I'm wonder if making an outer quilt for my hammock of a sheet of Reflectix will be as warm as one from Pertex and Primaloft?

Costwise? Durability-wise? Weatherproofness?

I am very happy with my Clark Jungle Hammock, and would like a lightweight solution to eliminating the convection problems of wind and cold.

I can see two potential problems. Tailoring the Reflectix to fit the shape of you in the hammock would be difficult. Any void between you and the Reflectix will become a cold spot. This is a challenge even with my down under quilt. The other issue is breathability. Reflectix doesn't breath so you may have condensation problems.

I'm happy with my quilts so I haven't tried Reflectix. I've read of people using it successfully inside the hammock, so my potential issues may not be issues at all. One benefit of a quilt is that I can also use it as insulation around camp or on the trail. My quilts are JacksRBetter No Snivellers.

Edited by ericnoble on 11/29/2006 12:43:11 MST.

I'd like to reinforce that an IR reflector is no substitute for insulation. It certainly couldn't hurt to add it, but hammocks are especially notable for convective heat loss and insulation is required for a warm night's sleep.

IMHO, the above comments are right on. The Reflectix will need to fit you closely, at least around the edges, so the trapped warm air can't escape when wind pushes on it. (In still air, voids between your body and the Reflectix won't be too much problem if all of the edges seal well to your body, but a wind will continually pump the warm air out of those voids.) This is one advantage of a thick outer quilt: some areas will be thinner and others full-loft, but it will conform fairly well to your body shape. I found at least a 20F difference in comfort range just by adjusting my under-quilt to fit my body more closely.

An advantage of Reflectix is that it is easy to shape (cut out a thin wedge then tape the edges together), but I'm not sure you could shape it close enough to a body in a hammock, especially if you change position. You could sew velcro dots all over the underside of your hammock, glue or sew velcro dots onto the Reflectix, then stick it tight; it might hold the Reflectix close to the hammock, or maybe the dots would all tear open when you got in (it's just an untried idea.)

Using the Hennessy Hammock SuperShelter underpad/undercover combination (the underpad is open-cell foam and the undercover is silnylon), I found condensation inside the undercover every morning. So Reflectix is sure to have condensation if the weather conditions permit it to form. However, as long as it didn't touch you, it shouldn't be a problem. But my guess is that when you sit up in the morning you're going to get a wet butt.

I use a JacksRBetter Nest down quilt (same as Eric's No Sniveller except the slit is cut to match a Hennessy Hammock) as my cold-weather under-quilt. This quilt, with or without the JRB Weather Shield bottom, has never accumulated condensation; it weighs 20 oz. and keeps me warm well below 0F. The Hennessy Hammock SuperShelter Explorer-size underpad/undercover weighs 20 oz (claimed 14 oz) and keeps me warm to below freezing. A body-sized GossamerGear ThinLight 1/8" sleeping pad (2.5 oz) or an Axium Jumbo "Blockade" autoshade (similar to Reflectix; 1/4" thick; 4.4 oz; it tore) inside the hammock keeps me warm down to about +40F. Reflectix equal to the surface area of the JRB Nest would weigh about 15 oz., and I would be surprised if it would keep me warm much below +40F. There could be a 40F or greater difference in warmth for the extra 5 oz of a JRB down underquilt compared to a Reflectix underquilt. I would certainly expect that a thin Pertex/Primaloft underquilt would be warmer, lighter, more compressible, less condensing and just as durable as a Reflectix underquilt. A Reflectix underquilt might be a bit cheaper than one made from Pertex/Primaloft, but cheap Walmart nylon and batting would probably be about as cheap and still work better than Reflectix. As long as your tarp sufficiently covers your hammock, weather shouldn't be much of an issue for your bottom quilt. Mine hasn't become wet from rain or driven snow.

YMMV. The above is based on my experience with down and SuperShelter underquilts, and various sleeping pads (including something like Reflectix) in the hammock. I've never used Reflectix as an underquilt.

Edited by Otter on 01/12/2007 13:12:55 MST.

