Aug 29, 2012 at 3:42 pm #1293495
So i recently poseted a question about down for making a quilt and got great responses. Then i had a brainwave if you cut up a space blanket to half inch squares and mixed it with the down could you use less down in a quit/sleeping bag/anything?
Space blankets reflect heat back so if there are a load of tiny reflective squares reflecting heat back would that make a quilt or whatever more effective?
Any thoughts/experience with this?Aug 29, 2012 at 3:50 pm #1907335
drowning in spamMember
A big part of why a space blanket works is because it doesn't breathe. I doubt throwing a bunch of space blanket shrapnel into your quilt will make a measurable difference other than increased weight.Aug 29, 2012 at 4:14 pm #1907347
just a guess…
What pops into my mind is an image of a piece of space blanked coated in condensation.
But do keep in mind that I don't do anything "scientifically" , just a guess
FrancoAug 29, 2012 at 4:25 pm #1907355
You could get perforated foil, used in many hydronic heating systems (not sure of technical brand name, though i have a huge roll of it sitting in my basement). Heavy, but it would prevent condensation buildup. Seems better use of weight might be to add more down.Aug 29, 2012 at 9:35 pm #1907464
I apologize if this sounds a little patronizing; it's not meant to.
However, while you're techincally correct about the space blanket reflecting heat back towards you, the vast majority of what a space blanket does to keep you warm is prevent evaporation (i.e.: it's a vapor barrier).
Something like 95% of your radiated heat (what the mylar in the space blanket actually reflects) is absorbed by your clothing. Which means that only ~5% of your radiated heat actually reaches the space blanket to be reflected. And, the majority of heat loss across normal temperature gradients (from your body temperature to the surrounding environment, say a 120* F difference at most) is going to be through convection and conduction, not radiation.
What a space blanket does, though, is prevent the evaporation of vapor from your skin. It's the same as wearing a trash bag (or non-breathable rain wear); it makes you feel hotter since the state change of the water to vapor isn't carrying away any heat from your skin.
So, mixing in pieces of space blanket into a down quilt won't do much, if anything at all. More down would be a better use of weight and bulk at that point.
However, if you were to make the inside of the quilt one large space blanket (or other non-permeable material like sil or cuben), you would get perceived warmth out of it. You'd also wake up wet, which is the downside to vapor barriers. They take a lot of skill to use properly (and I don't pretend to have that skill).
Hope it helps!Aug 29, 2012 at 10:00 pm #1907475
@riemanniaLocale: Northeast Georgia
Met a fisherman/hammocker in Alpine Lakes Wilderness who had taken an old Kelty sleeping bag, removed all the down, then put it in a 3 mil painter's plastic (polycryo-type stuff?) shell and taped it up (having no sewing experience), no baffles, not sewn through. Just a non-breathable bag of down. Seemed weird, but he loved it; I think it was sub-10 ounces.Aug 29, 2012 at 10:37 pm #1907483
drowning in spamMember
Polycryo (window film) is more like 0.75 mil.
That's crazy that that plastic down bag worked for him.Aug 30, 2012 at 6:22 am #1907516
@riemanniaLocale: Northeast Georgia
He was a strange bird, admittedly. I didn't see the quilt myself. He also said he once tried to store his denatured alcohol in a balloon. Apparently it did not stay in the balloon.
But I suppose standard UL items I use like tarps and quilts and frameless packs seem pretty strange to many other backpackers as well. Experimentation is at the heart of it all!
It seems I misread the initial post. I had assumed the space blanket would be the shell of the quilt. I have my doubts about mixing in the space blanket with the down; the best I could imagine it would do it terms of warmth would be by decreasing breathability (seems unlikely at that), which would increase vapor retention in the down and then result in a colder bag. It seems to be the only way one could use the space blanket to keep the quilt any warmer would be to use it as a true vapor barrier with the shell (keeping your smelly vapors out of the down), as I had originally misread. But still, that seems like a bad idea – those things are not known for durability.Aug 30, 2012 at 6:54 am #1907522
Not too odd. As long as you have non-moving air (so that convection currents don't defeat the warmth), that works. A Garlington Insulator works on this principle. The only reason it hasn't caught on widely is that it prevents evaporation of condensation (making your back feel sweaty under certain conditions) in the same way a pad can. Since pads are about the same price and readily available…well…Aug 30, 2012 at 7:54 am #1907547
Thanks for the replies guys, was just doing some brain storming and was wondering if it had been done :DAug 30, 2012 at 8:48 am #1907555
"Something like 95% of your radiated heat (what the mylar in the space blanket actually reflects) is absorbed by your clothing"
Absorbed AND then emitted back, apprx half back to your skin from the inside clothing surface and half away from you (towards the next surface). Though you typically have enough layers that adding a space blanket BETWEEN the layers will make nill difference. This application is one of those. This doesn't mean that space blanket, used correctly, can increase insulation value.
In order for a space blanket to help, it needs to go btw two surfaces exchanging meaningful radiative heat (imagine your bare skin to clear sky). If you add a layer (say down jacket). The outside layer of the jacket is still losing radiative heat to the sky. A space blanket on top of that would cut a heat radiated to the sky and allow the outside layer of jacket to increase its temp.
Now, If you put the space blanket between your skin and the jacket inner…it wouldn't make much of a difference b/c there is not much heat loss occuring via radaition btw your skin and the jacket inner (the are relatively close in temperature). The OPs idea is akin to adding the space blanket btw the jacket and your skin and therefore wouldn't result in any meaninful gain in insulation value.
