By the Numbers: If You Carry a Space Blanket, Buyer Beware

[Stephen Seeber]

Companion forum thread to: By the Numbers: If You Carry a Space Blanket, Buyer Beware

Stephen Seeber tests the infrared reflective properties of space blankets.

[Jerry Adams]

thanks, that makes sense

funny how the two look the same in visible light, with our eyes, but they have different reflectiveness in IR

I wonder how that equates to R value.  What R value of regular insulation will give the same warmth as the space blanket?

And then, what is the R value per ounce?  Compared to down and synthetic.

What is the volume required?

[Erik G]

That’s very interesting! I really like those SOL blankets for groundsheets as well – they’re surprisingly durable, more so than window film in my experience. Totally different from the crinkly space blankets of old.

[Stumphges]

Thank you, Stephen.

I had one of the SOL blankets tested. Like Erik, I also found it tough and durable. However, after storage for some time (~1 year) folded up, it stuck to itself so badly that peeling it apart ripped loose the orange coating. Messy enough that I tossed it. Maybe a disposable item.

[Dave @ Oware]

does NASA put their space blankets shiny side out on their space suits?

[Brad P]

But which one prevents aliens from reading my mind?

[Todd T]

I wonder how that equates to R value. What R value of regular insulation will give the same warmth as the space blanket?

The space blanket itself will have an R-value of approximately zero.  That’s not what you asked, but it’s important to understand that an R-value relates to conductive heat transfer and characterizes heat loss (Q) as proportional to the temperature difference between two points:  Q ~ (T1 – T2).  For the particular case of a typical R-value rated material, Q = (1/R)*(T1 – T2).

How much a space blanket reduces heat loss from your body depends on the temperature(s) and properties of the surfaces “seen” by the space blanket and the view factor(s) between the blanket and those surfaces.  Are you facing a clear night sky (effective temperature very low), a cloudy night sky (temp not so low), or the leaves of the trees overhead (effective temperature close to ambient air)?  Further, radiant heat loss is proportional (in complicated ways) to the difference between temperatures raised to the 4th power:  Q ~ (T1^4 – T2^4).

R-value is just not a good metric to characterize these things’ performance.

[Jerry Adams]

yeah, exactly Todd

but at some particular condition, the space blanket will reduce heat loss by some number of watts

and there’s an R value of insulation that would reduce heat loss by the same amount.  That would be interesting to know

maybe two conditions – under a clear night sky, and under cover (clouds, tree, or tent – I think those would be similar)

or, another way to tie radiative heat loss to reality would be how many degrees colder would you be comfortable, or survive

a space blanket also reduces heat lost to wind or evaporation – it’s complicated

[Todd T]

maybe two conditions – under a clear night sky, and under cover (clouds, tree, or tent – I think those would be similar)

I’d think it’s more of a continuum.  What if you’re camped next to a tree that blocks about half your view of the clear sky?  Or a quarter.  Or 81.7%?  As someone said above, “it’s complicated.”  :-)

[Jerry Adams]

yeah, that’s why two cases are good, zero cover and 100% cover

then reality would be somewhere between those

[Jim Morrison]

Excellent article.  I have often wondered about such things.  I was surprised that the SOL blanket was considered more effective with the silver side out, orange side in.  I use one of those as a simple lean-to shelter in good weather.  I will reverse my color scheme.

thanks… Jim

[Dave Heiss]

IR reflective materials, when used in clothing or, in this case as a blanket, require an air gap between the skin and the ambient to perform correctly. If the space blanket is in contact with the skin, no IR reflection can occur and convection/conduction will take over as the predominant modes of heat loss. But if the space blanket is worn over clothing or as a layer within clothing, it can work rather well. It was a few years ago and I don’t remember the exact results but when our lab tested IR reflective materials on a guarded hotplate they found that having at least a 0.25″ air gap between the skin and the environment was necessary to generate any kind of reduction in radiant heat loss rates.

[Ken (summmitboy)]

Maybe I’m missing something, but if the silver side of the SOL blanket is more reflective than the orange side, wouldn’t you want that to face the warm body that is emitting the IR, i.e. inwards, rather than the ambient (outwards)? You’d want the more reflective side facing out in a hot environment, e.g. desert with no shade…

[Michael B]

I understand the difference between the two products being significant, especially if they cost the same. It seemed the F&S was still successful at reflecting some of the heat though. If the scale on your images is an indication, you mentioned the kettle is at 120, while the ambient is 72. The temp in the image with the F&S blanket looked to be in the 82-85 range, only a little higher than ambient. You didn’t show the control image, which I assume would have shown a bright red 120F kettle in the middle. Furthermore, you mention a good air gap between the blanket and the hot body is required for good performance. What was the air gap used for this experiment?

