There is a lot of literature on how to stay warm doing outdoor activities. But I am not satisfied because most of the information about heat loss is qualitative, anecdotal, or based on some ancient source; thus I have tried to better measure heat loss.
I wrote an article about
Thanks to Roger for reviewing this article and giving me a bunch of ideas. Like my previous article, there are limits to what you can do with home-made instrumentation. To do this properly you need to use expensive lab equipment certified by some standards laboratory, which I don't have. Thus, there is uncertainty about my results, but this at least raises some questions that I think are interesting. There is both geeky analysis and practical information. Also I am conducting my testing outside on my patio. If I was in an environmentally controlled lab I would get more consistent results since there are uncontrolled variables on my patio. For example, the wind varies during testing so all the variables are not always consistent and the results are less repeatable. I can largely eliminate this by, for example, repeating measurements and making sure I get the same result. On the other hand, my patio is actually much closer to the actual conditions I sleep in so my results are more useful. My main interest is how to stay warm enough when I'm (trying to) sleep at night. Usually the temperature gradually decreases until just before I get up in the morning. Also, my metabolism gradually slows until then. I am focused on that most critical point, just before dawn. According to Wikipedia "Metabolic Equivalent", 1 MET = 58.2 Watts per square meter. While sleeping, a human body emits 0.9 MET = 52.4 W/m2 so that's what I assume for my metabolism. Different people have different metabolisms so your results may be a little more or less than what I measured. I have noticed that I am warm enough to sleep well when my skin temperature is 93 degrees F, which is consistent with assumptions made by others. My interest in measurement is to have enough insulation to maintain this temperature. During the day I normally don't worry about being cold. If I have enough insulation to stay warm while sleeping, I'll have enough for the daytime. Before going to bed, my metabolism is twice or more what it is when I'm sleeping. I wear half of my sleeping insulation, a synthetic vest, before going to bed, so I am warm enough. If I'm cold I can exercise more vigorously or get into my sleeping bag. So, my investigation focuses on what it takes for me to stay warm using those assumptions - 52.4 W/m2 and 93 F. ARTICLE OUTLINE # WORDS: 8400 A Premium or Unlimited Membership* is required to view the rest of this article. * A Basic Membership is required to view Member Q&A events
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Torso Simulator to test Heat Loss in Outdoor Gear â Radiation
I'm not a physicist. Just trying to figure out how to stay warm with the minimum weight. And get some measurements rather than anecdotal information. I think some of you know more about all this than me – I'm not claiming to be the authority and have much to learn.
And sorry if I mix terms like "heat" and "radiation" – it's difficult to be consistent all the way through an article like this.
"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"
If you look at "Difference in Heat Loss for Several Different Materials"
The "white" and "black" cases are white and black eVent, so there should be no convective heat loss. The air to torso temperature difference is about 46 degree F.
The "mylar shiny side out" and "foil" cases are about the same, about 59 degree F air to torso temperature difference.
I think a painters drop cloth would produce about the same result as the black and white eVent. There is a 13 degree F increase in temperature difference that you get by eliminating radiative heat loss. A shiny surface is 13 degree F better than a painter's dropcloth.
For Stephenson tents, they say regularly they have double wall with outside of inner wall aluminized.
That makes sense given my measurements. There's an air gap above the inner aluminized surface so that the inner wall will be warmer. Since it's warmer, there will be less condensation from the humans inside breathing.
I'll have to check that out, see if they sell fabric. They sold me some "fuzzy stuff" a few years ago. I wonder how reflective it is.
Jerry, I have a tiny scrap of aluminized fabric (from OWFinc.) Not really sure why I kept it, but I would be happy to send it along. Send me a PM with your address. Maybe there is enough to stitch a couple pieces together for testing.
The mixed terms is OK. Once I got the hang of what you were writing about and why you were righting it. Often tests report results in terms of effect, it was just slightly confusing, though consistent. I believe a better test of thermal emisivity would be to directly measure the radiation, via, a detector, though. Like those used to measure houshold insulation. This should give you cleaner numbers to work with. There are a lot of questions, to me, about your Boundary Layer numbers, and insulations above and below. This looks more like cumulative convective heating of the air rather than pure IR absorption. From your standpoint, ie camping out, it is nice to know what will be most effective at keeping you warm.