"Correct" application (for a cold backpacker) will place the space blanket material on the outermost layer facing a significantly colder surface (eg, a bivy, outermost layer of insulation, tent surface). Also, I would want the reflective surface (if only aluminized on one side) to face away from me NOT towards (so alumanumized surface would face the sky).Aug 30, 2012 at 6:54 pm #1907773
@ckrusorLocale: Northwest US
James, your assessment sounds right to me, except in one detail. I don't think a layer of clothing would radiate half of absorbed body heat to the environment (or to the next layer) and half back to the skin. That's a theoretical concept that makes assumptions that do not hold in the case of apparel (it works much better for applications like thin film insulation in a vacuum).
A layer of clothing is not the same temperature on the inside surface and on the outside surface, even for base layer clothing. Radiation of heat energy is determined only by temperature and emissivity, so those two surfaces cannot radiate the same amount of heat. Much more IR emission will occur from the warm inside surface back to the skin than from the cool outside surface to the environment.
I agree about using a space blanket so the aluminized surface faces out. People understand the high reflectivity of the aluminized surface but forget about its low emissivity.Aug 31, 2012 at 11:08 am #1907960
Colin, you are correct. For a warm body in a colder environment the clothing will always send more heat back to you than it will away from you bc of the temperature gradient.
On first reading, I interpreted John's post to mean a baselayer worn inside a sleeping bag will keep 95% of IR emitted from skin from making it to the insulation…but on re-reading I think he was speaking more generically (ie all layers of clothing insulation).
So even though I wasn't clear I was thinking about a baselayer…which I can't imagine would be more than 5C warmer on the inside than the outside in typical scenarios. Even at about 10C differcial I calculate the split to be 54/46% inside/outside emission.Aug 31, 2012 at 9:46 pm #1908124
Yeah, I was speaking as if all of the clothing and insulation in total would probably absorb something like 95% of the radiated heat.
And (forgive me here; high school physics was a decade back) most of the heat given off by one's body is carried away through conduction or convection, no? That's the premise that vacuum Thermos containers operate on and the reason that manned spacecraft need such huge honking heat radiators.
So, the majority of what a space blanket is doing is preventing the movement of water vapor away from one's skin (carrying away the heat that was used for the state change of the water), if I understand things correctly.
I could be wrong, though…it's been known to happen…more times than I'd like…Sep 1, 2012 at 10:47 am #1908210
@ckrusorLocale: Northwest US
John, I think all of that sounds correct. It's consistent with the good arguments I've always heard about radiant barriers (that most of the effect comes from reduced evaporative cooling).
I wonder about it, though. Would Neoair pads and Blizzard Bags be just as effective (or almost) without aluminization? A few large air spaces, divided by a few (<5) layers of film, shouldn't, by itself, be a very effective insulator. But the current generation of Neoair pads are just that, and they have among the highest claimed R-value per unit weight of any sleeping pads. The same is true for the Blizzard Bags. This causes me to have nagging doubts about the good, educated arguments I've heard that assert that any benefit from radiant barriers is minimal.Sep 1, 2012 at 7:07 pm #1908341
"most of the heat given off by one's body is carried away through conduction or convection, no?"
Once we start talking up %'s of heat loss by mode, the figures become very situational dependant…so it is tough to generalize. But in general :), the combo of evaporative losses and wind driven convection losses (forced convection) are the most important to protect against. This is typically the purpose of your shelter.
I don't have a good refence but generally speaking heat loss via radiation is on par with that off "natural" convection — I calculate a break even at about 20C temperature differencial.
If you are camping out in the open on a clear night, the radiative mode of loss is over a much larger temp differencial than the convective mode (you are xfering IR heat w/ ~ negative 70C autmosphere). This is a large part of why a desert can have such huge temperature swings.
If you wrap a space blanket around you skin inside a quilt its probably doing little more than keeping some moisture out of insulation. If you add it to the outmost layer, I would wager it is probably, nearly, worth its weight in down.Sep 2, 2012 at 7:50 pm #1908575
@tonymullLocale: Western Washington
back in like 1972 I bought from Campmoor an "Astronaut Sleeping Bag" Nylon shell, very small amount of some sort of fiber and stuck in and on the fiber tiny pieces of space blanket material. $17.99. I still have that thing. Total fail as a stand alone bag, but as a liner or with something over it, even a sheet, it actually works pretty good. Far better than an liner of similar weight or thickness. It's melted in a couple of places and stained here and there, but it has lasted many hundreds of uses. My wife has loved it as a warmer for her side of sleeping bags and I've tossed it in many times and been glad I did. Completely breathable.
Now SOL makes a 'breathable space blanket bivy' that in my use of it is nicely breathable, light, strong…and unfortunately a little snug for me. Paired with a very light sleeping bag or just with some extra clothes, it's great for summer here in the Northwest…and for $50, well….Sep 4, 2012 at 1:14 pm #1909015
@retiredjerryLocale: Oregon and Washington
I think Colin and James are correct
The aluminum layer has to be on the outside
I've been fooling around measuring this
The heat loss is the 4th power of the temperature of your body and the ambient temperature. The temperature of a cloudless sky is near absolute zero. That's the case when you're going to lose a lot of heat, is when there are few clouds or trees blocking the night sky.
In this case, an aluminum shell will allow you to be comfortable at maybe 10 or 15 degrees colder air temperature.
A piece of fabric blocks maybe 50% of the IR. If you put a layer of fabric over the space blanket you lose about half the radiant heat advantage.
I don't see how aluminum layers, like in a Neo Air would be much better than any other material. There's only a few degrees difference between inside and outside each aluminum layer, so there will be little radiative heat loss.
But, when I measure, it seems like the aluminum layers are doing something. And aluminum is a light material to make a layer with…
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