reflectix and condensation?

[Shane]

No, it's just that trying to explain a very complicated subject in terms a layperson can understand, and in a length you'll actually read, leaves gaps in the explanation. I had to take 3 three different college courses to understand the basics of this stuff, not to mention all the background math and science courses to be able to understand that.



The point we are making is this: Your down insulation doesn't keep you warm because it warms up itself. It keeps you warm because it prevents thermal energy from escaping to the outside air. The inch of down closest to the outside air will be colder than the inch of down right next to your body, but they both prevent the same amount of thermal energy from escaping.

Now let's look at the relfectx pad situation. Putting a barrier between the down and the body is the same as adding more down. You're just adding a different material which may reduce transfer of thermal energy at a different rate. The down on the other side of the barrier will be at a lower temperature, but still it will prevent the same amount of thermal energy from escaping as before.

Never said down heats itself. That's ridiculous. Perhaps I should shorten my responses.

[Bill_Mead]

Thanks Boston and rbinhood for the explanations on insulation, appreciate you taking the time to post.

[gnarus8429]

Refletix is neat stuff. It has caused condensation issues for me when I have used large portions of it in my hammock. The temp didn't seem to matter. I did find it to be warm though. I have used smaller pieces for foot pads with a shorter UQ. That seems to work better as vapor can escape there easier. Shug noted in one of his videos that he uses a small piece to address cold spots and as a sit pad. I think that it is best to use smaller pieces to allow air to circulate more freely. If you seal in your whole back side with the stuff things will get wet. No fun when it is cold out.

[rbinhood]

Not a problem. When I was an engineering student in college, I tried to apply a lot of what I was learning to my outside camping pursuits, in particular, staying warm while sleeping, and while sitting on a deer stand.

There is a great deal of information on the internet that can be found with google, applying heat transfer concepts to the design of outdoor clothing. Also, the U.S. Army has done a great deal of testing on this subject, and some of their research can be accessed online. Many of the clothing manufacturers have used the Army research to help them with cutting edge designs. The latest designs by Columbia employ a reflective, silverized layer in their clothing.

Years ago, while attending UW Madison, I used to go into Land's End stores in town and every now and then I would find some experimental clothing on a closeout rack. This was before Land's End became yuppy and trendy. I still have a silver mylar suit, top and bottom, that I bought there, and that I have worn in really cold weather to stay warm while hunting. The mylar is punched with thousands of small holes to let water vapor escape, and the inside is covered with a very fine nylon netting to increase air movement through the fabric. The problems with the Reflectix in hammocks that we are talking about were considered by whoever designed that mylar suit.

In response to Shane's comments about planes and bridges, both fail because of human error. Either they are not designed right to begin with, or not maintained properly. The laws of physics are immutable. Life, on the other hand, is full of human error, and it is human error that keeps lawyers fully employed.

[Boston]

Never said down heats itself. That's ridiculous. Perhaps I should shorten my responses.

That's not what I said..or I guess a misinterpretation of what I said. I wasn't trying to imply that you think the down itself creates heat. I should have said because the down FEELS warm isn't the reason it insulates and keeps ypu warm.

[Meerkat]

Excellent thread. Although not an engineer, I am surrounded by them. My son is a mechanical but wanted to go computer. My father in law and a nephew are EE's. I'm an airline mechanic so deal with these things regularly. The OP about Reflectix sums IMO, keeps you warm , can slow or stop moisture transfer, and if between user and insulation act as a VB to retain loft. If outside of insulation the VB will eventually saturate insulation. If curious, could try it out in the backyard or some place close to home if you have to bail out.

[FlySubCompact]

I totally agree with the engineerios on this subject. I also use a 3/4 length pad of reflectix, with no ill effect, with an UQ.

One of Shane's views about "feeling" warmer with just his UQ using no vapor barrier does make me wonder about what might be happening to vapor, even in really cold weather. Body moisture is probably being wicked away into the down. Maybe being evaporated, later during the warm of the day. Also, I seem to recall that moisture can still evaporate in extreme cold air. (I read once that Antarctica is considered the worlds largest desert.) Maybe the moisture that is released from a "down only" sleeper is slowly passing through the down and is still being absorbed by the cold dry air? I'm thinking that the term "freeze drying" may apply here?

