UltralightCold Weather Hammock Pad Idea

> a cheap space blanket and some other light material as backing and then having a fleece top

A couple of comments – please don't be offended.

First, if this idea was good you can be sure someone would have already tried it and marketed it. (I know, there has to be a first time, but …)

Second: the whole space blanket thing is a failure anyhow. It will reflect radiation from a fire, but a fire is very hot and a human body is not. About the only benefit you get from it is as a windbreak.

Third: if it is cold 'outside', you can be sure there will be lots of condensation against the space blanket, making the fleece wet over time.

Sorry. But this is an idea which gets resurrected every year or two.

No offense. That is why I come to this message board. I like to test my theories against the "gurus" (or the people that know more about MYOG) than me. Thanks for the insight.

Needle holes will start a space blanket to shreading or at least tearing…

You are putting a lot of weight into that design…

It will become a bathtub of nonsensible sweat that will become sensible as it gathers at the bottom… under insulation is best when it breathes since it has to wrap around unlike a ground pad which is flat with relatively little contact… also the fact that hammockers seldom move around (unlike ground sleepers) the opportunity to vent the wrapped bottom problem is exaccerbated…

Just my 0.02…. and I may be biased.

Besure to report out your results if you undertake the project.

Pan

Cameron,
I did something similar to this. I glued 1/8" foam from the hardware store to a space blanket and cut it to fit me. I never tested it outdoors because it was feeling damp when I used it indoors. I have slept on a pad in my hammock but I had to use a bivy to keep it from moving. Getting into a sleeping bag, and a bivy with a pad, in a hammock is a real feat! The zipper on my bag and the bivy are on different sides as well. I now use 2 Jacks R Better No Sniveller quilts and I'm very happy. The money was well spent. The quilts get used on outings and around the house. The quilt is my 3 season bag if I'm in a hammock or not.

Besides the comments above about low temperature differential and moisture accumulation (which are accurate), the space blanket won’t work as you desire because it will have insufficient air gap to perform as a radiant reflector. The reason Tom used windshield reflector was because it contains its own incompressible air gap, which is slightly augmented by the fleece. Since fleece is very compressible you’d be lucky to get 1/8″ of air gap above the space blanket, which is insufficient. The fleece-mylar-nylon would provide only wind resistance, while conducting heat away from your body fairly easily. I can confirm this because I did exactly that for several freezing nights, spreading a 100-wt fleece jacket on a space blanket on a tent footprint inside the hammock. It was very cold and didn’t perform as well as a 3/8″ sleeping pad (which is also damp and insufficient for temps below +40F). It would have worked better slung under the hammock, but I didn’t know that then.

It was a good thought though, and we certainly need more ideas for hammock under-insulation because it is such a significant problem. I don’t use in-hammock pads any more because they’re such a hassle to keep arranged underneath me. Instead I use a down quilt (Jacks R Better Nest) or open-cell foam (HH SuperShelter) slung under the hammock. So far these have kept me warm from +50F to below -5F, but I’d certainly like to reduce the weight and volume with a lighter, thinner idea!

Here is a recent thread that touches on the subject of hammock under-insulation, and here is a long thread from last winter on the same subject.

"it will have insufficient air gap to perform as a radiant reflector"

Where is this concept about coming from? Why must there be an "air gap" to reflect radiant heat?

>Why must there be an "air gap" to reflect radiant heat?

Because if there is no gap then heat will be _conducted_ directly from the warm body to the mylar film.

"heat will be _conducted_ directly from the warm body to the mylar film."

…assuming that it's mylar, however the new heatsheets are made of a different material. Either way, heat can only be conducted away as much as the material is a conductor of heat. How well does polyethelene or mylar conduct heat?

>Either way, heat can only be conducted away as much as the material is a conductor of heat.

It hardly matters what material the space blanket is made from for thermal conductivity because it is both very thin and very solid. I don't have the material specs handy so instead we'll try a real-world test. Go outside and let the space blanket cool down to ambient temperature (say, +30F), then put it against your bare stomach. Touch the space blanket with a thermometer. Does it remain at +30F or does it go to skin temp? Answer: skin temp. Thus, it is sufficiently thermally conductive for it to conduct heat to the hammock material, which we already know loses heat quickly through convection and radiation (the original loss the space blanket was supposed to stop).

