Freezing Temps and Open Cell Foam

[east_stingray]

These bolded statements seem contradictory. So which is it? Does the water vapor passing through the bag pass into the cold air and freeze ABOVE the bag's surface? Or does it condense ON the bag's surface and freeze?

If the latter, I would suspect you'd eventually seal your bag's surface with ice, thereby no longer allowing water vapor to pass through, and the foam (which would contain water vapor unable to escape) which would eventually freeze as well.

BER: Water vapor is going to condense once it hits a certain temperature (the dew point). When it's colder outside that temperature will be reached closer and closer to (and eventually inside) your bag. It's not a function of the material, it's a function of dew point (which is dependent on humidity and temperature).

[Rat]

These bolded statements seem contradictory. So which is it? Does the water vapor passing through the bag pass into the cold air and freeze ABOVE the bag's surface? Or does it condense ON the bag's surface and freeze?

If the latter, I would suspect you'd eventually seal your bag's surface with ice, thereby no longer allowing water vapor to pass through, and the foam (which would contain water vapor unable to escape) which would eventually freeze as well.

Both can be correct for any number of situations.

A layer of ice will not impede the migration of water vapor through a substrate. Instead the water vapor will migrate until it contacts the cold surface and condenses, once condensed it will quickly freeze. The layer of ice would need to be extremely thick to allow water to condense, but not freeze; or enough heat from within the bag to keep a layer of water next to the bags outer layer. A layer of water would impede the migration of water vapor through the outer layer/foam. I'm not saying it could never happen, but it would take a perfect storm.

I have seen the same effect in walk in freezers built inside another building where the installer did not insulate the floor but left it bare concrete slab. Eventually ice will build up to the level of the freezer floor, and begin to push it upward. Condensation would form between the layer of ice next to the floor and freeze; little by little it would jack the freezer up, I have even seen it pull out concrete anchors used to secure the wall rail.

[steene]

I am grateful for your insight and patience in answering questions. This forum has been extremely helpful to many and your input is appreciated.

[Pitch]

First, water vapor (a gas) doesn't move from higher temperature to lower temperature, it moves from a higher pressure (higher RH%) to lower pressure; this is Dalton's Law of Partial Pressures. You could in fact have a cooler area with higher RH% and a warmer area with lower RH% and the water vapor will move to the lower RH% area. Having said that, most of the time, in the Winter, the RH% level outside or sleeping system is lower than inside our sleeping system. That being a given, water vapor that is inside inside our sleep system migrates out through it, to the outside air.

My guess is that the plastic sleeve on the ground pad is selectively permeable; it will allow water vapor (gas) through, but not liquid water. The water vapor migrates to the outer layer against the ground, where it condenses and then, unable to migrate back into the insulation, freezes.

Water vapor on the top side will migrate through the bag, then condense on the outside and freeze; this is covered in the video. Ice formation on the surface will not result in water infiltration of the insulation due to the fact that water vapor migrates out, contacts the ice, condenses and freezes; never having a chance to migrate back into the insulation as water.

I see the benefits of this type system for prolonged use in sub freezing climates, but for my style it is too bulky and heavy. Adapting it to hammock use will be a fun challenge I am sure.

Good stuff!

The "critical" point is the region in (or outside of) the sleep system where the temperature is at or below the dew point. At this point the vapor re-condenses into liquid water. Hopefully this is outside of anything that will trap it inside your system (or in the case of a vapor barrier, allows it to condense and possibly freeze on the surface of said barrier so it can be removed later).

This is all theory to me... I've never played in this sort of cold successfully. I'm really glad you all are here to share this stuff so I don't have to get cold and wet.

EDIT:

BER: Water vapor is going to condense once it hits a certain temperature (the dew point). When it's colder outside that temperature will be reached closer and closer to (and eventually inside) your bag. It's not a function of the material, it's a function of dew point (which is dependent on humidity and temperature).

Just caught up and realized you got this point out already!

[BER]

A layer of ice will not impede the migration of water vapor through a substrate. Instead the water vapor will migrate until it contacts the cold surface and condenses, once condensed it will quickly freeze.

