Torso Simulator to test Heat Loss in Outdoor Gear – 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.

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.

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.

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

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

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.

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.

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

> 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 ?

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.

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.

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.