With all due respect, your experiment actually proves the opposite. Take a steel plate and a piece of plastic (both of equal thickness) and perform the same test. I'm sure you will find the plastic to not only 'feel' warmer against the skin, it will also undoubtedly have a higher temperature reading. This does not mean plastic is a better conductor of heat than steel.

Here's a better experiment: Take two ice cubes from your freezer (equal in size/mass, equal in temperature). Place one in each hand (both hands being of equal temperature and equal size). Wrap one ice cube in a single layer of a space blanket (shiny side toward the hand, not the cube) and the other ice cube in a single layer of the thinnest silnylon. The silnylon will actually have a slight advantage of being slightly thicker, hence potentially more insulative capability, yet both are waterproof to prevent direct water contact. Squeeze both at equal pressure until you can no longer bear the cold or until one ice cube melts first.

Guess what? After about 5-6 minutes, the ice cube in the silnylon becomes unbearably cold while the one in the space blanket actually was tolerable for an indefinite period of time (for me anyway). To rule out any bias on my part, I enlisted the assistance of my inquisitive and nonjudgemental 5-year-old son. I asked him which ice cube was bigger – he pointed to the one that was in the space blanket. I asked him which one had more water – he pointed to the one that was in the silnylon. I then placed my palms on both his cheeks and asked him which hand was colder – he pointed to the hand that had been holding the space blanket. He knew nothing of what I was doing, nor could he even comprehend the experiment, yet all 3 answers confirmed what I had observed.

About 5 minutes after the experiment, the palm that had been holding the ice cube in the silnylon began to hurt as it rewarmed, unlike the other one. Again, confirming that that hand WAS actually colder. Try it and see.

Conclusion: a mylar space blanket conducts less heat and is actually a far better insulator than silnylon.

Was this due to the properties of the mylar material itself? Was this due to the reflectivity of it, providing a radiant heat barrier thereby insulating my hand more?

I would love to see the good doctor Ryan J. himself perhaps employ some of his highly analytical and scientific approach to obtaining actual comparative data and maybe prove one way or another the validity of radiant heat barriers. Roger had stated above that this subject comes up every year or two, but every time it gets shot down without a fair fight. People spout off about clouds in the atmosphere or sleeping under a tree – both of which have nothing to do with the radiant heat given off by a warm, living body. No one has any actual evidence to back their statements. Even Ryan himself gave radiant heat barriers the shaft in an article he wrote about heat loss. But where is the science? Where is the proof? There's plenty of science out there for other applications of radiant heat loss…

Roger said: "First, if this idea was good you can be sure someone would have already tried it and marketed it. (I know, there has to be a first time, but …)"

Yes, this is a good idea and, yes, someone has marketed it… and has been doing it for many decades. That would be Stephenson Warmlite. (www.warmlite.com)

So, Cameron, your idea is actually pretty good IMO!
However, the uncoated nylon base doesn't seem to serve any purpose other than to protect or strengthen the space blanket.

I would recommend using a more insulative backing, like maybe a simple closed-cell foam pad (the blue ones from REI are pretty cheap). This would add R-value and 'support' the space blanket, while the fleece also protects the space blanket and prevents direct contact with your skin (assuming you're not completely clothed under bag). If you are fully clothed, you might even skip the fleece.

The fact that it doesn't "breathe" is not an issue – i.e. no condensation problems here… IF you're using it only as a bottom layer. I have yet to hear of anyone complaining about or wishing for a "breathable" sleeping pad.

> Conclusion: a mylar space blanket conducts less heat and is actually a far better insulator than silnylon.
Seems like a reasonable experiment and conclusion to me.

As to the bit about a reflector having to be some distance away from anything else to reflect … I can't see any logic there. Granted, if the space blanket is touching something it will sure conduct heat away, but that does not affect the reflection properties. Me, I think the whole reflection business is a great marketing gimmick for 'space blankets on Earth' with no real science behind it.
For the purist: yes, a space blanket will always reflect IR radiation from a body, but at body temp the amount of radiation is insignificant compared to conduction and convection.