A layer of ice on the outside of the bag would not impede migration through the foam. But the water vapor would not migrate through the ice, correct? It would condense on the inner side of the ice and then freeze. So over time you'd build thicker ice inwards.

BER: Water vapor is going to condense once it hits a certain temperature (the dew point). When it's colder outside that temperature will be reached closer and closer to (and eventually inside) your bag. It's not a function of the material, it's a function of dew point (which is dependent on humidity and temperature).

Yes, I follow. But then, under a constant low temperature (and constant atm. pressure), wouldn't the water vapor condense and then freeze on the surface of the bag or in the foam itself leading to the same issues as the poor gents in the arctic with 60# sleeping bags?

Understand that I am not arguing with either of you, I am just trying to figure out how the foam would be better than down or synth fiber. The claim by CITC is that it is.

[Cannibal]

A layer of ice on the outside of the bag would not impede migration through the foam. But the water vapor would not migrate through the ice, correct? It would condense on the inner side of the ice and then freeze. So over time you'd build thicker ice inwards.



Yes, I follow. But then, under a constant low temperature (and constant atm. pressure), wouldn't the water vapor condense and then freeze on the surface of the bag or in the foam itself leading to the same issues as the poor gents in the arctic with 60# sleeping bags?

Understand that I am not arguing with either of you, I am just trying to figure out how the foam would be better than down or synth fiber. The claim by CITC is that it is.

The difference being, when the water vapor does condense it is easily removed by just 'shaking' the foam or cover sheet. With a traditional quilt, you don't have access to the insulation that is full of ice, leading to a constant build-up to the point that it becomes a danger.

I think.

[east_stingray]

The difference being, when the water vapor does condense it is easily removed by just 'shaking' the foam or cover sheet. With a traditional quilt, you don't have access to the insulation that is full of ice, leading to a constant build-up to the point that it becomes a danger.

I think.

I think that's the point the system is based on, but I can see some problems with it. Where the water condenses is going to change depending on the humidity and temperature. If it's relatively warmer and drier, it'll condense somewhere out in the air. There would be a certain temperature cold enough that it would cause the condensation to happen inside the foam. I can't calculate that exact value or anything, because it depends on how much you sweat, humidity, temperature outside, and the properties of this particular foam. It could be that this would happen at some unrealistically low temperature only, making the whole point a non-issue.

[Pitch]

east_stingray I think you nailed it ...

I think that's the point the system is based on, but I can see some problems with it. Where the water condenses is going to change depending on the humidity and temperature.

In a nutshell:

The secret to any system would be to FORCE the water to condense at a particular point (or beyond that point - or not at all if the temp is high enough)...

That is, ensure that the entire system stays ABOVE the dew point until you are in a region of the system that you WANT the water to condense...

I dunno how this can be done (if it is) but would love to see some models or hear some ideas on how this can be achieved (or to hear some clarification on how this is done as I don't understand enough about it to deduce any mechanism from the mass of information above)

(Does this make any sense?)

[Pitch]

I have no idea if this would work... but it is one idea on how to force condensation in a particular region of the system...

That is, to have a large enough region of "poor" insulation up against a region of "good" insulation so that the likelihood of dew-point occurring inside the "poor" insulation region is exaggerated. The addition of a bottom-most-layer air-tight layer prevents drafts from penetrating forcing internal convection to be the only mechanism of heat transfer to the outermost layer.

I guess the whole entire thing could be moved inside the layers but then the ultra-low density region (poor insulation) would need to be mechanically strengthened (maybe composite striping of stiffer foam) to prevent compression and total loss of insulation (and condensation region)



Or... I'm coo-coo for cocoa crisps!

[CITC]

I am grateful for your insight and patience in answering questions. This forum has been extremely helpful to many and your input is appreciated.

You are most welcome, Steene. I hope the information helps you guys out. Hogn8tr is doing a better job of explaining the physics than I, and I believe that he understands how the system works in more technical terms.