Would the F&S (or Walmart brand, in my case) be more effective than nothing, and if so, by how much? Enough to warrant keeping it around rather than going out and buying an SOL blanket (nice name, btw). These are after all, survival tools, not typically items I am going to use frequently, as as such am less interested in what seems like a relatively minor difference in performance from a survivability stand point (72f vs 82-85f with a 120f hot body). A human body is not 120F, so it is safe to say that the differences between blankets will be less noticeable with a colder hot body.

Also, as was already asked, why would you place the foil side out rather than in, if the intention is to reflect heat back to the person wearing the blanket. Seems counterintuitive.

[Matt Dirksen]

“wouldn’t you want that to face the warm body that is emitting the IR, i.e. inwards, rather than the ambient (outwards)?”

+1 to that.

On that note, with the blankets stretched on top of the kettle, isn’t the emitted energy we’re observing through the thermal camera a result of conduction through the material?

[Jerry Adams]

interesting – Dave answered the question then Ken and Michael asked it, usually, people answer questions after they’re asked : )

Neo air mattresses have a reflective layer inside with air spaces on both sides, that works best

I think one of the SOL products has several layers of reflective material with elastic to create internal air layers

The SOL package shows the person wearing it with the orange side out.  I’ve measured crudely and both sides were about equally effective.  If I wanted SAR to find me and I was in the open, I’d put the orange side out, otherwise the foil side out because it may be more effective.

They give out space blankets to runners after a race, and to refugees being rescued, so they must be somewhat effective

I think a useful analogy is like if you have a light bulb, it’s hot.  You can see the visible radiation and there’s also IR.  If you put a space blanket on, it becomes dark.  It quits radiating heat away.

[Michael B]

Jerry, this forum is weird in that when I replied, for whatever reason, Dave’s response and subsequent ones were not shown. I am not certain what that is all about, but it has happened several times in other threads for me also.

[John Conley]

Great string of information and comments. There are so many possible variables that the “correct” answer for any particular situation may still be a bit ambiguous. I find these sorts of articles very useful. Thanks,

[Greg Mihalik]

^^^

“They give out space blankets to runners after a race, and to refugees being rescued, so they must be somewhat effective”

Or the marketing has been effective.

They would deflect the wind/breeze to reduce evporative cooling, but in direct contact with back, shoulders and arms would provide no radiant loss mitigation. If were sunny a black garbage bag might be more effective.

 

 

[Jerry Adams]

Michael, maybe it has something to do with the changes in site software, we should expect weirdness

[Jerry Adams]

I like your skepticism Greg : )

I think the analogy of a hot lightbulb radiating heat is good.  When you put a space blanket over it it no longer radiates away heat like that.

[Jerry Adams]

I’ve carried a space blanket around for decades.  Never used it.  I’ve opened it up enough to verify it’s not sticking to itself, ready to shred into strips.  I forget what brand.  Pre SOL.

[Mark Verber]

Some additional math for the party:

https://openoregon.pressbooks.pub/bodyphysics/chapter/space-blankets/

[Nancy K]

Just curious…..would using a space blanket hinder SAR because it would block or lessen thermal imaging?

[Jerry Adams]

That’s really good, thanks Mark

“Chapter 93 SPACE BLANKETS”

Oh no, you’ve just given me days of reading material.  93 chapters????

“Space blankets reflect the electromagnetic radiation emitted by your body back to you, rather than letting it escape, thereby reducing the rate at which your body loses thermal energy to the environment.”

No, it doesn’t reflect back, it just prevents you from radiating, but that’s okay, this is just an analogy to help us humans understand.  No reason to argue about this : )

“Therefore the space blanket saves you 200 W of radiative heat loss and 1100 W of convective heat loss, leaving only the 160 W of conductive heat loss.”

That’s an answer to my question.  What is the real world effect?

By far, in this example, the biggest effect is it stops convection – the wind blowing away heat.  A sheet of plastic would do that just as well.

A body generates maybe 200 W of heat if you’re a little active (MET = 2).  If the space blanket prevents 200 W of heat loss that’s significant.

160 W was lost due to conduction.  Calculated in previous chapter.  I don’t know what clothing was worn for that example.  But, in this case, with the space blanket eliminated radiative heat loss, you may actually be warm.

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