In By the Numbers, Stephen Seeber turns a critical eye towards fabrics and materials by testing for claims, degradation, and more.
Introduction
Not all space blankets are created equal. Even if they look identical to the eye, their infrared wavelength performance may be inadequate. I am in the process of building an instrument that requires radiant heat barriers that are reasonably robust. So, I thought I could cut up space blanket fabric and install it on the device. This provided me with an opportunity to test some space blankets and determine which one had the best infrared reflective properties.
I went to Dick’s Sporting Goods. They had two different space blankets available. They both sold for around $15. The first is made by Field and Stream and is called Survival Reflect Blanket. The package claims it conserves up to 90% of body heat. The second is made by Survival Outdoors Longer (SOL) and is called the SOL Survival Blanket. The same material is used in the slightly larger, and more usable, SOL Emergency Blanket. The package claims it reflects 90% of heat. I purchased both for my test.
How a Space Blanket Works
There are several ways that heat is transferred from your body to the environment. When you are sitting on a cold rock, heat is transferred via conduction through your clothes that are in contact with the rock. The heat from your body can also be transferred by convection which means that heat from your body warms adjacent air, which then carries the heat away. When you are sweating, the sweat can evaporate. Evaporation is the process of changing from liquid to gas, and it requires heat energy which can be extracted from the skin.
Finally, any surfaces (skin or clothing) can transfer energy through radiant exchange with the ambient (as long as the ambient temperature is lower than your body or clothing surfaces). Radiant heat exchange moves heat from a hot surface to a cool surface by means of electromagnetic energy in the infrared (IR) spectrum. This spectrum extends from 0.7 to 1,000 microns wavelength. Heat transfer from the body occurs primarily in the near and middle portions of the infrared spectrum—0.7 to 25 microns.
Infrared energy, like visible light energy, can be reflected. The ability of an object to reflect infrared energy is described by a physical characteristic called emissivity, and it can be measured. A material with an emissivity of 0 is a perfect reflector—it emits no infrared energy as a result of its own temperature, but will reflect any infrared energy that strikes its surface. A material with an emissivity of 1 provides no heat reflection but is a perfect emitter of infrared energy resulting from its own temperature. All real materials have an emissivity of greater than 0 but less than 1. Aluminum foil can have an emissivity of 0.05. This means that it emits almost no infrared energy as a result of its surface temperature, but it reflects nearly all incident infrared energy. Electrical tape can have an emissivity of 0.96. It will reflect almost no incident infrared energy.
If a material that is reflective of infrared is placed between the source of heat (your skin, for example) and the ambient, transmission of heat to the ambient can be minimized and your rate of cooling will be reduced. If the infrared material is placed directly on the skin with the reflective surface facing the skin, reflection does not occur. Instead, conductive heat transfer from the skin will heat the material. If the outward surface of the material has a high emissivity, the heat conducted into the material from the skin will be emitted to the ambient and you will continue to lose heat. Space blankets are often reflective on both sides. This means, where the blanket is warmed through direct contact with a body or clothing surface, the reflective outer surface of the fabric will limit or prevent radiant transfer that results from conductive heat transfer into the space blanket.
The most effective radiant blanket will be highly reflective of infrared on both sides. A less effective heat blanket will be highly reflective on just one side and that side should usually face the cold ambient. An ineffective space blanket will have moderate to high emissivity and will not prevent the maximum transfer of radiant energy.
Well, how can you tell which is better when you buy one? You cannot. Space blankets can have an identical appearance in visible light but can still perform very differently in the infrared wavelengths. The only way to know is to measure the emissivity, which requires, at minimum, a thermal imager.
Analysis
To test them, I mounted the space blankets on a permeation kettle with water heated to 120 F (48.9 C), while the room ambient is around 72 F (22.2 C). Visual inspection shows that both materials have the same appearance. With a thermal imager, a qualitative comparison is simple: A highly reflective object will show the ambient temperature when observed through the thermal imager. To that end, a piece of crinkled aluminum foil is placed on each space blanket. If the infrared image appears warmer than the aluminum foil, then the emissivity is too high for effective blanket performance. We see from the images that the Field and Stream blanket material is not a great reflector.