>With all due respect, your experiment actually proves the opposite.

And with all due respect in return, I think you misunderstood my simple experiment. Your question was, how well does a space blanket conduct heat? And the answer is ‘quite well’, as the experiment showed. “Feel” had nothing to do with it (that’s why I used a thermometer instead of my hand on the other side of the mylar). Your experiment also showed that the mylar space blanket conducts heat readily, since your ice cube melted, and thus we agree.

>Conclusion: a mylar space blanket conducts less heat and is actually a far better insulator than silnylon.

But that wasn’t the question. It is irrelevant that silnylon is not a good insulator. A mylar space blanket is not a good contact insulator either, even if it is better than silnylon. Anybody who’s slept in a hammock on a cold night on only a space blanket can attest to that. It’s not theory: I’ve done that for several nights at temps ranging from +40F to +27F and a space blanket is simply insufficient. (I related that experience in my first post in this thread. I assume my observations are not unique.)

>Was this due to the properties of the mylar material itself? Was this due to the reflectivity of it, providing a radiant heat barrier thereby insulating my hand more?

The relevant property is the thermal conductivity of the mylar material. The reflectivity is not a factor because your hand and the mylar are in direct contact and and thus the two _surfaces_ are at the same temperature (no temperature differential, no net radiant heat loss). The materials behind the surfaces, the interiors of the cube and your hand, are not at equilibrium and so heat transfer is occurring through conduction.

>People spout off about clouds in the atmosphere or sleeping under a tree – both of which have nothing to do with the radiant heat given off by a warm, living body. No one has any actual evidence to back their statements.

On what grounds do you make that claim? I would really like to know why you consider this to be untrue and unproven. This is supported by both theory and actual observation. I suppose your statement is literally true because other objects don’t have anything to do with the radiant heat given off by a warm living body (about 500 watts), but that’s just sophistry. _Net_ radiant heat transfer is a function of temperature differential between two bodies, and water vapor in the atmosphere (clouds) or trees are much warmer than the cold of space. The sky radiant temperature in dry air at night can be _much_ colder (a difference of -40*C or more in arctic regions) than the ambient air temperature. Thus net radiant heat loss under clouds, trees or bushes is significantly lower than under open sky, since these are rarely colder than ambient air temp. (Google “sky radiant temperature”.)

This sounds familiar, and I realize now this just came up a month ago in this thread. PJ posted the relevant equations (Stefan-Boltzmann Law of Radiation) a year or so ago in another thread, and I’m sure Richard Nisley could carry this on better than I in the formal domain if you need those equations explained.

Here’s the basic point: net radiant heat loss is all about temperature differential. If you’re wearing clothes, especially appropriate body wrappings for the cold outdoors including sleeping bags and pads, the _outside_ of your wrapped body is not going to be significantly warmer than the objects surrounding you (e.g., a hammock, tent, tarp, forest, etc.). Thus there is little opportunity for net radiant heat loss. (Space blanket use in an emergency situation with insufficient clothing is not relevant here.) Nobody is saying that there is _no_ radiant heat loss, but appropriate measures to prevent convective and conductive heat loss have the side effect of covering the body with a wrapping that has an essentially ambient surface temperature when exposed.

>But where is the science? Where is the proof? There’s plenty of science out there for other applications of radiant heat loss…

The math (if not the science) is simple: the relevant part of the Stefan-Boltzmann Law of Radiation states that the net radiant heat transfer is a function of the difference of the fourth powers of the absolute temperature of the radiant bodies. Taking an example of a wrapped body that is 5*C warmer than its surroundings, the net radiant heat transfer is about 25 watts; not much.

