Interesting Article on Reflectix

[rbinhood]

I recently read this article on foil faced bubble wrap insulation: http://www.energyvanguard.com/blog-b...diant-Barriers

Since many people, myself included, have used Reflectix or some form of it as an underpad, I tought the article made some good points.

First, the R value of reflectix is only about 1, not 6 as some of the manufacturers claim. That is because R value is a measure of conductive insulation value, not reflected heat value. For Reflectix to have much R value, there has to be a dead air space created between it and whatever it is insulating. If you are laying on top of it as we do in a hammock, it has little R value.

So what is the value of Reflectix and could it be achieved at lighter weight in another way? I think the answer is yes.

Reflectix's value is that it stops radiant heat loss. Heat loss in a human being occurs in several ways--conduction, convection, radiation, and evaporation.

Reflectix is a radiant barrier and a vapor barrier. Both of these values could be achieved with a simple space blanket, at some weight savings.

I'm switching to a space blanket next time I winter hang to compare it to the Reflectix.

[gmcttr]

While I agree with the argument on R value, try this simple test at home and let us know how it comes out...

Make a cozy out of reflectix and another out of the "space blanket" material. Put a ziplock freezer bag in both and add 2 cups of boiling water. Grab hold of both with your bare hands and see which one insulates the best.

I'm guessing the "R1" of Reflectix will be feeling pretty good.

[Knotty]

While Reflectix will insulate better than a space blanket, thanks to the air space in the bubble wrap inner layer, I believe both are overrated for the very reasons explained in the article rbinhood linked to. It's a quick and easy read, so better to follow the link than me trying to paraphrase it.

[HomeMadeHiker]

I would be interested in the results of the test stated above but instead compare RFLTX of comparable thickness with Blue Wally pad and the gossamer gear gray pads. But maybe I little more objective manner.
Like this:
1. boil 1 cup water - record temp
2. Pour in Freezer bag - place in cozy
3. Place cozy into freezer for 1hr (or some other amount of time)
4. After time is up record temp
5. Repeat for each material
It Seems that each material has a little something different going for it. So this way we could compare them side by side in manner similar to how it would be used in a hammock.
I would be willing to do this but I do not have either of the foam pads. If any one has some scraps to spare I would love to try this out.

Oops, Did I hijack a thread? Not sure what exactly constitutes a hijacking.

[mophead]

Very good point in the article, but I think the conditions are more optimal on the other side of the pad compared to the space blanket. I use a truck windshield reflector which is similar- foil on both sides and pockets/bubbles in between.

I think most of the benefit of this pad comes from blocking the cold that would otherwise rush in. With the bubbles, although they would compress some, the air gap between the outside and what touches your body would remain warm longer than a space blanked that is simply one layer with no air gap.


(Sleeping bag)
------foil-layer-------
+++air gap+++++
------foil-layer--------
******cold*******

VS

(Sleeping bag)
------foil-layer--------
******cold*******

No doubt, air is the name of the game in insulation. So I would think a pad would give an additional benefit over a space blanket. When carrying multiple pads you may get diminishing returns, I have not tried both. The pad/bag has always done the job.

[rbinhood]

One of the things I neglected to say in my initial comment is that I would pair the space blanket with a closed cell foam pad. I would not use just a space blanket, because much of the heat loss is conductive and convective.

The heat loss science involved here is fairly complicated and can't be reduced to some of the simple tests that are being advocated, although I have to say that HMH's proposed comparisons would be a good place to start.

There may be no right answer for everyone. Try out various combinations and see what works best for you. I simply put this information out there so that people could make informed decisions.

[gmcttr]

Whoosh.....right over the top of your heads...sorry.

My "test" was a tongue-in-cheek suggestion with what I thought to be obvious results...one hand comfy and one hand burned.

[SGT Rock]

Years ago Youngblood wrote a great piece about why radiant heat reflection systems do poorly. I don't know where that went to in the cloud - it was originally over at one of the Yahoo groups. But to paraphrase what I remember:

Radiant heat loss is how the heat from the sun transfers to the earth without a conductor or air currents to get it here. Basically it is heat seeing the cold spots and transmitting the heat energy. If you were to stand naked in a freezer, then you loose lots of heat from radiant heat loss. And your body is an excellent transmitter of heat which would allow your heat to rapidly dissipate. But if you were to surround your body with a close Mylar curtain, it would drastically (about 50%) cut down this heat loss - that is the theory behind the radiant heat reflector like a space blanket. Unfortunately for most of our systems, you must have space between you and the reflector, touching it negates the benefit.

