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.