Freezing Temps and Open Cell Foam

[TeeDee]

Given my disdain for pads and the associated ignorance I could easily be wrong, but aren't the groundlings using CCF pads?

True, but then what difference does it make?

OCF or CCF - I'm not aware that one is inherently firmer than the other.

The primary use of the pad underneath is insulation. So if the OCF pad insulates as well, what's the difference?

[CITC]

The question I would have for CITC is if the performance of the OCF he uses is adversely affected if it is compressed for long periods of time, says for months, 8 or 9 months? Does it become permanently compressed? If so how much as a percentage?

Yes. Foam compressed over time will lose its loft. There are foams that do VERY well under prolonged compression, but the density and weight of these foams make their use in camping gear prohibitive. So, I have to strike a balance between compression durability and weight, and it's not an easy balance to strike.

The foam that I use currently will regain it's loft after being compressed for a day-long hike, but if left for over 4 days in that state you are in danger of causing permanent damage. The percentage is variable. How long did you leave it compressed? How compressed was it? There is no simple answer to this question.

I recommend that people loosely roll them up, lay them flat, or hang them for prolonged storage.

[Cannibal]

True, but then what difference does it make?

OCF or CCF - I'm not aware that one is inherently firmer than the other.

The primary use of the pad underneath is insulation. So if the OCF pad insulates as well, what's the difference?

My guess is that the CCF would resist compression much better than the OCF. If I understand correctly, that would mean it would take much less (lighter, less bulk) CCF to insulate to the same degree as OCF. Course, the down side is the moisture that builds-up between the sleeper and the pad when using CCF.

[Rat]

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.

[Mustardman]

Nope - long before you joined I'm pretty sure.

I don't even remember you making any comments about the SS .

Sweet, had me worried there I have had some heated discussions about foams, but I don't believe they were specifically related to the supershelter, which I think has the right combination of stuff to make a perfectly adequate insulation system.

OCF or CCF - I'm not aware that one is inherently firmer than the other.

The distinction between the "firmness" of the two is right there in the description - open cell foam means the "bubbles" inside of the foam matrix are open and interconnected - this is why the foam can "breathe" and why the foam is more compressible (in general!) - put your weight on the pad, and it squeezes air out of the pad. So the firmness of the pad is determined primarily by the stiffness and denseness of the polymer matrix that makes up the foam, with some small contribution from the permeability of the foam determining how easily air can escape from it.

In closed cell foam, on the other hand, the cells are sealed up, so the air that's trapped inside of them can't escape. That's both why they are excellent insulators (no worry of convection or air flow moving heat away from you) and why they aren't as compressible - the individual air pockets act like little cushions, and the only compressibility comes from actually compressing the gas and making it more dense, rather than forcing the air out of the pad.



The increased compressibility is probably the MAIN reason Tom H uses open cell foam in the supershelter. The same thing made from CCF would be harder to pack because it would be a nightmare to try to roll it up with that weird shape. In the supershelter, since your weight isn't on the foam, the extra compressibility isn't an issue. In between the pad layers in the blackbird, or underneath you in a sleeping bag, it would become more of an issue, although obviously workarounds like using a heavier foam or using more of the foam can compensate for the compression.

[BER]

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.

Thank you Hogn8r for restating that in a more scientific manner. Note that although I did not specifically reference Dalton's law, I did mention partial pressures. I guess I assumed the relative humidity, i.e. the partial pressure of water vapor, in the winter air would be lower.

Thanks John for your reply. So it seems that the issue of the additional pad/de-icing bag on the bottom would be a factor of laying on a cold, impermeable surface like the ground, and would not be needed in a hammock who's bottom is not usually waterproof.

[Rat]

Thank you Hogn8r for restating that in a more scientific manner. Note that although I did not specifically reference Dalton's law, I did mention partial pressures. I guess I assumed the relative humidity, i.e. the partial pressure of water vapor, in the winter air would be lower.

Not trying to rebuttal, just trying to make it so we all understand at the same level what is happening. I just happen to have a good understanding of how it happens and am trying to help others...

[Mustardman]

The de-icing cloth collects the moisture from the underside, and every 2 days or so, you strip it off, crumple it up, shake out the ice crystals and then put it back on, ready for another few days of moisture collection.

This to me is the most clever part of this whole system. The idea of intentionally trapping the moisture in a place where it's easy to remove is very different from most approaches to the problem I've heard about. It's certainly more appealing to me than the "wetsuit" approach of full body vapor barriers where you lay in warm puddles of your own sweat that, while they won't cause hypothermia because they are trapped against your skin by the vapor barrier, sure do make a fat guy like me smell bad

[BER]

The moisture that comes from the underside of your sleeping bag needs someplace to go that is NOT your sleeping bag. If you did not have somewhere for it to go, it would turn to ice against the frozen surface, in this case (without the pad and cloth) it would be the cloth of your sleeping bag, and then it would continue to freeze inside the foam of your bag until you have saturated the system and are sleeping in what is essentially a block of ice.

Above you from the bag, the moisture freezes in the air and settles back as frost, or is blown away by the breezes.

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.

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.

[kohburn]

looks like a good option for long term durability and a non-backpacking situation.