However, we see that the apparent blanket temperature of the SOL material matches the apparent temperature of the aluminum foil. Thus, the SOL blanket will be the better reflector and can be expected to provide good performance.
When we measure the emissivity in 8-12 microns, we find the emissivity of the Field and Stream blanket is 0.64, and the SOL emissivity is 0.13. Therefore, the SOL blanket is a much better reflector.
How to Use the SOL Survival/Emergency Blanket
Both sides of the Field and Stream blanket have a similar finish. The SOL blanket has an orange foil appearance on one side and silver on the other. This is seen in the following image:
The measured emissivity of the orange side is similar to that of the Field and Stream Blanket. So, the orange side is a poor reflector, compared with the silver side.
I suggest, when using the SOL blanket, the silver (shiny) side should face the ambient (outside) environment. This will provide the lowest possible radiant heat transfer. The product photograph on the package shows two people huddled together wrapped in the blanket with the orange side out. One clear advantage of the orange side out is that visibility is improved if someone is searching for you. A minor benefit of using the blanket with the orange side out is the absorption of some solar gain. However, since the orange side is still somewhat reflective, it may take some very strong sunshine to feel its heating effect.
Never use a space blanket under another layer (unless you want it to function as a vapor barrier). A layer placed over the space blanket would eliminate most of the reduction in radiant heat transfer.
Related Content
- More by Stephen Seeber.
- Read Rex Sanders’ interview with Stephen for the Standards Watch column.
- In the forums: Should I Bother With a Space Blanket?
Discussion
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starting at chapter 1
this is really good. Open source textbook, intended for schools. They have test questions to see if you understand…
this is sort of like wikipedia or Kahn Academy. Everything doesn’t have to be from a for profit company trying the make everyone rich.
Not that I have anything against for profit companies trying to make everyone rich. I’ve worked for several. Never got rich though. Did make a living…
all in perspective…
But back on topic – seem like from the following comments, my $1 Walmart space blanket (provide it opens up and doesn’t stick to itself) should provide me with an effective emergency option to keep warm, even if it isn’t the least emissive material. I put it, along with a $1 Walmart poncho, together into my fanny pack. Takes up less space than my wallet.
The material used for space blankets is not “foil.” It has vaporized aluminum sputtered onto the surface, resulting in ~10 angstrom thick islands of Al.
If you hold space blankets (potato chip or Chips Ahoy pouches) up to a bright light, you can see light filtering through. Instruments measure this “optical density,” which will depend on the metallization process and any subsequent abuse.
The metallized film is typically laminated to second ply to bury this fragile layer in the middle for added protection.
Comparing two blankets under a bright light can be a quick way to compare the effectiveness of two films.
The conduction vs. radiation heat loss subject is most interesting. In a survival class I took years ago they handed out thin yellow plastic bivy bags. They seemed effective and I always assumed this was because of the envelope they made that held the pocket of warm air around your body. I.e, prevented convection to some extent. The yellow color, while not perfect, also contributed to less heat through radiation, I think. Some mountaineers used to wrap up in coated nylon tarp (they called it a “bivy sheet”) with surprising success.
Jerry,
care to elaborate on blankets preventing you from radiating rather than reflecting radiation? I think you have it reversed
It seems In my opinion to be semantics. A hot body emits IR. The blanket can do 3 things: reflect the IR, absorb the ID, or allow the IR to pass through. Preventing a hot body from losing this energy to the atmosphere seems to be accomplished by the first two, but in practical application, the space blanket has so little mass that it is unlikely to absorb much energy. It is more likely to either let it pass through, or reflect it back into the environment surrounding the hot body.
yeah, it’s really semantics. Bottom line – if you put on a space blanket you’ll lose less heat due to radiative heat loss. 200W less in that one example.
the space blanket will heat up to the same temperature as your outer surface due to conduction (ignoring that there is a little air space so it’ll be a bit cooler)
then, since the space blanket has low emissivity, it will not radiate IR
as compared to whatever you’re wearing if you don’t have a space blanket, which will radiate IR
I suppose the space blanket reflects IR back to your body but that’s moot, since the space blanket is (almost) touching it will be the same temperature as your body due to conduction
if you look at someone with an IR camera, they’re glowing – emitting IR – that’s the heat that you’re losing
do the same while wearing a space blanket – they’re dark – not emitting IR – not losing heat
So still have the question about NASA space suits, do they have the shiny side to the inside or out?
