Jun 25, 2013 at 5:34 pm #1304613
@maiaLocale: Rocky Mountains
Companion forum thread to:Jun 25, 2013 at 7:16 pm #1999777
@richard295Locale: San Francisco Bay Area
Your testing methodology is MUCH improved from your first published BPL article… congratulations!
I just did a speed read as opposed to a detailed analysis. The first thing that raised a flag was your comment, " But I’ve noticed that every time I measure down, I get a different value."
The ambient humidity is the most probable cause of your down insulation measurement variance. You used a Brunton ADC Wind Compass Pro (?) to calibrate your temperature sensors and this device has a humidity logging function. Did you use it to record the ambient humidity level for the duration of each test? If so, do a correlation analysis. If not, in the future using this sensor should show you an inverse correlation between the ambient humidity and the down clo value.Jun 25, 2013 at 8:05 pm #1999790
My ADC Wind has no humidity measurement. Maybe I have a humidity measurement device somewhere – next testJun 25, 2013 at 9:24 pm #1999819
Big fun. Two comments: on the perforated reflective material, it used to exist and was called Texolite. Needlepunched aluminized Mylar. Don't know if this is still made but it was used in sleeping bags by Moonstone Mountaineering among others. I have a MYOG bag made with nylon lining, polarguard insulation, and texolite shell. Known to my kids as the "Special Sparkle Bag".
Texolite had issues with durability – the aluminization wore off and eventually washed off.
On the reflective tent fabric – Stephenson's has it as an option, and they may still sell the fabric (they used to)Jun 26, 2013 at 1:10 am #1999857
@canonshooterLocale: Western Washington
So, if I am understanding your report, a person could really improve the warmth of their quilt or even their tent by something as simple as a loose fitting piece of mylar laying just laying on top of their snug fitting quilt without seriously increasing condensation or pack weight. I don't have a technical background so maybe I am misunderstanding what you wrote.Jun 26, 2013 at 4:38 am #1999867
GOOD testing! Just on a more theoretical concern, I am not sure that multiple layers of foil/mylar space blanket will change your numbers significantly during measurement. Once the IR radiation has been reflected, there will be nothing left for a second layer to reflect. You would still get some, of course, due to the space blankets own warmth, more on that part below. A second layer will help slightly, but I doubt you would be able to measure the difference with your equipment. I suspect a NeoAir Xtherm is simply holding dead air better, hence, increasing it's insulation effect, not so much the second layer reflecting more heat back. Besides, the reflected radiation would be trapped between the two layers, effectivly just warming the air molecules slightly.
"Both combined gave a 20 degrees F difference, but I was pushing the limits of my instrument because there was so much temperature difference, that the torso temperature was much greater than 93 degrees F, so there was more radiant and convective heat loss around the edges and my guard plate wasn’t able to get warm enough, so I question this result."
Yes, I would question this result, too. But, you are ignoring the actual warming of the "space blanket", as above. This will cause it to radiate IR the same as if it were reflected. Containing little mass, it will re-radiate warmth almost as quick as it absorbes heat. Heat is defined as the movement of molecules. IR is defined as a radiation. Heat is what we get when IR is absorbed. IR is what we get when heat is lost (if not by conduction or convection.)
Outdoor Wilderness Fabric (http://www.owfinc.com/) still has some UV reflective silver and white.
As far as mylar films go, they aluminize the mylar films by passing through a chamber that is evacuated. A jolt of electricity flashes off aluminum from a bar stock that coats the film, 2-5 molecules thick…read very thin. It is not physically adhered to the film, so it will wipe off with use. Not very durable.I suspect that aluminized cuben would be no more durable, since it would prevent the inside surfaes from bonding with pressure. It would need to be applied to the outside, much like a typical space blanket, and, have the same durability issues.
For breathable, perforated mylar, a sheet *could* be folded quite small and clamped between a couple layers of 1/4" scraps, then using a 1/16" (a bit more than 1mm) just punch a series of holes through it. I haven't tried it, but I would guess it would take no more than 15 minutes to do.Jun 26, 2013 at 7:03 am #1999892
Shawn – exactly – an aluminized Mylar on top will make a big difference. 15 degree F temperature difference in cloudy or sheltered location, 20 degree F under clear sky. For maybe 2 ounces.
But there's that condensation problem.Jun 26, 2013 at 7:33 am #1999903
I usually use a heatsheets from AMK for my summer camping where it is 60F+. Nice, light, easy to use. I can always try to push it some further by wear all my clothes and rain gear and doing things like sleeping under trees and bushes but it is mainly for summer. Condensation is a problem but not really when it is above 70F during the days and you can dry off just by walking around. I am going to try the needle punch thing to increase the breathability of it and will try to use it as you quoted Ray Jardine by flipping it over to reflect the heat during hot parts of the day. Any other alternative insulation ideas in the pipe? Thanks!Jun 26, 2013 at 7:34 am #1999904
I think for internal layers of foil, you get 15 degree F for each air gap due to the boundary layer of air. It would be 10 degree F for non IR reflective surfaces, but if either surface facing the air gap is aluminized, then it's 15 F.
