Which way do I orient a reflective pad?

I imagine it's up, so your body heat is reflected, but the Cascade Designs site happens to be down at the moment and I can't see what they might have to say about it.

To reflect IR. Shiny up when it is cold, shiny down in the special case where the ground is hot… or up to keep the sun's heat off your noggin.

Your answer here

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=40865

"I hypothesize it's down so your body heat hits the foam "twice" and gets stored."

LOL I have a perpetual motion machine I'd like to sell you.

Imagine a regular foam pad on top of a space blanket. Now the reflected heat can gather in the foam.

You guys are thinking of the coating as an outward-facing mirror, but what's stopping it from working in reverse?

I think silver side up would be warmer, but just barely

In order to reflect heat you need an air space next to the reflective surface, otherwise conduction will make the reflective surface and whatever's next to it the same temperature – reflective surface does no good.

If the reflective side was down, it would be directly against ground so no effect

If the reflective side was up, there might be places where the sleeping bag wasn't against surface so it would actually do something

I sleep directly on my sleeping pad, so there are many places where I'm not touching it so it would have a little effect.

I think mostly it's just a gimmick

Now with a Neo-air, inside, there are reflective surfaces with air spaces next to them so it should actually do something.

If the way the pad worked was to "hold" heat, then is would also *conduct* heat straight into the ground very efficiently. The pad works by minimizing the conduction of heat. Unfortunately can't have it both ways. Laws of thermodynamics and all that.

"You guys are thinking of the coating as an outward-facing mirror, but what's stopping it from working in reverse?"

I does! But it is not an equilibrium situation.

It's not that it reflects heat, it prevents you from radiating heat.

"In order to reflect heat you need an air space next to the reflective surface, otherwise conduction will make the reflective surface and whatever's next to it the same temperature – reflective surface does no good."

Nope. IR hit human skin, human skin gets warmer, surface of pad (with low heat capacity) also gets warmer.

Though I would agree its not a gigantic effect, it will increase the effective R value since the IR was a near total loss before.

"It's not that it reflects heat, it prevents you from radiating heat."

If what you said is taken literally, then nope, you are gonna radiate heat anyway.

Sheezus. Physics.

The way I hear people (and many of my past students) describe how they think things work sometimes sounds indistinguishable from magic to me. LOL

If your skin is touching the reflective surface, then it will conduct heat. If you touch a piece of aluminum foil, it will be the same temperature as your skin.

If you cover yourself with aluminum foil, you will radiate no heat. The emissivity is close to zero. Using Stefan-Boltzman equation, no IR radiation.

You'll still lose heat from conduction.

Point it out sideways so the heat bounces off the tent walls and back to the top of the sleeping bag…

"If your skin is touching the reflective surface, then it will conduct heat. If you touch a piece of aluminum foil, it will be the same temperature as your skin."

Yep, all true. If the surface really gets to the same temperature then it will radiate about the same energy per unit area. Fortunately your skin does not touch it directly, but your bag does get pretty compressed. Probably you could get a better effect with a shape like a Z-rest of something, but I totally agree, basically a gimmick.

Now maybe it it was designed more like a bed of nails…..

"If you cover yourself with aluminum foil, you will radiate no heat. The emissivity is close to zero. Using Stefan-Boltzman equation, no IR radiation."

This statement contains vaguely defined terms and half-truths (the calling the foil-human composite "you" for instance, undefined temperatures of the parts of this "you", and so on) 6/10

It is worth pointing of that IR by itself can not escape (if you were enclosed), but until it gets converted into molecular thermal energy it doesn't heat you either. But it does get more chances to do that (instead of just escaping) therefore increased R values. Works much better as a blanket.

I like the idea of a Mylar coating for the tent. Now that would be toasty!

Maybe a gimmick, but Cascade Designs does claim an increase in R value at the same price and weight as the original Ridge-Rest, so what's to lose unless you really don't need a new pad?

I do believe it. And if this isn't the site where a few percent make all the difference in the world,then nowhere is! LOL

A lot of people, I think, have advocated using a Mylar ground sheet in winter, and so on.

Magnets, how do they work????

:o)

Turn the tent into a solar oven!