But there is a better way - wear clothing. If you put on long sleeve clothing you would dramatically reduce radiant heat loss (about 98%), and the clothing itself makes a poor source for radiant heat loss (about 5% effective as your skin). To add to this, each layer you add further cuts this effect down. So by just wrapping yourself in a hammock and a sleeping bag you negate the need for a reflector. If you were to wear a single layer of clothing, total radiant heat loss is less than 10%. Add your sleeping bag and it is less than 1%, add the hammock... well, you get the picture.

As I recall, Yougblood was an aerospace engineer, so his advice came with some background in the field of which he wrote.

[Meriadoc]

I started a comprehensive response but so far I'm up to 500 words and still going.

So for right now I'm just going to weigh in with this opinion:
If a radiation barrier is going to help (and there are a lot of physics questions behind that) the space blanket is lighter but wears out very quickly. I'd rather have reflectix simply for durability and because it won't bunch up as easily. If the space blanket bunches or the coating wears off - as inevitably happens - then it's use is limited.

Whether the radiation barrier is going to help depends on whether you can get an open space on even one side. If there is open space on both sides that's great. But even if there is open space on only one side it can either be used to absorb less radiation or to emit less radiation. If there is no space on either side, conduction will overpower radiation as a method of heat transfer.

So, if there is space between you and your underquilt, by all means place a radiative barrier there. (I'm thinking that there might be small spaces since the radiative barrier isn't completely form fitting.) If at the same time you can get a vapor barrier close to you, that's even better.

And then to reduce the amount of heat radiating from your quilt, you might want to place a radiative barrier on the outside (bottom) of your underquilt so that it can't radiate as much heat to the outside. At the same time you are reducing the flow of air through your insulation. (This is only good as long as you are using a vapor barrier close to you! Otherwise the quilt will simply get full of all the water from your evaporative cooling.)

Or if you use a pad, then try a radiative barrier on the outside of your hammock to reduce the amount of heat you radiate. This one may have a dual air barrier if you wrap it high enough around you.

[rbinhood]

I too, have a degree in engineering, and have done advance level coursework in thermodynamics and heat transfer.

I like and tend to agree with the summary offered by Sgt. Rock, but I must stress that the heat transfer analysis here is far more complicated than it is simple.

Both Reflectix and a space blanket are impermeable to air, so both will cut convective heat loss from the wind. Where in your layers you position them can make a difference. Go to any number of the green building sites online and read the various opinions about vapor barriers and draft sealing in building construction. The principles for insulating a home and your body are relatively the same, as are keeping insulation dry, windproof, and intact if it is to be effective. The major difference is your house stays in one place, so insulation weight doesn't matter, but most hammock campers move and carry their insulation, so weight usually matters.

Reflectix will be better at stopping conductive heat loss than the space blanket, but not by much(Its R-Value is only 1).

Both Reflectix and the space blanket are vapor barriers, so both will stop evaporative heat loss equally well. They will also trap moisture in your insulation equally well, so where you place them is critical.

Both Reflectix and the space blanket reflect heat equally well. They are equal radiant barriers. You are not feeling radiant heat when you pick up the pot wrapped in a space blanket, you are feeling conducted heat.

A space blanket weighs less than an equal size piece of Reflectix. The difference in weight may or may not matter to you.

Reflectix may wear better than the space blanket, but even Reflectix breaks down eventually.

Reflectix may have a better lie than a space blanket, but that would vary with application.

An equal size piece of Reflectix cannot be packed as small as an equal size space blanket.

And finally, most radiant heat transfer takes place at the visible end of the spectrum, so, if you can't see light through it, chances of radiant heat loss being stopped by it are pretty good. Clothes alone may stop radiant heat loss at the visible end of the spectrum.

So, I end where I did last time. Experiment and use what works best for you for your given situation. It ain't all the same if you are a gram weenie vs. a car camper.