I have an extremely warm jacket-New Balance Fugo, that Richard Nisley tested as one of the warmest for it’s weight. It has a reflective finish on the lining fabric.
Some of the biggest heat loss as mentioned happens from convection and sometimes evaporation. Even a cheap space blanket, poncho or trash bag reduces those a lot. The place I have seen cheap space blankets fall down on the job was in stormy weather where the wind just tore them apart. A leaf bag makes a better shelter in those situations as it is tougher and can form either a vest, jacket or mini tent.
It is interesting how this topic comes around every few years.
If I recall correctly, many years ago it was determined (via thread talk and analysis) that the “regular space blankets” were basically worthless (other than a lightweight, fragil ground cloth). I know the product hasn’t changed- has the science changed or gotten better?
great…
now I’m getting ads for SOL products : )
(which is fine, I’d just as soon see SOL ads as something else)
“I’ve carried a space blanket around for decades. Never used it. ” – Jerry
Yeah, me too. I didn’t always have one with me, but often did. And I never spent $15 on one – they’re $1 each online.
And I’ve never used one for anything.
But when I pack a trash-compactor bag, sometimes I use it. Most often to haul trash with, from camp or the litter I’ve picked up along the trail. But I’ve also gnawed a head- and arm-holes out of it and worn it as a make-shift poncho. And I’ve dug a hole and lined it with the trash bag to create a sink to wash my hair and other bits in.
So two UL principles:
1) don’t bring what you don’t use and
2) take multi-use items
have caused me to abandon space blankets for Hefty Garden or Trash-Compactor bags.
“great…now I’m getting ads for SOL products : )”
I periodically post about solar showers, solar showers, solar showers, solar showers because the models in those ads are cuter.
I’m a bit unclear if men are even allowed to use them.
There has been a lot of good discussion here and I think a good number of questions have been answered. If it is unclear why I recommend the orange side in, you might want to reread, I cover this in two different places. But, as many have inferred, it is all about contact with a surface producing conductive transfer and preventing radiant transfer. Concerning space suits. The radiant barriers used in space suits and satellites are typically multilayer insulations. You get multiple layers of thin, low emissivity fabrics, separated by a scrims to minimize physical contact between each layer. As long as the scrim grid is offset a bit between each successive layer, conduction between layers is minimized. In space, one need not be concerned with convective losses, at least for a satellite. I don’t know how sealing is accomplished in a space suit and where the air seal is in relation to the radiant barrier. Suffice it to say that if the radiant barrier sits in a vacuum, it will perform with greater efficiency. Air in a tiny space does not support convection but still supports conduction. Concerning R value beneath a tree, a cloud or the open sky. Each of these describe different ambient conditions. The clear sky will have a very low radiant temperature. The cloudy sky will have a warmer temperature. The clouds trap radiant energy from the earth that would otherwise travel out to space. which results in higher radiant temperatures than a clear sky. Locating your self beneath a tree, especially a with leaves, would have the warmest radiant temperature. At night, the tree ensures the least heat loss because the branches and leaves capture heat radiating from the ground to the sky after the sun sets so they have relatively high radiant temperatures. The tree structure shades the ground from the heat sink that is the sky dome. The heating at the tree is part of a diurnal heating and cooling process, so the radiant temperature of the underside of the tree might be more or less than ground depending on time of day and cloud cover. In each case, the delta T from the space blanket surface to the ambient will be different for each case, so the heat loss will be different. So, how does this impact the measured R value? I am not sure. Fibrous insulation thermal resistance is typically impacted by changes in their average temperatures. This is largely due to the thermal properties of the trapped air. Thermal conductivity of air increases with temperature. It is also impacted by air pressure changes. Effective emissivity of a space blanket may also change with temperature, as can conductivity of the sheet material. One can search the internet to find these characteristics. However, I would guess that the impact of temperature differences from extremes of environment will have a far greater impact on heat transfer than changes in emissivity or conductivity of the space blanket itself. Convective losses due to wind speed will also have a substantial impact on the effectiveness of your space blanket. So, how effective will a space blanket be in real life? As we can all agree, that is a complex question. But, you will be happier in a location shielded from the wind and shaded from the clear night sky. If the surface is kept dry, you will keep the effective temperature of the blanket from dropping due to evaporative cooling. Minimize contact with the ground to minimize conductive losses. Now, some have referred to missing solar gain due to the reflectivity of the blanket. That could occur. There are always tradeoffs and in that case, you’d probably want the orange side out. Understanding some fundamental heat transfer mechanisms will at least help you use your space blanket as effectively as possible under the conditions of your emergency.
