O.K., now I understand there are three ways to transfer heat (conduction, convection, and radiation). Coat manufacturers have always strived to minimize heat loss via conduction by using insulation with a high R-value per unit thickness. Makes sense to me.
Now let’s look at “emergency blankets”. These blankets are very thin, and incorporate a shiny, metalized material on the inside surface of the blanket. They work by reflecting heat (radiation) back to the body. A friend of mine tried one out a couple of weeks ago. “It worked great. I was burning up after 10 minutes,” he said.
So if the “reflect back the radiation” concept works so well, why don’t coat manufacturers exploit this effect, and thereby make coats less bulky? Why don’t we simply make the outside of refrigerators super shiny to reflect heat away from them? Instead of using thick Fiberglas insulation, why don’t we install aluminized or Mylar sheets inside walls to reflect heat back into a room?
Does an emergency blanket have an R-rating? What is it, and how does it compare to regular 'ol “anti-conduction” insulation?
There’s something very basic I must be missing here…
It’s been tried to varying success, particularly for hiking gear. Those “emergency blankets” are waterproof or nearly so, and so would get clammy very quickly if they were used with conventional insulation. The hiking gear companies tried using them in strips, sort of like tinsel.
I suspect the mylar surfaced “space blankets” work effectively with a body because they are so close to the radiating source (the warm radiating human body). A silvery fridge would have a negligible effect on overall fridge efficiency in reflecting away ambient heat radiating from the warm(er) air mass in the kitchen.
The blankets do reflect radiant energy back to the wearer.
The blankets trap heated air next to your body, which insulates.
Once the trapped air is moisture-saturated, sweat evaporation slows down or stops, reducing the (huge) heat-loss due to evaporation.
After a marathon (where emergency blankets get good use), the body is still warm, just losing heat fast. So, you slow the rate of heat loss due to #1-3 above with your emergency blanket. Works very well.
These factors are either not present or working at reduced effectiveness for a refrigerator.
Insulation needs to “breathe”, that is to pass insensible perspiration or vapor through the material. Otherwise the insulation becomes saturated over the course of a few days, rendering it considerably less effective. Not a big deal in the backyard but is critical on long trips or high altitude. About 1 pint of water overnight is one figure I’ve read, outside that exhaled through breathing. That’s why waterproof sleeping bags have been rare to nonexistent over the years. Even Gore-Tex, whose manufacturer claims items to be both waterproof and breatheable, has had trouble over the years incorporated into sleeping bags. I’m told that certain “moisture barriers” used in housing insulation contributes to condensation buildup and subsequent mold problems.
Also, don’t forget, someone that’s using an emergency blanket isn’t under it naked. They’re most likely fully clothed, so they already have a layer of ‘conventional’ insulation.
Also as good as the mylar may be, it’s also used becuase it saves space and money. If possible I’m sure most poeple would prefer a down blanket to mylar.
With the emergency blanket, you have a big reflective surface right up next to a thermal source (you) radiating in the low-to-mid-90F. If you don’t have it wrapped tightly then you’ll lose more heat by convection and it won’t be nearly as effective. In a house, however, the ambient radiant temperature is much lower (we’ll say 70F for talking purposes) and most of the heat loss will be by conduction. The nasty secret about house insultation is of what little effect it is; you can cram twice as much insulation into your walls and get barely more result from it. This is why thermal pane windows (with an evacuated layer between panes) really are such an improvement to a house’s overall insulative capability. We could put Mylar[sup]TM[/sup] sheets in walls (and if you notice some board-type insulation actually has a thin layer of Mylar[sup]TM[/sup] in the inner face) but it’s just not all that effective by itself.
With clothing, I suspect that a good portion of the reason it isn’t used more is because of how effective it would be in reflecting heat, and how poorly it does with allowing moisture to escape. The watchword in modern outdoor gear in the last twenty years has been about breathable/waterproof fabric like Gore-Tex[sup]TM[/sup], Conduit[sup]TM[/sup], Drilite[sup]TM[/sup], et cetera. That said, one of the old tricks to improve the thermal rating of a sleeping bag was to put a layer of emergency blanket and polyester or wool material inside, like a liner. This got awfully sweaty, though; a down bag is much more comfortable.
The emergency blanket needs to have an air gap around it to work. If you lay a sheet of emergency blanket on a block of ice and put your naked butt on it, your butt will freeze pretty fast. But if you are wearing pants made from porous material (i.e. almost any fabric), you create an air gap filled with trapped air (thus minimizing conduction) and lined with reflective material on one side (thus minimizing radiative transfer). Just like a thermos bottle. If you only minimize one of those heat paths, the heat will simply use the other path to get past that layer.
I imagine a coat with an aluminized mylar layer incorporated in the middle would be pretty effective. But as already pointed out, you’d have a problem with humidity building up inside. And it would probably make a crinkling noise, and won’t be very soft.
As mentioned, a lot of the effectiveness of the space blanket is due to trapping air and moisture, and I wouldn’t be surprised if a clear plastic blanket wouldn’t be just as effective (as far as human senses could tell).
In fact, in cold-weather survival situations, putting a layer of waterproof plastic as a middle clothes layer is recommended, as trapping sweaty air will reduce heat loss a lot.
The problem is that it’s not very comfortable having all that sweat trapped up against you. Think sweaty clammy, think vinyl car seat in summer, think tropical jungle and “how long before I get crotch rot?” kind of feeling. That’s why this is only a survival tactic, and everyday clothes are made more breathable.
On the other hand, refrigerators are already moisture tight (more or less), and modern house walls are, too (that’s what Tyvek™ is for.