IR is radiation. This is the same as light, UV, and gamma rays.

Convection is a form of conduuction. Heat one end of a nail, the other end gets hot, too.

Silver reflects the most, it also radiats the least when it is hot.

Black absorbs the most, and, radiats the most. han input

Molecular movement is what we interpret as heat. The temperture of "space" is quite high, but, because there is so few molecules,ie, output is greater than input, so we say it is cold. This is a wierdism that sort of points up the fact that molecular motion is interpretted as heat.

Generally when an atom gets hit by IR it absorbs it by shifting electrons to a higher state. They move a bit faster. The increased movement is interpretted as heat by a human. More IR will force the electrons to fly away from an atom, leaving it ionized (reactive.) It may combine with other things: like 2H2 + O2-> 2 H2O. This also releases IR heat causing a chain reaction, ie a big boom. Anyway, the absorption is often in one step or one photon. The molecule may have several steps it can *loose* heat from, so, I think of this a shifting IR radiation down the spectrum, making the object warmer. Slap something with an IR frequency and it radiats a lower IR frequency. How does an object select which frequency to radiate? It doesn't. Basically, it is random.

Often the IR is simply lost again when the electrons drop back in an atom. It has several specific quanta of heat that it will eject and absorb. This is the basis for lasers.

Anyway, placing a low density material, like foam, in front of an IR reflector means that the foam will get two chances to absorb heat. Since the silver radiats less, it will generally be interpretted as radiating the other way.

Conduction happens when the whole molecule, nucleus and electrons, start shaking. This causes the next molecule(s) to vibrate a bit more, and so on. A very low order heat.

IR doesn't need an air space to work. It doesn't need anything to do with conduction to work. The balance between in and out is what we look at to determine how *warm feeling* something is. Example: a man in space would get roasted by the heat there, but the density of the heat is not enough to hurt him. The balance between input heat by conduction and output heat by radiation is not skewed enough to cook the astronaut.

A “clear air” or vacuum gap is required for an IR reflector to function.

That might be the gap between your nose and the insulator, but it is not the intimate contact between your hip and the insulator.

“Reflective foils without an air space have an equivalent R-value of 0.”
[Near the end of the 2nd page]

[Edited to add Link to wiki] And, of course, wikipedia.

Personally I think it is hype, or inconsequential. Too many other factors enter into picture.
In a physics lab, sure. In the woods, not so sure. In the marketing department, Absoultely!

Wait, so if you have it silver-side down, can't the foam act as the air space? Was my hypothesis correct?!

Greg, not really. For conduction/convection, yes. IR is radiation. If you have a piece of reflective material it will ALWAYS reflect the same…well, untill contact is intermingled. IR only composes about 10-12% of lost heat on your body. The back comprises about 35% of your suface area. Call it 50% for easy calculation, soo, it only comprises about 5% of heat lost. It doesn't really matter to IR if it is contacted or not. What you are refering to is the total insulation value of a blanket or pad, not just IR. Convection/conduction don't count to IR. I suspect that some of the foam is of increased density near the aluminized layer. Between this and the IR reflection, you get a higher R value with the pad. But, R value measures TOTAL heat loss. Not just convective, conductive or IR alone.

There are some secondary (and tertiary) reflections going on, too. These are often discounted because they comprise less than 1%.

Contact would mean that you loose all convective insulation and only reflect the IR. With a small gap, say a 1/2", you find you automatically pick up a .5 R value. It is all subjective. If you put it on you skin, it feels cooler than if you have a gap. IR and heat are intertwined enough to prevent seperation, but understanding how they work lets you grab the extra 10% from stuff. It is sort of like relative humidity. You feel cooler at 100F and 0% relative humidity than at 90F and 100% relative humidity even though humidity is not measure of heat.

“Radiant barriers must have airspace to function correctly. The Department of Energy / Building Science Report recommends a 3.5” air space above the reflective barrier in ceilings.”

“Radiant reflective barriers have a proven track record in hot weather where their main application is for reducing the heat from solar gain getting into attic spaces. The physics that allow these products to work in hot weather do not correspond to cold weather climate performance, or to applications without appropriate airspaces.”

Reference