The reason radiant heat loss is significant in other applications is that those generally involve large areas and/or high temperature differentials, e.g., houses. Nobody wraps houses in a foot-thick down sleeping bag, and although high-R thermal insulation is used to insulate a house the outside of the house still ends up being much warmer than the surroundings, which is generally open space, not a tent. Thus, radiant heat barriers are useful in reducing heat transfer to the siding. Although these radiant heat barriers do use a radiant-reflective surface, they also have some kind of ‘air gap’ (whether air, nitrogen, vacuum or whatever in the form of an open space, large bubbles or a foam) in order to allow some space for the internal body to radiate and the reflector to reflect, while not ‘touching’ to a significant degree which would allow heat loss through conduction. As I see it, the point of the ‘air gap’ is to allow the heat from the warm radiant body to radiate and then reflect from the colder radiant barrier, without signficant transfer of heat to the radiant barrier though conduction or convection. (That’s why Reflectix does work somewhat as a hammock pad while a mylar space blanket does not.)

And now we’re right back where we started. No air gap, heat loss through conduction, no significant reduction of heat transfer to the atmosphere. A cold night in a hammock is the result.

I don’t think anybody has claimed that radiant heat loss isn’t possible, it’s just that it isn’t significant with proper and sufficient insulation to prevent conductive and convective modes of heat loss. Further, radiant heat barriers like space blankets are also vapor barriers. Moisture accumulation is a significant problem on its own, possibly greater in its own way than radiant heat loss.

>Yes, this is a good idea and, yes, someone has marketed it… and has been doing it for many decades. That would be Stephenson Warmlite.

Warmlite vapor barriers are not used on their own without a sleeping bag or air mattress. ‘Nuff said.

>I would recommend using a more insulative backing, like maybe a simple closed-cell foam pad (the blue ones from REI are pretty cheap). This would add R-value and ‘support’ the space blanket, while the fleece also protects the space blanket and prevents direct contact with your skin (assuming you’re not completely clothed under bag). If your fully clothed, you might even skip the fleece.

And you might also skip the space blanket, since the foam pad will significantly insulate the body and it doesn’t radiate much heat. However, if the space blanket is placed between the foam pad and the hammock then it might help a little. (That is actually hard to quantify without lab equipment.)

>The fact that it doesn’t “breathe” is not an issue – i.e. no condensation problems here… IF you’re using it only as a bottom layer. I have yet to hear of anyone complaining about or wishing for a “breathable” sleeping pad.

I complain. When ground sleeping there isn’t quite as large an area of contact between the sleeping pad and the body as there is between the body and material in a hammock, which also limits air/moisture circulation. After a single night in a hammock with a space blanket I noticed a significant decrease in the loft of the down on the underside of my sleeping bag; I have never noticed this when ground sleeping. Subsequently, when using a layered pad similar to that originally proposed in this thread, I found that the space blanket actually had drops of water on it in the morning, sufficient that I felt it necessary to shake it off before folding it. So condensation, and resulting damage to the compressed loft, is a factor with non-breathing underlayers in a hammock.

"The fact that it doesn't "breathe" is not an issue – i.e. no condensation problems here… IF you're using it only as a bottom layer. I have yet to hear of anyone complaining about or wishing for a "breathable" sleeping pad."

Oh contrare…. There are hundreds of threads and comments on general hammock threads at Whiteblze.net and Hammockforum.net that both mention and complain about the bottom lack of breathability and the resultant moisture issues and temp loss because of this condensed non sensible sweat on the bottom of large wrapping pads and other "creative" non breathable hammock bottom systems…

This issue is largely unnoticed by ground dwellers because ones curved body has only limited points of contact with a pad, which is flat on the ground… Further bottom venting occurs to a much larger extent when the ground sleep changes postion…. several to many times in a night.

Wide non breathable pads and other "creative" non-breathable material bottoms that wrap up the sides of the snuggly encasing hammock do not vent and become a vapor/condensing trap…. The issue is exaccerbated by the fact that once a hammocker has settled into the sweet spot, most do not move until morning or the mid night bio break…While some sweat less, for a great many if not most, this is a very real issue.

Pan

>As to the bit about a reflector having to be some distance away from anything else to reflect … I can't see any logic there.

I had to think about it a bit when I saw a post (a long time ago) that mentioned a required "air gap". I didn't remember anything about that from my thermo classes. But after looking at the common use of that term in heat engineering it became clear that it was simply quantifying the technical definition of "body" as a distinct object. If there is no area of significantly lower density between two objects then they probably cannot be considered to be distinct bodies. Further, if they are in direct contact then I'm not sure that the radiant heat transfer mode is applicable. E.g., the rate of net heat transfer from working muscle to the bone probably doesn't include a radiant heat transfer factor because they aren't distinct 'bodies'. A space blanket in direct contact with skin would be in the same situation, once the initial thermal shock issues leveled out and thermal conduction was taking place across the interface.

