reflectix and condensation?

[billvann]

The original post asked about placing the Reflectx between the UQ and hammock. There was a brief mention about placing it outside of the UQ with a UQP. But that post preceded the engineers' replies. My understanding from the reading is that condensation will occur and water will pool up at the low points.

Taking the mylar suit example of small holes hilt in to allow water vapor a path to escape, and other's observations that partial pads present minimal problems with condensation, what if you constructed a Relectx pad with pores or holes at potential low points buy the butt and shoulders to allow condensation to drip out? A full pad with holes would be easier to keep in place than a partial pad.

A lighter version would be to add a space blanket to an UPQ. It would lack the extra insulation from the bubble wrap but would be easier to manage. Or with the silver lined material Columbia is using (although I suspect that may not be available).

BTW, I have a synthetic Potomac UQ. It would still suffer from getting wet but would retain much of it's insulate properties, mores than down under similar conditions, correct?

[Alamosa]

To me, the whole topic of vapor barrier is specific to cold winter camping when temperatures are well below freezing and going to stay there for significant portions of the trip. Those arethe only conditions which would allow vapor to freeze into the insulation and be a problem.

Under normal 3-season hiking conditions, vapor barriers should not even be a consideration, IMO.

[Boston]

Just seems to me that there is more going on in the relationship of how down insulation keeps you warm than just the retarding of heat transfer,

Can you explain what you mean by this?

There are only 2 ways to maintain a temperature in a system:
1) Prevent heat from leaving (insulating)
2) Add heat at the same rate it is leaving (generating or transferring from storage)

An example of heat generation is a fire. It uses chemical combustion to create heat.

An example of storing heat is a hot water bottle. You heat the water from a heat generator, and it stays hot for a while (high specific heat). As the heat from the bottle transfers out of the water (due to the high temperature differential with it's surroundings) it warms the air surrounding it.

Down doesn't create heat, and it doesn't start off hot, so the only thing it can do is prevent heat from leaving.

No insulation is perfect, so heat must be added in some way. In a hammock this is your body heat. Your body burns calories at some rate, and part of that goes to maintaining body temperature.

In relation to the moisture issue, and down's effectiveness, the down feather itself isn't what insulates you. It's the air trapped by the down. Basically you have millions of tiny, tiny pockets of air that can't form convection currents. Air has very low thermal conductivity, so transfer of heat through that air is very slow, from a conduction stand point. This is what actually insulates you. When the down get's wet is clumps, and the air pockets go away.


Also, please realize I'm not trying to say your observations or experience is wrong. What you're saying you experience is true, obviously, but the reason that's happening is different than what you seem to believe.

One thing that may be a point of confusion in this discussion is that HEAT and TEMPERATURE are not the same thing. Heat is thermal energy, and is measured in Joules, or BTU's, or Calories. Temperature is a measure of hot and cold. You raise temperature by adding heat (energy) to a system. You decrease temperature by removing it.

[dakotaross]

Can you explain what you mean by this?

There are only 2 ways to maintain a temperature in a system:
1) Prevent heat from leaving (insulating)
2) Add heat (generating or transferring from storage)

Would it not be correct to say that heat is maintained by the down, or is at least done so with such effectiveness that the heat produced by the body is somewhat at equilibrium with heat transfer to the outside? I know you're on record of saying it would be an inhuman furnace to create temps that stay above dewpoint in the quilt, but is it unreasonable to say that at some point of maximum efficiency, this kind of equilibrium can exist? That ordinary body heat is at some point enough to "transfer in" to replace what's going out?

I don't know, but I can "feel" the answer there somewhere. I'm sure math would explain it if we could agree on some assumptions.

[dakotaross]

...In relation to the moisture issue, and down's effectiveness, the down feather itself isn't what insulates you. It's the air trapped by the down. Basically you have millions of tiny, tiny pockets of air that can't form convection currents. Air has very low thermal conductivity, so transfer of heat through that air is very slow, from a conduction stand point. This is what actually insulates you. When the down get's wet is clumps, and the air pockets go away...