I used to work for a company that had invented soft fabric coolers. They used a reflective mylar with polyester needle punched thru it for the insulation. This kept the mylar film from touching the fabrics on either side. I proved very insulating for the thickness compared to foams and other synthetic fills. Just like in our sleep systems, the bottom of the coolers used closed cell foam for resistance to compression. That insulation wouldn’t work well for clothing or sleeping bags because the twisting and turning would have torn the mylar film, where in the cooler bags it was supported and protected by heavy duty nylon fabric. I still have a couple of the bags and they work well even tho 35 years old. I think the YETI bags and stuff just use foam today, but are much thicker.
At one time someone offered a bivy sack with several internal layers of space blanket held apart by noseeum netting.
I think it was the Rain Shed that also sold the mylar/netting already quilted together for making your own projects. I tried some in overmitts, but could not tell it made enough difference and it was fragile.
Both of these are crap to use unless in a real emergency. Go with the SOL Escape.
[edited – MK]
Like at least two of the above, I’m thinking, why orange on the inside…
“Space blankets are often reflective on both sides. This means, where the blanket is warmed through direct contact with a body or clothing surface, the reflective outer surface of the fabric will limit or prevent radiant transfer that results from conductive heat transfer into the space blanket.
The most effective radiant blanket will be highly reflective of infrared on both sides. A less effective heat blanket will be highly reflective on just one side and that side should usually face the cold ambient. ”
In the field, the space blanket will be in contact with clothing / but also stretched over gaps as you hug it around you. This means then, I think, if I am following the science, that heat loss on the inside (from you, wrapped in the blanket) will not largely be radiant but conductive, so the internal surface emissivity is not so important. The space blanket will ‘acquire’ heat from your body via conduction. A low emissibity on the external surface of the space blanket means that little of that heat now residing in the fabric of the space blanket will be lost to the outside, to quote “A material with an emissivity of 0 is a perfect reflector—it emits no infrared energy as a result of its own temperature, but will reflect any infrared energy that strikes its surface.
So keeping orange on the inside will reduce heat loss that travels via conduction into the space blanket, and potentially thereafter into the air via radiance. The low emissivity of the silver side should reduce that.
Is there also a convection transfer of heat from the space blanket or does that require radiance (to transfer energy to the molecules of air) which will be impeded by the low emissivity?
“…heat loss on the inside (from you, wrapped in the blanket) will not largely be radiant but conductive, so the internal surface emissivity is not so important…”
I think that all makes sense
convective transfer from outer surface – the wind blows air at the surface, which warms up due to conduction (doesn’t matter what the emissivity is), and is then carried away
With No Space Blanket:
1. Conduction -> environment => Personal Heat Loss via Conduction
2. Convection -> environment => Personal Heat Loss via Convection
3. Radiation -> environment => Personal Heat Loss via Radiation
With no Blanket you lose heat from 3/3 types of heat transfer
With Space Blanket IR Reflective Side In
1. Conduction -> Blanket -> environment => Personal Heat Loss via Conduction, blanket loss to environment via radiation, conduction, and convection.
