Some anecdotal comparison:

Back in 03 I went through my hazmat level a certification. We had to put on an enclosed tyvek suit (not level A, but close) with an SCBA inside. This was in the middle of winter, and we were experiencing a frigid arctic blast at the time. Outside temps were around 15 degrees F, and winds out of the northeaast at about 15 mph. After we were suited up we had to walk from the classroom to the simulator, maybe 200 yards. Our escorts are wearing the full complement of winter gear; parka, watch cap, gloves, boots. Inside the tyvek suit, I am wearing a t shirt, jeans, and my work boots.

Now the tyvek suits are notorious for not venting well, and I am an air hog with an SCBA (I consume about twice as much as as the average joe over time, go figure), so my suit is inflating some (the suit vents better once a little pressure differential is established). Anyways, the whole time I am walking across the parking lot, I notice that the temp inside that suit is still warm enough that I am perspiring. Now once we get into the simulator and start crawling around I am sweating and getting uncomfortably hot. Once the simulation is over, I punch the bypass valve on the SCBA to get the cold compressed air blowing on my face continuously to help me cool down. After I have doffed the suit, I take a step outside, and my warm sweaty body jolts from the frigid cold blast.

So I am thinking that way more than 1/8 inch of air gap between my body and the suit did a lot to keep me insulated from the cold and trapped a lot of body heat in the process. From an experiential point of view, I'd say the vest must be similar.

Even though the air in the vest can circulate more or less freely within the individual chambers, I don't think convective loss is going to be a factor because the circulating air doesn't have a good thermocouple to the outer layer of the vest, based on how the air in that tyvek suit stayed so toasty warm. If the outer layer of the vest were metal or wet fabric then probably convective loss would be an issue because the circulating air in the chamber would be in contact with a much better thermocouple.

Come to think of it, once my suit inflated, I did get cold spots where my skin came into direct contact with the tyvek, such as at my elbows and wrists when I had to bend my arms. I suppose that, had the suit not inflated as it did, I might've ended up much cooler than I was. That seems counter-intuitive to the 1/8" principle.

Sounds like a job for mythbusters to me.

I've got to think that a vest like this would add warmth, but how much I have no clue about.

Seems like something made out of material similar to a sleeping pad might be a winner. I doubt it would capable of being packed into something the size of a deck of cards, but it should be fairly small.

Interesting product, and good discussion to be sure. I stumbled across this vest recently (online) and was thinking that while it had to be more insulative than a plain space blanket, the convective losses would steal a lot of heat, especially when you're sitting still. However Benjammin's experiences seem to refute that to a degree.

I agree that you'd be hard pressed to find a fleece vest that weighs this little & packs this small.

Like many things on the market, I wish I could wave a magic wand and make tweaks to what is a reasonably promising design:

- construct it from the same quiet, strong plastic as HeatSheets
- add just a little baffling of some kind to the chambers
- put a little bungy cord/elastic around the arm holes & bottom hem to keep warm air from being pumped out each time you move

Of course, by the time you did all this it would weigh twice as much and cost $30.

Here is the product's webpage & it has some more details. Here is a detailed review from Backpacking Light that I think is freely available without a subscription.

There's a lot of heat energy in the evaporated water that makes up sensible perspiration (sweating) and insensible perspiration (which happens continously from our skin).

If you can trap that evaporated water, you create a warm, moist envelope around yourself. That's why some hardcore outdoorspeople put sealed vapour barriers inside their sleeping bags to get more warmth for less weight.

But unless you have insulation outside of the vapour barrier, that warmth would be very short term. The determining factor would be your activity level. If you slow down, after your clothes have become saturated with moisture, convection and conduction would catch up with you. And then you would be really cold.

The Tyvek suit is a good example; the same would apply to oft-repeated advice of "crawl into an oversize trash bag and you'll be fine." I'd hate to spend a night like that; without additional insulation, it would be miserable.

The vest under discussion is a bit different, in that the air chambers won't contain moisture except that from your breath while inflating. If the wearer is moderately active, it will reflect and retain some warmth, though convection losses will be much more than an insulated garment. I think the most effective use would still be to wear it next to the skin, and put any other clothing over top.

This seems reasonable to me. I am thinking that directly on the skin might be too uncomfortable, but over the top of a synthetic t shirt (like the under-armor type), then with the remaining clothing outside of the vest it should be pretty effective.

From my experiences in Iraq with battle armor, I can say that everything under the vest is going to get wet from perspiration collecting, but the arm, torso and neck openings should serve to mitigate that under moderate conditions. A good synthetic undergarment will help to keep the comfort level up, without retaining too much moisture I would hope. You'll probably end up with fairly wet armpits if you exert yourself in this vest at all.

Step one is to get one of these vests, step two will be to find a cold place to try it out in (looks like I am gonna have to invest in a big chest freezer next).

I would like to see some thermal imaging studies performed before I would count on this product do what the manufacture’s claims. This product cannot fully encapsulate you the way you can with the heatsheet (covering one’s head and trunk, effectively blocking the wind – convection loss). We have discussed the use of an inflatable air mattress in terms of heat retention, which they do not, although heat loss is through conduction (contact with the ground) more then from radiation or convection loss that the vest would theoretically block. It seems the claim to fame is that you are inflating the chambers with air warmed by the body via one’s breath and that might have an initial warming effect, but as soon as the heat is transferred from the trapped air to the outside air, this short term benefit would be quickly lost. Trapped air with that degree of volume will still allow for body heat to be transferred from the body to the outside air as the chambers cool off and start robbing the body of heat. Is it better then nothing? Sure.

Pete

Here's another good review, with pics even:

Be Prepared To Survive Review

http://www.montane.co.uk/productdetails.php



Montane Jetstream Jacket

Weighs 75 grams and packs about the same size as an apple. Fits inside my small Snowpeak 300ml Titanium cup.



The breathable nylon shell material of the Jetstream jacket when worn can be stuffed and insulated with dry grass, heather, Cattail heads, shredded newspaper etc or anything else that can be used as an insulator to reduce convective heat loss in very cold environments. The Jetstream jacket is also remarkably warm in for its weight its own right, as it can cut down dramatically any convective heat loss being windproof and somewhat weather/water resistant but still remaining a MVP material. This allows the air space directly above the skin and between the undergarments to remain dry and thereby stops the undergarments from wetting out and stopping excessive evaporative cooling of the skin.

I would say a large garbage bag would be as warm as that vest
and would provide better coverage from the elements. A tyvek
suit, especially the coated models would be better yet.

The air mattress comparison is a great one.

Micro convection larger voids in still air occurs whether sealed off or not in larger voids. Why do you put fiberglass in your walls, or special gas in thermo's.

This vest is stopping heat loss by convection, evaporation, helps in the short term with stopping some wetting of the clothes underneath by rain or snow and so is useful, but the air pockets are kinda gimmicky without some method of stopping internal convection.

I would take a trash bag over that vest given the choice, even if they were the same price, due to the better coverage of the garbage
bag. You can make a small tent from a garbage bag and even use a heat source under the hem as the cavers do. I carry one in the lining of my climbing helmet.

Okay, then I guess I am going to have to find a decent garbage bag and buy one of these vests and do a comparison.

Now the question is, where will I find a place in central Florida cold enough to test them both?

Looks like I am still going to have to purchase a big chest freezer too.