Those IR measurement devices are pretty cheap. Cooking stores. Harbour Freight had one for $40 or something. I've thought about getting one.
All I really care about is how warm I am. It doesn't really matter what the emissivity or IR radiation is. What matters is the temperature difference if I sleep under a nylon surface, reflective surface, etc. What is the temperature of the air away from me vs the temperature at the surface of the insulation?
I wonder if my temperature sensors are effected by radiation? Will they measure temperature of the air or will they have radiative cooling themselves and thus be cooler than the air?
This may be controversial but I agree with David Thomas's analysis of the foil placed on top of insulation. It works because it stops convection heat loss, conduction heat loss and, for a human, evaporative heat loss. The amount of radiant heat loss stopped is insignificant because the body produces very very little radiant heat.
FOR THE HUMAN BODY, RADIANT HEAT BARRIERS DO NOT WORK WELL AT STOPPING RADIANT HEAT LOSS. Since there is very little radiant heat produced, there is essentially VERY LITTLE heat to stop. Don't believe the sales hype that radiant heat barriers keep you warm by stopping radiant heat. The barriers may work but they do it by stopping convection, conduction and evaporation. A painter's tarp, bivy bag, or other cover will work the same way.
" convection as a heat transmission cause " …this is one of those things that have been a thorn in my side for some time. the statment is as taught by every science book they ever forced upon me, and i suspect those books are wrong.
convection does not cause heat loss. convection itself is a movement Caused by heat, (or differentials of it.) convection to me looks more like an effect of soemthing else, not the something else that they claim it is. those books are wrong.
and also, "heat loss", if you look at it as drop in temperature, can be caused by expansion.
is an interesting thread.
cheers,
v.
http://www.homedepot.com/p/Reflectix-4-ft-x-125-ft-Heavy-Duty-50-g-Perforated-Radiant-Barrier-RB4812550/203927012#.UcyfwPlZd8E
Many companies which sell reflective barrier products for homes typically have perforated versions. These are due in-part to reducing the risk of condensation between the rafters of a house and the attic envelope beneath them.
Matt
Well written exploration of the topic.
Thanks.
I recommend using a Kestrel Weather Meter (4200 series).
It measures relative humidity, temp, air density, etc. for a much better, more accurate take on atmospheric conditions
The diving industry likes to sell aluminized stuff. At least they used to. Haven't looked in a while. I think one brand marketed titanium something. Really, I can't remember if I bought the hype then or not… I know I couldn't afford it. Minimizing water flow through a wet suit is most important. Titanium or not, don't want to heat up a new volume of water if the warm water flushes out.
> jerry adams:
> > Omni-heat: http://www.columbia.com/Omni-Heat-Reflective/Technology_Omni-Heat_Reflective,default,pg.html"
> 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.
This site has some interesting information:
http://www.windowoutdoors.com/WindowOutdoors/Window%20Outdoors.htm
I wonder that the painter's tarp might also transfer less heat than a foil blanket. Both are impermeable barriers. The tarp doesn't reflect radiation but the foil blanket does. However, I imagine the foil blanket will conduct heat better/faster than a plastic tarp.
"convection does not cause heat loss…"
Convection can disrupt the boundary layer of air on the outside of your sleeping bag which adds about 10 degrees of temperature.
Convection can also blow through your fabric and carry away some of the warm air inside your insulation.
Convection can increase the minimum temperature you'll be comfortable. An outfit could be comfortable in still air, but if it was windy you could be cold.
Jerry,
Nice article. Wow, you have been busy with this topic.
Do you think you resolved the question of whether a radiant barrier, like Heatsheets, is worth its weight in terms of heat retention. It weighs a little less than one oz/sq/yd. When placed most effectively in a bag or quilt, Will it provide more heat retention for its weight than Primaloft One or high loft down?