I hadn't seen that fabric at owfinc. If it's like the silver fabric from Seattle Fabrics, it's only half refelctive so hardly worth screwing with.
Yeah, aluminized not very durable, comes off with use. Good for emergency blanket one time use. Good in Neo-air internal layer where it's protected.
Good idea about folding and then putting holes in. I have to try that the next time it's 20 F where my sleeping bag is too cold for, then maybe I'll try that (next January).Jun 26, 2013 at 9:53 am #1999938
Very interesting article. I go back to the statement by Mammot, "Radiation is a very small part of our heat loss (despite the myths put about by sellers of space blankets)". If one holds their hand close to their face, they feel virtually no heat from radiation. When they touch their face they feel warmth by conduction.
Radiation by heat loss is relative to the temperature to the third power. Thus, radiation heat loss is very low at cooler temperatures. Radiation heat loss is significant when there are temperatures at high temperatures such as pot belly stoves and industrial machinery.
I have tried to sleep under a space blanket alone and darn near froze. Yet, I have a Stephenson tent with a metallized inner layer that does seem to hold in the heat very well. This article is certainly something to think about.Jun 26, 2013 at 10:17 am #1999944
>"Radiation by heat loss is relative to the temperature to the third power."
It's actually a stronger dependency than that. Radiant heat loss is proportional to the absolute temperature of the object to the fourth power.
Two data points: If your skin is cool, 20C / 68F as it would be when your body is conserving heat in cold weather; and if your skin is at body core temperature of 37C / 98.6F.
In absolute temps, that's 293.1 Kelvin and 310.1 K (528 and 558 Rakine for my warm-beer-drinking ancestors).
Comparing the radiant heat losses off of that skin: 293.1^4 / 310.1^4 = .80
So there is 20% less heat loss from radiation from cool skin than from hot skin – a pretty minor difference.
I notice radiation effects much more from my overall surroundings, specifically the imbalance of IR between myself and the environment:
Sun-baked rocks all around me in the Grand Canyon definitely heat me up.
A clear night causes the top of a sleeping bag, or tent, or car to cool below the air temps (it radiants IR to deep space and gets essentially no IR back from the sky) resulting in dew or frost. Frost can form on a radiating object in air temps of 45F or more if there is no wind.Jun 26, 2013 at 10:20 am #1999946
Radiation heat loss is relative to temperature to the 4th power, but it's relative to absolute zero, so there is some heat loss at ambient temperatures.
The space blanket will give you 15 or 20 F temperature increase according to my measurements, but if you need much more than that you will still be cold. You will have to increase your metabolism to keep your body temperature up, but that doesn't make for good sleep.
Is the Stephenson tent wearing out quickly?
Does it look shiny, like aluminum foil, or is more gray? Based on the fabric I got from Seattle Fabric which was more gray, and was urethane coated nylon, I think if it's gray it's only half reflective (emissivity = about 0.5).Jun 26, 2013 at 10:50 am #1999958
I have two Stephenson tents. The one with the radiant liner is going on 30 years old and has been used more than a hundred times. The barrier is more of an oxidized aluminum color like aluminum paint from a can. It is not chrome colored.
I think the question is: If the human body does not produce that much radiant heat, why does the space blanket add that much heat?Jun 26, 2013 at 11:36 am #1999982
Does condensation build up on foil effect the reflection of radiation?Jun 26, 2013 at 11:47 am #1999984
"If the human body does not produce that much radiant heat, why does the space blanket add that much heat?"
A space blanket, by itself, does NOT add heat. All the heat comes from your body.
If you sleep comfortably at ~93F comfortably, naked, then a space blanket will only add 15F, ie you will maintain in about 78F weather. That is about how much radiant heat loss that it will reflect back to you. As Jerry was saying, the tent is only about half of that. It would help with about 7-8F. The full space blanket would only help with about 7-8F under those conditions.
Without other insulation, the space blanket alone would NOT keep you warm. You need insulation for that. Space blankets are only a 15F help. They are not additive, except slightly, due to their low mass/insulative value and the nature of the radiation.Jun 26, 2013 at 12:32 pm #1999998
"Does condensation build up on foil effect the reflection of radiation?"
Good question, probably it would
On the other hand, if you had a foil outer layer, then it won't cool from radiant heat loss, so maybe there wouldn't be any condensation
If you're talking about condensation under a space blanket, I don't think that mattera. Won't affect what's radiated from the front surface. And the bottom of space blanket doesn't radiate much heat because it's so close to what's underneath.Jun 26, 2013 at 2:48 pm #2000037
Water is a highly effective absorber of IR, even water vapor is (that's why IR telescopes are at 13,000 feet, not near sea level). If a space blanket was half covered with water film or droplets, it's emissivity would increase from about 0 to 0.5. ( 0 = perfect mirror; 1 = perfect black body).Jun 26, 2013 at 2:54 pm #2000038
>>"If the human body does not produce that much radiant heat, why does the space blanket add that much heat?"