No quibble there. My thought is that the specifically "hot" air that is trapped by the down is what reduces convection heat loss. Would it be correct to say that the "velocity" of convection currents is reduced as a result of the trapped hot air? It seems to me that this describes what I'm trying to get at about efficiency. Heat is drawn from your body at a lower rate making it easier for your body to be comfortable and to remain in sleep state.

[Boston]

Would it not be correct to say that heat is maintained by the down, or is at least done so with such effectiveness that the heat produced by the body is somewhat at equilibrium with heat transfer to the outside?

Yes, that's exactly what I am saying. At steady state (nothing changing) The HEAT (energy) transferred through the quilt to the outside air is leaving at the same rate the body is generating it at.

The temperature of the quilt, however, will vary. Next to your skin it will be the same as skin temperature. Next to the outside air it will be the same temp as outside. This is pretty easy to test if you want to try it out (use an IR thermometer to measure the outside of the quilt, after someone has warmed up fully).

I know you're on record of saying it would be an inhuman furnace to create temps that stay above dewpoint in the quilt, but is it unreasonable to say that at some point of maximum efficiency, this kind of equilibrium can exist? That ordinary body heat is at some point enough to "transfer in" to replace what's going out?

I think I understand what you're asking, and I'm pretty sure the answer is no. Either your insulation is effective, and very little thermal energy is getting to the outside of the quilt, and the air pulls that away, thus it's temperature isn't raising, but you're warm. Or a lot of heat energy is getting to the outside of the quilt, because it doesn't insulate effectively, the air pulls it away, and you're cold.

The only caveat would be if air temp was very similar to body/skin temp. In which case there is little heat transfer because the temperature differential is low anyway. Possibly the the quilt would heat soak, but you'd be way to hot to want it anyway

[steveflinn]

yeah. i use an adequate UQ and if it's cool-windy i use a doored tarp. add a sock in dry snow. canvas sock in freezes.

and sleeping naked? i like the capilene longies better. year-round. then the hammock and UQ hardly need washing.

plus, navy training...need to be ready to go from the last exhale of sleep to the first inhale of action during an emergent situation. usually sleeping with the VFF toe-shoes on, too, so i am functionally dressed and ready to fly. at 10.1oz they're the best camp/sleeping/fording/backup shoes ever.

[cwford]

good article on vapor barriers by someone with lots of experience

[LakeSuperior777]

I was impressed with how much water was trapped between my mylar coating foam pad and the top layer of my hammock. Temp outside was 15F and I never really felt the dampness through the clothes I was wearing. Of course as soon as I got out it all turned to ice made it easy to remove .

Engineers are doing a good job on explaining insulation

[Boston]

No quibble there. My thought is that the specifically "hot" air that is trapped by the down is what reduces convection heat loss.

Nope. Actually if the the temperature difference between the quilt and the outside air increases, heat transfer's at a higher rate out of the quilt.

Would it be correct to say that the "velocity" of convection currents is reduced as a result of the trapped hot air?

As for convection velocity - that wont be impacted by the air in the quilt - as the down traps it and prevents it from moving - see my previous post. The air movement causing convection is the outside air. If the air is still it will still develop natural convection currents due to the buoyancy of the air changing as it heats up (heat rises, as they say). As velocity increases, so does the amount of heat convection can pull away. That's part of the reason you feel colder when the wind blows through your quilt.

It seems to me that this describes what I'm trying to get at about efficiency. Heat is drawn from your body at a lower rate making it easier for your body to be comfortable and to remain in sleep state.

What your describing about heating the quilt material's is partially correct. The quilt material, the air inside your sleeping space, it will all warm up to a stable temperature. While it does that, it's drawing heat from your body to do it! However, the heat transferred from your body to the outside air, through the insulation, remains constant while this is happening, and afterward (assuming your core temp doesn't change). It's just that you have to warm up the insulating material first, so your body loses heat at a greater rate, until the temp stabilizes and you reach steady state. The point I've been trying to make is that your heat loss to the outside air isn't impacted by this temperature change in the quilt. Going back to the original question: this is why reflectx doesn't "negate" an under quilt.

Btw, I just want to say that this has been a great dialog, and I'm really enjoying having this discussion. I'm starting a small section of the AT tomorrow, so I wont be able to respond again until late Sunday.