2. Convection -> Blanket -> environment => Personal Heat Loss via Convection, blanket loss to environment via radiation, conduction, and convection.
3. Radiation -> Blanket reflects and absorbs no energy -> Person => No Personal Heat Loss via Radiation
With shiny side in you lose heat from 2/3 types of heat transfer, conduction and convection. The blanket loses heat to the environment from 3/3 types of heat transfer, so some of the heat it gets from you via conduction or convection is lost via radiation.
With Space Blanket IR Reflective Side Out
1. Conduction -> Blanket -> environment => Personal Heat Loss via Conduction, blanket loss to environment via conduction, and convection.
2. Convection -> Blanket -> environment => Personal Heat Loss via Convection, blanket loss to environment via conduction, and convection.
3. Radiation -> Blanket -> environment => Personal Heat Loss via Radiation, blanket loss to environment via conduction, and convection.
With shiny side out you lose heat from 3/3 types of heat transfer, but the blanket dumps the heat it got from you via radiation to the environment via conduction and convection
So with the reflective side out the blanket wont let out heat via radiation, but you still do. Since the blanket can still lose heat via conduction and convection, with the shiny side out there is still a pathway for the energy from personal radiation to escape to the environment. With shiny side in you are in theory blocking that pathway so that your body can only lose heat by conduction and convection.
Shiny side in: will the heat saved via blocking radiant loss simply be redirected out via conduction?
I suppose its not the number of avenues that’s crucially important but the overall energy lost, which requires measuring in the field or replicated in the lab.
Too many absolutes in the foregoing descriptions. These shiny things don’t block radiative heat transfer, they merely lessen it. Just like insulation doesn’t block conduction, it lessens it. I think there are too many variables involved for generalizations to be very useful.
Or going out prepared (warm clothes and a way to keep them dry) so you won’t need emergency shiny things. :-)
I think it folly to not have a certain reasonable amount of emergency items. You cannot plan for everything and also carry a reasonably light pack. Pack for what is expected, and then this extra one ounce item may help in the chance what was expected turns into something more than what was expected. If you want to pack extra, then that is your choice. I prefer to keep as little as is necessary in my pack/on my person.
+1 – I carry space blanket for unexpected
I think the orange side is somewhat reflective, just not as much as the shiny side. Regardless, put shiny side out
You can not tell emissivity by looking at it, you have to measure it
Stephen,
Thank you for your article. It will greatly help with future purchases.
In the mean time, what I have are called “heat sheets,” for emergencies. They were what was being sold in the shops when purchased. The company’s website states:
“Heatsheets blankets can be used silver side in for cold weather, reflecting up to 90 percent of the user’s body heat back to them. When facing an emergency situation in the heat, Heatsheets can be used silver side out, deflecting heat away from the user to allow the body to stay cool.”
Would very much like to see your comments about the above. It does not seem entirely consistent with your article. Thanks.
“Heatsheets blankets can be used silver side in for cold weather, reflecting up to 90 percent of the user’s body heat back to them. When facing an emergency situation in the heat, Heatsheets can be used silver side out, deflecting heat away from the user to allow the body to stay cool.”
Although you didn’t ask me : )
That reflects a common misperception about space blankets.
And if the non silver side is less reflective in IR (higher emissivity) then it’s wrong advice.
If it was hot I think you’d be better off not using space blanket. At least if you could get in shade. If not, maybe you could hold the space blanket up so you could get it in its shade
Another source is this article, that claims that conduction and convenction makes the intuition somewhat more complicated. The conclusion is: put the reflective side directed to the outside if the weather is calm, and put it directed to inside if the weather is windy and wet. It does contains some experiments of putting a hot bottle in the freezer (a calm environment), on which putting the reflective side inward facing indeed does helps retains the heat better.
https://cdn.ymaws.com/www.eps.org/resource/collection/016775D4-8888-474D-887F-3E33AEA5E6D0/EPSPED_SurvivalBlanket.pdf
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