Looking at the AMK website I see two different AMK Heatsheets. One is larger and is for two people, I guess, although it says its only 20% larger. Is there any other differences between them? Also, I can;t find out how small they pack. Anyone know?

20% larger than competitors.

Haven't seen the small one, but the two-person version packs smaller than a video cassette. Fits in your back pocket like nobody's business.

Thanks for the clarification. That makes more sense.

The smaller one is just like the regular silver space blanket - about the size of a cigarette pack. The larger 2 person one, is a bit larger then the 1 person one. I would say its about 3x5x1 inches when folded.

I believe the emergency bivy they sell (the smaller package of the two bivy bags on their site) is made of the heatsheet material.

What is surprising is that, the way their advertising reads anyways, the smaller packaged bivy seems to be higher rated for keeping you warm. That seems counter-intuitive to me. Even if it isn't as I perceive it to be, I still prefer the smaller packaged bag for packing convenience. I bought some last Xmas as stocking stuffers for the girls, and as more gear for me.

Those yellow whistles look a lot like the Fox Micro 40 ones I own.

I have yet to try out the heatsheet material in cold climate. I would like to see how effective the thinner bag really is.

I thought the same thing until I used one of the Heatsheets blankets in the Sierras. I ended up making a bivvy of sorts by taping the edges of it with duct tape. It keeps more of the heat inside. (Of course it keeps more moisture too, but at least I was somewhat warm)

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What I ended up using was a combination of a Thermo Lite and a Heatsheets. I climbed inside a Thermo Lite and then wrapped Heatsheets blanket (now a bivvy) over it. Temperatures at night dropped to about 40. I was wearing socks, pants, t-shirt, light fleece and rain shell, while sleeping inside a tent. I was relatively warm, but would get chilled if there was even the slightest wind.

Overall, I would recommend this, or similar set up, only when temperature stays above 50. Anything below makes things uncomfortable. (Then again, I do tend to get cold rather easily )

Well, comfort aside, I reckon the idea with these things is to keep you alive more than anything else. I guess long as it keeps you dry in a rainstorm and keeps the wind off your back, you got a better chance than without it. If the reflective heat claim is at all realistic, that'd be an improvement over garbage bags enough to warrant the added cost I suppose.

I haven't done a lot of testing with the Heatsheets and Thermolite bivvies, but my understanding is that the Heatsheets has better reflectivity because of the material. However, there is no way for moisture to escape, so you get condensation on the inside.
The Thermolite has a vent down by your feet to prevent this. The material is more durable, making it last longer in a survival situation or even reusable as a lightweight bag when doing ultralight backpacking in warmer weather.

Either is greatly augmented by some fill material (fleece sleeping bag, fleece jacket, blankets, etc).

Ew-w-w-w-w. Maybe they should put the vent somewhere else to protect the innocent.

Last winter I used the bivy as an inexpensive vapor barrier liner to my down bag. The bivy worked ok, but I did tear a small hole in it and you'll never get packed down as small once you open it up. For my next winter trip I'm going to buy a real vbl for my bag and I'll put the AMK product (with hole duct taped) in my emerengy gear stash.

I know Vellux; I used to have one as the blanket on my twin bed. Never tried it outdoors though. Very warm, but don't let your little brother put a lollipop down on it; it'll rip all the fibers off that patch.

I'm not sure if it is more reflective, but the heat sheet I tested seemed warmer than the mylar sheets I've used. It was also stronger and quieter. Plus it is a better signal, IMO- I'm always worried that a mylar sheet will be mistaken for water if used as a standing signal. Blaze orange on the other hand.

Wait a minute; wait a minute; wait a minute. The smaller bivy that AMK sells is warmer than the larger Thermolite2?

Could it be due to a less porous material? I've seen the thermolite, they seem more permeable, like a car cover type of material. Never seen the heatsheets. Are the heatsheets more of a plastic feeling like the mylar spaceblankets? I don't see any temperature ratings an them except for the thermolite.

I've been following this topic closely because I'm looking to replace the Outoodr Research bivy sack that I have with something a lot smaller and lighter. Does the bivy have any type of drawstring to close up the head opening?

In my personal experience - that is true. I think the reason for Thermo Lite being "less warm" is because there is a foot went and a sort of opening to get your hands out. It lets even the slightest wind in.

I did wake up somewhat sweaty when I used it them my ultralight backpacking trips. Both Therma Lite and Heatsheets offer decent alternatives to regular sleeping bags and I certainly saved several pounds in weight. As far as comfort goes - that's a different story.

Interesting. I've used my old Thermolite (Thermolite1?) with a fleece sleeping bag liner with good results in warm weather, but I haven't taken it into much wind. I was wearing socks, long johns (top & bottom), and a fleece cap. I guess if it were really windy, one could duct tape the vent closed.

Perhaps the advantages of the Thermolite are:
-it can fit back into it's original container
-more durable
-vented (both an asset and a liability)

The Heatsheets bivvy is:
-smaller
-lighter
-(and apparently) warmer

Does that square with the experience of others?

That pretty much sums it up.

Next time I backpack in relatively cold (under 50) weather, I will probably bring a Blizzard Bag. Above 50, Therma Lite works OK

Interesting. One last thought: for sheltering-in-place in a car, I think the Thermalite2's would work well. The car acts as a shelter, forestalling the wind.

I bought a couple of Theralite2's for that purpose (one for my wife, one for me), and I keep my older, lighter Thermalite ("Thermalite1") in the bottom of my pack on day hikes.

Based on this thread, maybe I should get a smaller Heatsheet bivvy for my hikes. Hmmm.

What exactly is the Blizzard bag made out of? I looked at that, but from the picture it looks a lot like the amk heat bivy. Is it some sort of insulation, and can it be reused?

Looks like I'm also probably going to pick up a AMK bivy for day hiking

Chris Kavanaugh has tested the bag and wrote a report about it.
One thing to consider is that it is significantly heavier then Heatsheets or ThermoLite. Here is some info on the bags themselves.