I could be wrong about this, of course, but that's how I see it.

> Although these radiant heat barriers do use a radiant-reflective surface, they also have some kind of 'air gap' (whether air, nitrogen, vacuum or whatever in the form of an open space, large bubbles or a foam) in order to allow some space for the internal body to radiate and the reflector to reflect,

I'm on shaky ground here becaseu Australian houses do not use anything like the Tyvek sheeting inside the external wall of a house. We simply don't build houses that way.

But is it possible that the air gap in houses is actually there to allow a little convection to happen, to remove any build-up of moisture on the outer wall?

>But is it possible that the air gap in houses is actually there to allow a little convection to happen, to remove any build-up of moisture on the outer wall?

There are several 'gaps' in house design. Dupont Tyvek ThermaWrap, like Reflectix, is a combination of a reflective surface and a foam, so the gap is part of the product. This gap allows no air circulation at all, of course. Therefore ThermaWrap is perforated to allow water vapor to move through it into other air spaces where the moisture can be exchanged. So the technologies are not mutually exclusive.

[Edit: My description of ThermaWrap is incorrect. ThermaWrap does not contain a foam, although its stated R-value of 2 does include a 3/4" air gap. (I assumed, incorrectly, that ThermaWrap was a kind of sheathing.) ThermaWrap is also not perforated; unlike some other pin-perforated barrier products, its vapor permeability is a function of the material. Here is the technical description, which I didn't have when I posted the above. "Dupont Tyvek 3480M ThermaWrap is a nonwoven, spun bonded, nonperforated, white-colored polyethylene sheet with an aluminum coating on one face. The membrane has a nominal thickness of 0.010 inch (0.25 mm) and a basis weight of 2.4 oz/yd^2 (81g/m^2)." The air gap, then, must be provided through some other construction material or technique.]

OK..So if I made little perferations in the mylar sheet that may help the vapor from a body to pass through, but still get the radiant heat return, all be it diminished due to the small holes in the barrier.

Thoughts

>OK..So if I made little perferations in the mylar sheet that may help the vapor from a body to pass through, but still get the radiant heat return, all be it diminished due to the small holes in the barrier.

I don't know the details on Tyvek ThermaWrap, but a competitor, TVM Reflective Ruff Rap, claims "rFOIL™ RRR is a single layer of 99.9% pure aluminum foil laminated to a 6 mil layer of polyethylene and pin perforated. … It it also adds a radiant barrier to the exterior of the home blocking up to 96% of radiant heat." Evidently some breathability can be attained for a minimal cost (4%) in radiant reflectivity; both claim about 23 perms, or 160 g/(m^2 * 24 hours). (Does anybody know if that's enough?) Ruff Rap claims an R-value of 3.8, while ThermaWrap has an R-value of 2 (including a 3/4" air gap).

I suspect "pin perforated" means a whole lot of work if you're going to do it by hand. I suppose you could spend a few hours running a space blanket through an un-threaded sewing machine, or maybe a metal horse curry comb pressing the space blanket against a closed-cell sleeping pad? Just guessing. But if you could do something similar it should take care of the moisture problem while still leaving most of the surface area of the space blanket intact.

The problem remains that, with the fleece being highly compressed between the warm body and the space blanket, there will be significant thermal conduction through the space blanket resulting is significant loss of heat. The alternative (which I haven't tried) is to sling the fleece/space blanket combo below the hammock. If you use stretchy cords to hold the pad in place it shouldn't over-compress the fleece, which will allow it to do its job as insulation. (Fleece is a bit heavy, though.)

It seems a lot easier to buy ThermaWrap or Ruff Rap as a vapor-permeable radiant barrier, rather than try to manually perforate a space blanket. If it is sufficiently vapor permeable, it may be useful to add it to an underquilt system. If placed between the hammock and a suspended sleeping pad, it might allow the condensation to accumulate on the sleeping pad rather than being held against the hammock fabric, which could make a suspended sleeping pad a more viable solution for summer temps.