As usual, great post Douglas! The closest I came to using Reflectix was when I glued a space blanket to some 1/8" foam I bought at the hardware store. It turn out great but I've never really used it. My back felt a little damp when I tested it in my basement with the hammock.

"an IR reflector is no substitute for insulation"

I use Reflectix for my pot cozies, and It seems to "insulate" according to the common usage of that term; so I went to reflectix.com to research this..
It is not an insulator by design; it provides a 97%barrier to radiated energy. "R" values are only used to measure conductive heat flow, not radiated, so the foil alone has no R value.

However, the bubblewrap between the layers of aluminum does insulate as any stagnant airlayer would. The theoretical R value of the air layer is 5.5[ft2·hr·'F/Btu·in] according to the following document, which provided a great review of heat transfer basics for me, and has an equation for the combined R value of the foil-bubble-foil 'sandwich':
http://www.reflectixinc.com/PDF/RIMA_Handbook.pdf
However, it stops short of claiming any R value.

It seems to me that for a SUL hiker able to sleep on a sheet of buble wrap, reflectix would be a better option on cold ground.

Edit: After further research I understand why Reflectix does not post the R value of its product. The FTC prohibits radiant barrier manufacturers from posting this number because it varies by installation. Also, the industry group NAIMA suggests against it, and there were numerous law suits where the actual R value did not match claims.
Internet sources other than the OEM claim reflectix has an R value of .67 to as high as 5.6. Truth is probably in the middle? [google search on reflectix r value]

Edited by Brett1234 on 11/29/2006 20:28:00 MST.

>It seems to me that for a SUL hiker able to sleep on a sheet of buble wrap, reflectix would be a better option on cold ground.


Agreed. I consider Reflectix to be about as warm as a closed-cell sleeping pad of the same thickness (since Reflectix bubbles are closed cells). That's why I'm guessing a Reflectix underquilt would be good down to about +40F. But I'd sure rather use a synthetic quilt as a hammock underquilt, for the reasons previously stated.

Brett,

do you want to do an experiment?

your reflectix pot cozy vs. a non-reflectix pot cozy - both using the bubble wrap approach.

same pot; same volume of water; foil lid with a thermometer sticking out through it, or use an electronic temp. sensor stuck to the outside of the pot.

heat water to same starting temp each time.

take one minute or 30-sec. temp readings and log them.

compare the results.

how much longer did the reflectix cozy keep the water above a certain temp?

i'd be very interested in some real numbers to see how much difference it makes with water heated to 180 deg and then having the heat source removed and the foil lid covered pot placed in a cozy. to use Mike Barney's word it's a "simple" matter to calculate the heat loss in some given situation, or even to perform a series of calulations with one degree changes in the delta-T, but we need an accuarate measurement of the surface area involved. Besides working in a Test Engineering group, i ascribe to Rhiele's old dictum, "One test is worth a thousand expert opinions".


as far as radiant heat barriers for sleeping human bodies. IMHO, it's majoring in the minors as reducing convective heat loss, conductive heat loss, and evaporative heat loss (e.g. from wet clothes worn to bed - sometimes not much we can do about that) is effort much better spent.

if i'm not mistaken (and someone PLEASE correct me if you feel that i'm wrong) there is very little or no radiant heat loss from a human body inside of a sleeping bag. Why? After being in the bag a short time, the air temp inside the bag is close to or at our skin temp, so little or no delta-T for radiant heat loss to occur. Other than for "black body" emitters, a delta-T is required for heat transfer (cf. Stefan-Boltzman Law of Radiation).

Sure, if we don't have sufficient insulation, then there may be a small benefit (that's why, in part, Space Blankets are used for emergency situations), but based upon a youthful experience that i've related elsewhere on two occasions in these Forums - they don't work that well when used alone; at least not what i expected as a teenager.

Furthermore, IIRC, DrJ has previously mentioned that a small thickness of any insulation is sufficient to interfere/prevent radiant heat loss for the inner portion of the bag to the air outside of the bag.

Now, radiant heat loss from a burning flame, that's another story (large delta-T b/t the flame and the air around the flame).

[Note: delta-T means a difference in temperature.]