It was mentioned that Heatsheets are mylar. Doubt it, as it behaves more like polyethylene, and bonds like polyethylene, not at all like mylar.
From reading David Thomas' posts and your article, it seems that the answer to the above question is negative. If that is so, the better design would just add a little more Primaloft One or down rather than a reflective barrier.
Note: A meat tenderizer with lots of little spikes is one way to make Heatsheets vapor permeable.
I think a reflective layer facing out will provide more warmth for it's weight than primaloft or even down. Also a lot less volume packed. I think that's a great emergency gear item in case you get injured and can't continue walking.
The only problem is it doesn't breath so it will condense inside and get everything wet. So, better to just add a little more insulation. And radiation only accounts for a small amount of heat loss so you don't have to add that much insulation.
I was looking at heatsheets website. They said it reflects back 65% of the heat from your body. They have pictures of runners wearing heatsheets. If there was a one inch air gap between human and heat sheet, then it would reflect back all the radiation. It looks like there's an air gap some places but, for example, where it rests against the shoulders, there's no air gap, so it doesn't work. Maybe overall, it averages that 65% of the area has an air gap…
I think if they got rid of that obnoxious advertising on the outside of the heatsheets and just had a reflective layer, it would work better…
Meat tenderizer – good idea, lots of holes quickly. One thing is, I would like to be able to make a known amount, like 5% or 10% of the surface area be holes.
Please do not forget that relative humidity has an equal part to play with heat retention, when it comes to overall "comfort". This is what our psychometric charts have been offering us for over a century now. If our bodies are warm enough, but we are in an environment where we are too humid, we won't be comfortable, and will most likely not fall sleep.
It seems that RH is simply neglected in these quantitative measurements. I'd hate to see conclusions being formed based on half of a picture. If IR barriers actually worked a practical scale, all our cars, buildings, and homes would be shiny now. The reality is that IR barriers are more effective at reducing solar heat gain than preserving human generated warmth.
Fwiw, there were sleeping bags made with high emissive fabrics back in the 80's (Kelty comes to mind.) They are gone now because they ignored the importance of vapor permeability. Furthermore, you will find less building codes requiring vapor barriers, simply due to the fact that building science has now concluded that they cause more harm than good to our environments.
If you want to stay warmer in a sleeping bag, have an overbag, make sure you have a good conduction barrier (ground pad), a good draft coller, and don't toss and turn. The sudden air exchange in a bag will render any heat retention pretty much eliminated.
Matt
"If IR barriers actually worked a practical scale, all our cars, buildings, and homes would be shiny now."
Those are different cases with objects at different temperatures so not directly applicable. And there are lots of easy things we could do to reduce energy consumption that we don't because we're idiots. Uh oh, drifting back to the global warming flame war chaff topic…
I too have been skeptical about how much radiation heat loss is which is why I measured it – it's small, but noticeable – if you could eliminate radiation you could sleep with the same comfort at 15 degrees F colder according to my measure.
"Fwiw, there were sleeping bags made with high emissive fabrics back in the 80's (Kelty comes to mind.) They are gone now because they ignored the importance of vapor permeability."
Adventure Medical Kits has a bivy that has several layers of reflective including outer layer that are also permeable to water vapor. The reflective material is sort of acordion shaped with elastic to keep it in shape so there are air spaces between reflective surfaces. They say it's good down to 50 degree F at 8.5 ounces.
I agree with your comments about keeping warm – ground pad, draft collar, etc.
> Furthermore, you will find less building codes requiring vapor barriers, simply due to the fact that building science has now concluded that they cause more harm than good to our environments.
Vapor barriers cause more harm than good to the home environment because they trap moisture and that can cause rot ?
Or, they cause more damage to the environment at large because the resources required to produce the vapor barrier material exceed resources conserved by using the vapor barrier ?
"This Old House" continues to use vapor barriers.
You have to do it correctly.
