reflectix and condensation?

[rbinhood]

I am also an engineer and agree with everything Boston has said, particularly that this is a very complicated subject. Heat is transferred by convection, conduction, evaporation, and radiation.

Put a fan on yourself in the summer and the cooling effect you feel is the result of moving air carrying heat away from your body (convection).

The cooling will be greater if you are sweating because now the moving air combined with your body heat causes the sweat to evaporate, cooling you even faster (evaporation). It takes heat energy to evaporate the liquid sweat off of your body and turn it into a vapor, or gaseous form.

Put your hand 6" from a hot stove, and you will feel heat due to radiation. The red hot stove radiates heat through the air to your hand. Silver surfaces, like Reflectix, block radiant heat loss by reflecting it back towards the object radiating the heat. Put a piece of aluminum foil between your hand and the stove, and your hand will stay much cooler.

Touch the hot stove with your hand, and you will feel heat from the fire inside the stove conducted through the wall of the stove to your hand (conduction).

Now let's look at the original post and analyze what is going on, beginning with the Reflectix. First, Reflectix, as the name implies, and as it silver surface demonstrates, is designed to reflect radiant heat back towards the body. Second, Reflectix has a moisture impermeable surface, meaning water vapor can't go through. More on that in a second. Finally, Reflectix is just aluminized bubble wrap. All of the dead air spaces in the bubbles stop conductive heat loss. The dead air space is not a good medium for conducting heat. Water, on the other hand is a very good medium for conducting heat, which is why the radiator in your car is full of antifreeze and water.

When you sleep at night, your body gives off water vapor from your skin. Between breathing and transpiration through the skin, the average person loses up to 2 quarts of water a night. The heat of your body evaporates the water, turning it into a gas, water vapor. The water vapor gets trapped by the impermeable surface of the Reflectix. In theory, Reflectix should act like a vapor barrier. If you study the concept of vapor barriers in clothing and sleep systems, the idea is to create a micro climate next to the skin, which eventually saturates with water vapor. When the micro climate is saturated, it is harder for the body to evaporate more water off of your skin, and theoretically, the amount of vapor being evaporated off of your skin drops dramatically, and along with it the heat loss needed to evaporate the liquid also drops. The problem with vapor barriers is many people feel clammy inside them and don't like using them for that reason.

Another thing is happening with the Reflectix and the water vapor. If the inner surface of the Reflectix is below the dew point (temperature at which water vapor will condense into liquid water), then the water vapor will condense inside the Reflectix, and may pool, or create a wet inner surface.

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.

A hammock sock keeps you warmer by stopping convective heat loss due to blowing wind, and also creates a micro climate around the hammock, warmed by the sleepers breath and body heat. Think of it like a Goretex parka over a down jacket. The Goretex stops the wind, and the down inside insulates your body.

There are even more subtle complexities to the analysis, but this explanation touches on the main points.

[f k]

Boston, I think that you have it right. I think also what we may be missing talking about is that the reflictix does not stop all heat transfer otherwise it would be the cure. It is a radiant heat barrier, it still conducts heat. The down is a conductive barrier. It blocks heat transfer from touching something cold.

Or, just read the above. :-)

[Boston]

Thanks rbinhood, great explanation! That was much more than I was willing to type on my smart phone :-)

[Bubba]

Well colour me edumicated.

BTW, the OP seems to have a synthetic UQ (Jarbidge)

[Shane]

This is what happens when you argue with engineers. The theory doesn't always match the observations. If not, then planes would never crash, bridges would never fall, and lawyers would be out of business.

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.

[nothermark]

Good ones!! Thanks!!

I love some of the "explanations" of why insulation works. It does not make heat or cold. It just retards transfer. FWIW I think the sleep naked idea comes from Jack Stephenson who had a reason that escapes me now. They pioneered a lot of what the ultralight folks take for granted.

[nothermark]

This is what happens when you argue with engineers. The theory doesn't always match the observations. If not, then planes would never crash, bridges would never fall, and lawyers would be out of business.

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.

Actually it happens to everyone you know of. The question is whether or not it dries enough during the day to avoid problems. It has been known for a very long time that down loses loft over prolonged use. Today's method of hitting town every few days gives it a chance to dry out and avoid issues. The "old school" "heavy" camping techniques were developed around folks living for weeks at a time in the same gear with no town stops. That included rest days to clean clothes and dry gear every third or fourth day to avoid moisture buildup.

[canoebie]

Another variable with uq and tq is that warmth rises due to convection. Moisture travels accordingly. I think more condensate builds in a top quilt than an uq. I like reflectix as a supplement for my feet under me. I have had little problems with condensation. OTOH when I have IX on top of me, which also serves as a VP, I get all kinds of wet. Just another variable.

[Boston]

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

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.

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.

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.

[Scotty Von Porkchop]

This has turned into a great thread, keep it coming guys. FWIW I use reflectix as a backup, it adds heat, sweaty uncomfortable heat.