside, like this article. Apologies for that, but just trying to get to the part that isn't being covered – either from the physics end, or from the anecdotal end.

RS, moisture is water, from a less than a gram to a very large amount. Water vapour is a gas that is measured by humidity, though often people interchange the two. They will respond differently. Condesation happens at the "dew point" given a temperture and saturation of the air. Perspiration is both.

Moisture (water) in your sleeping bag down will always make you colder, given a constant temperture. It will speed up heat transfer and generally ruin the insulative properties. As was said, use a full VBL to avoid condesation (water.)

Even dry-down will NOT stop condensation in a bag. Glass is water proof, yet we see condensation droplets on it all the time. Drydown is only good for not soaking up water. Like sleeping in a lake, or sleeping in a rain storm. Neither is recomended for ANY down.

Generally a bag will dry itself. Any perspiration will tend to be moved through it to the outside air. Conditions will determine exactly where (how far from your skin) this will occur. It does happen inside the down. Basically equilibrum will allow so much for your body, outside temperture, humidity, and mineral content of your perspiration.

You are better off wearing at least a light set of long johns inside a bag. This will trap any moisture(water) allowing it to evaporate off (into a gas.)

Humidity does not really effect down. It will move out without a vapour barrier.

I leav my bag open in the morning, packing it last, to let it dry as much as possible.

Yes, James, I'm probably guilty of saying "moisture" when I mean water vapor. Of course, I don't believe water as solid is being transported through the down.

Appreciate the 101 on down and moisture. What I'd like to know is what role the temperature inside the down plays in the transport of water vapor. The article I put forth contends that down is more comfortable due to its ability to transport moisture more quickly, and therefore, allows us to be more regulated at skin level. Everything I've put forth revolves around "heated" trapped air in the down. I've contended that more clothing acts to absorb a great deal of heat so that the down doesn't get as hot as it ordinarily would, and that perhaps that has a lot to do with how well the down is transporting water vapor AND to what degree (sort of a pun) condensation might occur more readily.

Experience tells me if the down is hot, I've got a lot less to worry about with regard to moisture. But this is in what I would call milder temps – like high 20s and up. That's most of my experience. I understand this relationship might break down as temps go colder and others have already responded to that.

> what role the temperature inside the down plays in the transport of water vapor.
Very little, approaching zero, as long as the temperature is above the dew point. So most of your arguments (or the article's arguments) have no foundation.

> The article I put forth contends that down is more comfortable due to its ability to
> transport moisture more quickly,
I suspect the author of that article has limited knowledge of the physics of heat and mass transport, and may be confusing the nice soft feeling of a good down sleeping bag with something else.

One wears extra clothing inside a SB for extra insulation. When it is that cold outside, moisture transport is usually lower in priority than just being warm enough. However, below 20 F you can start considering using a VBL to good effect.

Cheers

Let's say for a second that moisture IS being transported from the body through the down and being evaporated outside. What would that mean? Wouldn't temperature affect where that dewpoint is going to occur? Is there some energy involved at higher temps where water vapor could be transported more easily and, therefore, wouldn't be accumulating in a way that might otherwise be expected? Let's just assume that water vapor IS being transported like the article suggests. What would we expect as the results?

And for the sake of consistency in what we're talking about, let's not talk about conditions where a VBL is appropriate. Let's say 25F-40F. I know one could use one in those conditions, but its not a given.

> Wouldn't temperature affect where that dewpoint is going to occur?
Of course.

> Is there some energy involved at higher temps where water vapor could be transported more easily
Not that I am aware of. And I have been involved in heat and mass transfer for a few decades.

Cheers