[dakotaross]

...Now lets go back to the down underquilt without Reflectix between the body inside the hammock and the quilt. The water vapor from the sleeper gets evaporated just like before. If the inner surface of the quilt is permeable, the vapor goes into the down. Somewhere inside the down, the temperature inside will likely drop to the dew point. The temperature gradient can be understood as follows: Surface temp of the skin of the sleeper is 98.6 deg. Fahrenheit. Outside temperature is below 32 deg. Fahrenheit. Somewhere between the two temps will be the dewpoint. When the water vapor hits the dewpoint, it condenses and turns to liquid, saturating the down. It is possible that the down may be warm enough inside the two surfaces of the quilt that the dewpoint is never hit, in which case the vapor will go through the outer layer of the quilt, and either condense on the outside, or evaporate off. This is a very unlikely scenario. More likely is the down will get saturated with water, and eventually lose its loft and insulating qualities. Wet down is a very poor insulator.

A sleep system with Reflectix inside a down underquilt will definitely be warmer than just a quilt. Over a period of many days, the Reflectix will also help keep the down from getting saturated with water, meaning the sleeper will stay warmer in the long term. A down quilt alone, without Reflectix, will not be as warm as the combination of the two. However, some people may not like the clammy feeling of sleeping on Reflectix. To some extent, in the end, it becomes a matter of personal taste...

This is what happens when you argue with engineers. The theory doesn't always match the observations...

If what you are saying is true, then everyone not using reflectix with an underquilt is sleeping in soggy down after a few days. That is simply not happening to anyone that I know of and I've been doing this a while - been on plenty of long trips in all kids of weather. It doesn't take much water to make down clump up or be soggy, and would be evident on long trips. Ever sat on a toilet seat? It feels warmer after you sit on it for a minute because you warmed it up. The body continues to add heat as an engine in a process. It's not a static equation.

We are talking about human perceptions of dryness and warmth, not strictly the laws of thermodynamics.

...Your down insulation doesn't keep you warm because it warms up itself...

...I should have said because the down FEELS warm isn't the reason it insulates and keeps you warm.

Gosh, I want to write so much more, but I have to work. Just want to say I'm with Shane on this one. Anecdotal evidence of condensation in down is mostly not there, except for a incident here and there. And I do think that vapor moving through the down to the outside is THE most likely reason why its not otherwise occurring more. A reflective barrier on the outside tends to warm the air immediately outside the quilt to dewpoint levels.

I do think that down performs a function that is not about technical heat transfer, but about the trapping of warm air around you that causes your body to relax in terms of circulation. When you're cold, your body limits circulation to the extremities to protect the core. When you feel warm - and down on skin is the best - this serves to bring your body to equilibrium.

Or course, I'm not an engineer, but I did go to Georgia Tech which causes me to think I'm smarter than I actually am.

[dakotaross]

I have read that you get condensation sleeping on reflectix. Some say yes, and some say no. What is it? I use an UQ and I plan to have the reflectix on top of the hammock and I will sleep on top of the reflectix. The temps will be in the 20s. What can I expect? I will also use a hammock sock out of 1.1, if that makes any difference. Another question, if the reflectix reflects body heat back, will the warmth of the UQ be blocked by the reflectix. Thanks.

So, getting back to your question, I think reflectix is thin enough to not be so much of a barrier to your heat transferring into the quilt. Not like wearing a bunch of clothes can be. So it likely does not negate the UQ.

However, radiative heat reflection requires some space for that to occur. It does not work directly on your skin. Extra clothes can provide that space, but that then prevents heat from getting into and trapped in your quilt insulation, which does mean your quilt will perform less efficiently. If you have a double layer hammock, put it between the layers, and then wear minimal clothing. This will provide some space, though not optimal.

If I were going to use reflectix, I think a thin foam pad with reflectix on the back of it makes more sense. My advice is to do everything possible to make your quilt more efficient, thereby utilizing all that its rated to. You do that, you can take a 30 degree quilt into the 20s. That would be ensuring its snug up against you and wearing minimal clothing. I'd also advise having separate substantial insulation for your feet, hands and head.

[nothermark]

Gosh, I want to write so much more, but I have to work. Just want to say I'm with Shane on this one. Anecdotal evidence of condensation in down is mostly not there, except for a incident here and there. And I do think that vapor moving through the down to the outside is THE most likely reason why its not otherwise occurring more. A reflective barrier on the outside tends to warm the air immediately outside the quilt to dewpoint levels.

I do think that down performs a function that is not about technical heat transfer, but about the trapping of warm air around you that causes your body to relax in terms of circulation. When you're cold, your body limits circulation to the extremities to protect the core. When you feel warm - and down on skin is the best - this serves to bring your body to equilibrium.

Or course, I'm not an engineer, but I did go to Georgia Tech which causes me to think I'm smarter than I actually am.

Local mountaineering society limits their winter trips to 3 nights max because the moisture load degrades sleeping bags that fast. The anecdotal evidence relies on people not weighing their gear every morning or getting water dripping out of it. That does not mean that there is no water collected. I one does a serious study the data always comes back to the moisture being there and getting trapped.