Edited by pj on 11/30/2006 04:44:56 MST.

pj,
Id love to try the test you suggested. I suppose the goal would be to determine the R value of a single layer of reflectix, and thus compute the fuel savings in grams for using a cozy. Simple algebra using the forumlas I found and linked in a previous post.
I am a test engineer by trade, but I do precious little of hands on testing these days. I suppose that's why I get a kick out of testing my gear and discussing it here.
Anyway, I'm overseas on a contract assignment with none of my tools, so Ill try and find a bimetalic thermocouple type instrument to run that test. Maybe I can find a cheap cooking thermometer with a digital readout. If you were to do it in your garage this weekend Im sure you'd beat me to the punch.
I am concerned my stove will melt the reflectix, but I'll figure out a controlled way to prevent that.
Exciting, I'll have to make a test card for this..

Future 'reader reviews':Today a few new items arrived by mail, a GG Torsolite, a Thinlite, an icebreaker wool top, a TNF Diad, REI coolmax socks, and montbell Thermawrap pants, all bought cheap or used. It was like an early Christmas, and I've got a full deck of 'test cards' for my next overnighter. What do you have on the back burner pj? (besides your cup of o-cha?)

Hey guys, I don't know how much time you spend in fabric stores, but there is a product called "Insul Bright" that is sold near the fabrics used to line quilts (insulation dept). It is sold by the yard. It is a thin fabric with a mylar center. I have made cozies with it, and it simply rocks! It is light, flexible, washable, and it breathes. It used in making insulated lunch boxes orginally. You might try it, if you can find it.
Btw, thermometers? Check the grilling section.

>>"I am concerned my stove will melt the reflectix"

Brett,

Maybe i'm missing something. Maybe i'm getting confused. i was thinking of a cozy with an interior (and usally also an exterior) radiant barrier vs. just a non-radiant insulative barrier.

So, just do the following:

Heat the water in the pot on your stove.

Remove the pot from the stove (no more heat & no more flame).

Place the pot of 180deg water into a cozy.

Take the temp. readings and repeat using the other cozy.

Compare results.

Many thanks for being willing to perform this test.

Edited by pj on 11/30/2006 09:28:59 MST.

pj,
Sorry, you were clear the first time, and thanks for the clarification; I imagined measuring the time to heat. Time to cool makes more sense. Ill report back..

Douglas, Eric, et al, Thanks for your input on the Reflectix insulation subject. I was trying to carry a dual use item by utilizing the pad under the hammock; and in the event I had to 'go to ground', as my ground pad.

The Clark Jungle Hammock has the advantage of not needing the sides, where the pockets are, to be covered with insulation if you have gear in them at night. Thusly, I would be able to cut a narrower, less complicated shape, needing less curve to match the bottom of the hammock.

Granted, there could at times be a space opened up underneath me creating a cold air void, if the edges were disrupted.

Using a down underquilt will not allow me to 'go to ground' with it as my ground pad. Though it would be warmer, lighter, and less bulky.

Can't have my UL cake and eat it too.

>I was trying to carry a dual use item by utilizing the pad under the hammock; and in the event I had to 'go to ground', as my ground pad.


I can't think of any 1/8"-3/8" insulation material that is vapor permeable and mostly non-compressible. About the only thing I can think of is to make a cheap synthetic quilt (don't bother with DWR) and use a Gossamer Gear 1/8" ThinLight sleeping pad to suspend it beneath the hammock. The ThinLight is pretty tough material (especially the 1/8" Wide/Long ThinLight, which was a different spec and is unfortunately no longer available) and it should be under less stress if sized to fit and suspended between the Jungle Hammock pockets rather than from the hammock ropes. This would provide a weather-proof cover for the quilt at a weight penalty of about 1.5 oz. There will be some condensation, but it should stay between the quilt and the ThinLight and hopefully not wet out the quilt. (I wouldn't recommend a down quilt with an impermeable cover.) The ThinLight works well as ground insulation, but it's not much padding if you're forced to sleep on compacted ground. (My sleeping pad system is a full-length 1/8" ThinLight supplemented with a thicker torso-length pad.)