>Thoughts

Good idea! I'm going to try to find some ThermaWrap, and I'll test it when I get a chance (might be a while, though).

DF, Ruff wrap has the 'higher' R-value when a 3/4" gap exists because of the ability to reflect more radiant heat. If there is not a gap it will have approximately the same thermal insulating properties of tyvek.

DF, you're also correct, radiant heat loss only occurs when there is separation of two SURFACES (note, simply stating bodies is not enough, see below) by some non-opaque substance (aka one able to transfer light waves readily, which includes IR), this is what is commonly referred to as the "air gap" required for radiant heat loss to enter the discussion. There are very few (fleece is NOT one of them) materials that will pass IR but not pass visible light. Therefor ANY opaque material will halt "radiant" heat loss (in other words there is ZERO radiant heat loss through felt too the mylar for the mylar to affect to begin with).

Also note that for radiant heat loss to occur, there has to be a significant difference in the skin temperature or the two objects (aka the two surface temps) in question.

With the fleece / mylar combination proposed the separated surfaces that the reflectively of the mylar could have an affect on the radiant heat loss between would be the fleece and the mylar itself.

Since A) the outer surface of the fleece and the inner surface of the mylar are going to be at roughly the same temperature, and B) they are in contact (and therefor there is little if any 'separation of surfaces'), there is, essentially, ZERO RADIANT HEAT LOSS for the reflectivity of the mylar to affect (perforated or not). Any similar thickness opaque plastic would work as well, and would function mostly as a wind block (note, mylar is probably the most readily available sort of plastic).

Joshua-I think that you overlooked the radiation re-emittance principal when you said, "… ANY opaque material will halt "radiant" heat loss ".

As an example, in perpetually warm climates, heating and cooling contractors routinely install reflective panels underneath the opaque roofs. They do this to reduce solar radiation into the attic space that is re-emitted from the bottom side of the roof.

This same re-emittance phenomenon applies to backpacking insulation. A non-reflective opaque membrane will absorb and then reemit a portion of the radiation it receives. Your high loft insulating jacket or sleeping bag will typically absorb most thermal radiation rather than re-emit it with an insulation thickness of ¾ to 1”. There is more or less a diffuse radiation field inside conventional insulation (down, Polarguard, Primaloft, etc.). The total absorbing surface of the fibers determines the net radiation transport. The interceptions are effectuated by a great number of fibers with a small cross section, partly absorbing, partly passing, and partly scattering the radiation.

In a high-loft, high-void space insulation (down, fiberfill, etc), yes, there will be some depth where IR can get in an "bounce around" if you will (similarly to what visible light will do). Anything solid, that's not so much the case. With most fleeces, there's not going to be a significant amount of space between the fibers for IR to pass through (just like fleece blocks visible light a lot better than 'fluffy' insulation does)

My understanding of "re-emmitance" in construction is that IR does not actually pass THROUGH the roofs (though some may penetrate), so much as the roof heat up to very high temps, hot enough to become radiation emmiters and they "re-emit" IR off of their undersides.

We really should be using two terms

"Re-emmitance" – aka the material absorbs IR energy, heats up and they re-emits IR energy from all surfaces

AND

"Pass-through" – materials that have the ability to allow some IR energy to pass through their physical structures.

Roger,
You say this idea is resurrected every year or two, but I haven't found any of those posts.

I have identified 2 issues with using these space blankets while sleeping:
a) rip easily when sewn or used heavily
b) non breathable

Therefore, two questions arise:

1) How effective are these blankets? Most say something like "reflect up to 90% of body heat" but I can't figure out what that means to a user. Could I survive in 30 degree weather with one wrapped around me?

2) I've come up with a plan, please tell me what you think:
a) Use a re-usable space blanket. These are heavier, but pretty tough.
a) cut out holes, maybe 2"x1" in the fabric in a plaid like pattern. Then sew the fabric around a sleeping bag liner, and use it as an alternative to a sleeping bag.

Am I dreaming too much? :D