Jerry, it seems your experimentation has led to more questions than answers. Not only in the area of theory vs. practice, but also in the sense of what your measurements actually show, and also in how they would translate to the real world of a human in a sleeping bag. In one sense I don't care about the theory or how it works if placing a reflective material over my sleeping bag would keep me warmer. But it would be interesting to be able to isolate the factors – though I expect that would require more expensive instrumentation to measure IR directly and a closer simulacrum that would include moisture production to mimic the human body – so that you could determine if it is indeed a reduction in radiant heat loss, and if so, can that be effected by other materials besides the aluminized mylar which we know has limited durability. Or, if it turns out to be another effect at work, what is the best way to make it happen? My experience with purported radiant heat barrier material (Texolite) tells me that it keeps heat in somehow. That was clear. Exactly how is the question.
>""This Old House" continues to use vapor barriers.
And Bob Vila continues to promote Edenpure heaters that put out an amazing 3,413 BTU per kilowatt-hour. (for you non-engineers, that's tongue-in-cheek, because every electric device, be it a toaster, a hair dryer or a length of wire puts out 3,413 BTU per kilowatt-hour (unless it is has an active refrigeration cycle – compressing and expanding some working fluid – then it can be more efficient)).
I always liked Norm better.
Paul – I agree, as many new questions as answers
"My experience with purported radiant heat barrier material (Texolite) tells me that it keeps heat in somehow"
That's what I measured. It's because you radiate less heat away. If you look at one of those IR camera pictures of a person, you will be fairly bright in IR. If you cover yourself in foil you won't be. My measurements says it increases the temperature difference by 15 degree F – the minimum temperature you can sleep at comfortably will be 15 degrees less.
"But it would be interesting to be able to isolate the factors – though I expect that would require more expensive instrumentation to measure IR directly and a closer simulacrum that would include moisture production to mimic the human body – so that you could determine if it is indeed a reduction in radiant heat loss, and if so, can that be effected by other materials besides the aluminized mylar which we know has limited durability."
Now that sounds like the evolution that led to EN 13537 and beyond. It becomes so complex as to be impractical.
Maybe use a crude instrument like mine to identify a few configurations, and then try them in the wilderness in real use.
Thanks for the article and excuse me if I'm repeating what others have said as I haven't read all the comments.
I've used Blizzard products quite a lot and found them remarkably warm with just a little condensation. I note that the inner and mid layers are perforated mylar and they may sell you some if you drop them a line. I've sometimes wondered about attaching some to my tent but haven't gotten around to it. As it's a separate layer it would be easy to replace when it begins to wear, as it inevitably will.
Hi Jerry,
Haven't read BPL for a year or so, and missed this article. You write:
"Another way to minimize radiant heat loss is to have an aluminized layer above you, for example your tent. Cuben is Mylar and is not fragile, so if there was an aluminized version that would be very effective."
I bought some aluminized cuben on gear swap a couple years ago (from Colin Krusor, IIRC). Base material was CT2K.18 (0.95 oz/yd^2, I think) and I weighed finished weight at 1.29 oz/yd^2. So aluminizing seems to add a fair bit of weight.
Still, if a small aluminized tarp would add 15* for only a couple ounces extra (vs. regular cuben) that might save 7 ounces or more in extra insulation. On the other hand, air circulation under the tarp might mean higher convection losses and a tent might use enough more material than a tarp to cancel out weight savings in insulation.
Long story short, I haven't made anything with the aluminized cuben yet. I've been thinking it might be a good bottom for a bivy–not much lighter than silnylon, but maybe the radiant barrier will still be an advantage. PM me if you'd like a small piece to test with your rig.
Sure. I need a piece 12.25" x 12.25". I'll PM you my address. No guarantees I'll ever measure it. I have a piece of material from James Marco also to test which he sent me 6 months ago.
I just happen to be resurecting my instrument, fixing a couple defects.
According to my theory, which I haven't got universal acceptance of, in order to get advantage of reflective layer, there has to be an air space either above or below, so bivy bottom won't do much.
I had a bivy bottom from aluminized silnylon. I don't know that the reflective layer did anything, but it seemed to maybe be a little more waterproof or durable
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