Edited by Otter on 01/12/2007 14:02:43 MST.

Douglas, I have a GG torso pad the one w/ the convoluted bumps that I use on the ground already, and some 1/8th" foam for legs.

I recognize the fact that if I use down as an insulator one side fabric needs to be breathable for the down to properly ventilate. Thanks

A "Black Body" or ultra simplified ideal radiating body emits its maximum energy at a wavelength given by Wein's law lLambda(microns) = 2.898 x 10-3 / T (Degrees Kelvin).

To make this simple, the sun has an apparent radiative temperature of about 6,000 degrees kelvin. This results in a peak radiative wavelength of about half a micron which falls in the visible spectrum pretty much near green. So, here you can see the peak radiative wavelengths. Almost a match made in Heaven, or by Evolution as you have a better chance to see if you look where the light is brightest.

A good reflector (at a given wavelength) is a poor absorber (for the same wavelength), and thus simple visible shine is a mark of a good reflector of the radiation of the sun.

Now a human being is only hot to about 300 Kelvin (about 28C + 272 = 300Kelvin). The human body had a peak radiation wavelength of about 10 microns.

10 microns is fairly far out in the infra-red spectrum and you can't see it with your eyes which can only see energy with a wavelength between about 0.4 microns (blue) and 0.7 microns (red).

Your eyes can't tell what would be a good reflector for those wavelengths. Lots of people think that if they want a good radiator for the human body, they should use black colored substances. But "Black" as we normally judge it is really only black in the visible, and could be anything in the far infra-red.

The bottom line is that when it comes to "radiative" heat loss from the human body, there is a substantial amount of BS floating around. Your eyes are of no value in figuring out what works and what doesn't.

There is another "law" called Stephan's Law which states that the radiation given off by a perfectly radiative body is proportional to the body temperature Kelvin) raised to the fourth power.

A visibly hot object (say 6000K) will radiate a lot more than a fairly cold human body of the same size (say 300K) and the amount is 160,000:1. A wood fire may have a flame temperature of 2000K and the wood fire would radiate about 1,975:1 relative to the human body.

So, the human body radiation is a pretty darned small piece of stuff, but then the human body only puts out about the equivalent of 100 watts of heat (or less) at rest.

At times, the radiation from the human body is probably over ly hyped compared to the losses from conduction and convection. You can feel those loses with your hands, and your hands probably can't detect the radiation from a human body.

Jim spaketh:"But "Black" as we normally judge it is really only black in the visible, and could be anything in the far infra-red."

We've done some work with some high end IR and thermal camera systems, and it's amazing what has a bright visible spectrum response may have a very small IR signature. On the flip side, there has been significant work done to fabricate clothing and coatings that have a very low - to no IR signature, both for coolness in hot climates, as well as making people and other targets not stand out when seen with thermal and IR viewers.

I learned some of my IR materials from thermal control of spacecraft. The spacecraft are bombarded by the sunlight and heat up from it and are cooled by radiation to space. The temperature of the sun is 6000K and the temperature of the spacecraft is about 300K and thus radiates in the IR at about 10 microns. If the spacecraft gets too much hotter than 300K, the electronics self destruct from excess heat.

People were good at trying to design materials systems that would, for example, absorb very little of the sunlight and radiate as perfectly as possible at 10 microns in the IR.

The material which would do that perhaps the best was a second surface quartz mirror with silver coating on the second surface. You could see the visible reflectivity of the silver that bounced the solar energy back to space, but the high IR radiative character of the visible light transparent quartz was impossible to see.

Laboratory instruments were used to design, and proof the whole concept.

Getting back to the original issue of Reflectex making a better cozy, somebody has got to do the measurements, or else we can just leave it as a perpetual debate that no one can do more than arm wave about.

Lab work.

But then too, the design of the cozy itself will have some bearing on comparative tests. A cozy made from an insulating/reflective polyester mat layer (insul brite?)will be physically different than the one from Reflectex.

I've sat around hypothesizing how to make really great cozies, but in the end, I just copied one of the pouch ones sold on the internet and was done in under an hour. It is easy to decide to just do something 'good enough'.