>A space blanket, by itself, does NOT add heat. All the heat comes from your body.
A space blanket also blocks air flow. As such, it reduces convective losses.
And there is an air film on each side of the space blanket except where it is pressed tightly to another object. That creates a layer of insulating air of R = 1 to 1.5, depending on wind speed.
A smaller effect is, since the space blanket blocks air flow, the humidity inside builds up a little and you have less evaporative heat loss off your skin.
A painter's "tarp" of 2-mil polyethylene would do much of what a space blanket does. And be quieter.Jun 26, 2013 at 3:02 pm #2000043
"There are two conditions:
No foil – under a clear sky, this should be what happens when I “cowboy camp” with no tent
Foil – placed on top of the insulation. Since the foil has an emissivity close to zero and it’s adjacent to the fabric underneath, radiant heat loss should be very low."
I am wondering if you might have tried a third condition which would be putting the foil under the insulation. If the foil is on top it would stop radiant, plus convection, plus some conduction heat. If the foil is under the insulation it would stop mostly radiant heat.
It would be interesting to see if you get the same 15 to 20 F saving of heat.
Great article and lots of work. I have always believed that outdoor manufacturer's should give us R values with clothing and bags like they do for sleeping pads.Jun 26, 2013 at 3:46 pm #2000055
@tjaardLocale: Minnesota, USA
Columbia is using some kind of breathable reflective lining in their gear this year.
Unfortunately, they are not known for being the lightest, most efficient brand, so any potential gains in insulation/weight are offset by the rest of their construction.Jun 26, 2013 at 4:16 pm #2000059
"I am wondering if you might have tried a third condition which would be putting the foil under the insulation. If the foil is on top it would stop radiant, plus convection, plus some conduction heat. If the foil is under the insulation it would stop mostly radiant heat."
I did try insulation/foil/fabric. When there's no air space against the foil, you get very little benefit. It would be the same with foil under the insulation. You do get a degree or two like you do with any fabric layer, because there is a small air space because the fabric or foil is wrinkled a little. If you smashed the fabric or foil flat, then there would be no heat advantage. This is somewhat speculative because it exceeds my measurement accuracy.
And like I said, my measurement set-up is crude so it raises as many questions as answers.Jun 26, 2013 at 4:36 pm #2000064
"Columbia is using some kind of breathable reflective lining in their gear this year.
They have dots of aluminized material with breathable in between. Good idea if the percentage of area covered by aluminized material is large enough. Cover more area to reduce radiant heat loss. Don't cover too much to prevent water vapor passage.
But, their reflective layer is inside the garment. Then you have no air space on either side of the reflective surface. I don't think that works. It will be little better than a non-reflective surface. The reflective surface needs to be on the outside, or if it's internal, there needs to be an air space next to it.
The concept that heat flows from your body to the outside and gets intercepted by a foil layer is not a useful model – that's just marketing spin…Jun 26, 2013 at 6:57 pm #2000106
I've read it claimed a few times that a radiant barrier such as aluminized fabric can actually fight condensation forming on shelter walls, because the IR reflectively causes the fabric itself to become warmer (typically approaching the temperature of the air, rather than much cooler as it would be under an open sky).
Any thoughts or observations on this?Jun 26, 2013 at 7:47 pm #2000124
"I've read it claimed a few times that a radiant barrier such as aluminized fabric can actually fight condensation forming on shelter walls, because the IR reflectively causes the fabric itself to become warmer (typically approaching the temperature of the air, rather than much cooler as it would be under an open sky)."
When I was testing on my patio under clear sky, the foil did not get dew but the eVent fabric did.
A surface, whether there's a human underneath or not, will radiate heat under a clear sky and get colder. As the night progresses, the temperature gradually drops, and at some point goes below the dew point, at which point some of the water comes out of the air and condenses on the fabric.
Of course it depends on the amount of water vapor in the air and the temperature and how clear it is.
If you're sleeping in a tent, you exhale water vapor which can similarly condense on the under side of the tent. If the tent was made of reflective material, it would be warmer so there would be less condensation.
Remember that my instrumentation is crude so this is all somewhat speculative, not a recommendation to do anything with any guarantee that anything will work.Jun 26, 2013 at 8:14 pm #2000131
David, I agree. There are other smaller components going on than simple IR. The difficulty is seperating the two components: ie heat and radiation. Heat is defined as molecular movement. Infra Red is defined as a band of radiation. Even Jerry mixes the two. Take a look at his "Boundary Layer" graph. I interpret this as 0 IR absorption at 0" distance from the film, with an exponentially decreasing amount absorbed. But the effect of absorption is heat which he then plots…a bit confusing since he says he is only doing an IR study in a previous paragraph: "As everyone knows, heat loss is caused by conduction, convection, radiation, and evaporation. This article focuses on radiation. I will look into convection and evaporation in the future and probably will write something up about those."
Yes a painters drop cloth will do much the same, but this is sort of off topic and ignores the IR (the radiative heat loss,) which is really the focus of what Jerry is writing in this article.
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