I did a mini test of the Adventure Medical Kits Heatsheets Emergency Bivvy.
Click here for product info for Adventure Medical Kits Heatsheets Emergency Bivvy 3.5 oz.

I also own the heavy duty version by the same company called Thermolite 2.0 Bivvy. I have not tested this one yet. Click here for information on Adventure Medical Kits Thermolite 2.0 Bivvy 6.5 oz.

The test I conducted was only a "mini" test, but the data is useful.

This test was conducted on my patio.
I have conducted many such tests on my patio with other products down to 4 degrees farhenheit because I get bored in January.
For this test I was wearing a cotton t shirt, cotton socks, and thin cotton pijama bottoms.
Underneath me was a Thermarest Dreamtime sleeping pad, which was awesome.
I put a temperature probe at my chest, one at my knee, and one at my feet. These probes were in the bag and outside of my clothing.

Ok here is the data:
Ambient temperature = 50 degrees
Wind 10 to 15 mph
After a 15 minute test of me laying there motionless:
Temp at chest = 83 degrees
Temp at knee = 77 degrees
Temp at feet = 64 degrees

I was impressed. The bivvy doesn't list a temperature rating, although the heavy duty one from the company is rated at 50f on the packaging. I'll test that one another time.

By the way, I am always a very cold person, and I felt pretty warm during this test, but my feet felt cold.

I have the heatsheets bivvy with me at all times since it is so small.

Let's discuss

Update:
For comparision purposes I did another test.
This test was with a synthetic fill mummy sleeping bag good down to about 25 degrees farenheit.
Here are the results:
Ambient temperature = 50 degrees
After a 15 minute test of me laying there motionless:
Temp at chest = 93 degrees
Temp at knee = 91 degrees
Temp at feet = 83 degrees

I felt very warm and toasty as expected.

For the super small pack size of the Heatsheets Emergency Bivvy I think it's a great tool for if/when the need arises.

Good information. How long did you stay out in the bivvy? Do you think you could've slept in the bivvy if the temp had stayed @ 50 degrees?

I think using the thermarest made a significant difference. Insulation from the ground is a huge benefit in retaining heat.

It will be interesting to see how the Thermolite 2.0 compares to the heatsheets under the same conditions.

"Let's discuss"

Ok

Hey Jeff, can you try it out again without the sleeping pad, the Thermarest Dreamtime sleeping pad is counter productive because it is reducing the thermal transfer from the ground. It is designed to help trap some of the heat from escaping to the ground and I'm wondering without it how fast you lose heat to the ground. In a survival situation, you may not have your pad.

Update, I guess Benjamin and I was typing at the same time, basically what he said about the pad making a difference.

I was going to test the other bivvy, but it stared raining.
I was in there for only 15 minutes.

I would not even try to sleep if I didn't have any pine bows or other insultion between me and the ground.

Obviously the experience would have been significantly different with nothing between me and the ground. Conduction would have won.

I used the thermarest because I wanted to eliminate the variable of conduction of my body heat to the earth. I wanted to see exactly how well the bivvy would trap the heat coming off my body.

I have slept out in the Thermarest Bivvy overnight, under cedar trees, also with a thermal pad (z-rest). I don't think its counter-productive because I was trying to simulate a real world bivouac, for which I do tend to carry my z-rest. If I didn't have any thermal layer from the ground I would have probably built up what protection I could, and shelter in the bivvy sitting up - not a very comfortable night. When I slept out in the bivvy under cedars it was fairly cold, mid-40s, with rain, but I was in normal outdoor clothing for me - a thermal layer, pants, long sleeve shirt, hat, gloves, fleece jacket, boots. I took off the gloves, too warm, and slept snug for the night. I didn't wake up in a pool of my own condensation, the outside of my fleece and pants were damp. The point is I slept through the night, comfortably, and woke up, then wrapped up the bivvy in its stuff sack, and went indoors (and slept some more as I recall). I don't recall my feet getting cold. I did this a year or two back, and I wish now I documented my tests better, but I think I reported on it somewhere earlier in this forum.

The Heatsheets Bivvy is one of a line of progressively improved products - small, lightweight, increasingly effective emergency shelter. Better than heatsheet's thermal blanket, which was better than the space blanket, which is better than the Tacoma Mountain Rescue emergency shelter I carried in packs for 25 years. It should help you get by if you are at least mildly prepared and seek appropriate shelter from the elements. Its not my bivvy of choice, I have a Montbell bivvy bag that is waterproof and much more durable. But I carry the Heatsheets Bivvy on every outing, on the odd chance that I need it or someone else does - to help recover from hypothermia or shelter them while injured. At $11-12 the Heatsheets bivvy is also much more likely to be slit down the side and wrapped around an immobile victim than my $100 Montbell.

Agreed, the use of the ground pad does greatly increase the effectiveness of any sleep system. However, if you can get nearly duplicate conditions, try the thermolite with the pad as well so you have consistancy, then try it again with each without a pad.

Personally, I like testing this kind of thing in an unheated outbuilding, so I don't have rain and wind, just cold and a nice, hard, heat conducting floor. I tried my Heatsheet bivy on that at around 50 and was fairly comfortable in socks, t-shirt and skivvies. Better than I would have been with a 55 gallon contractor bag, although I did have to suck in the gut a little to get in there. Problem is, it is a perfect vapor barrier and I perspire a lot, although with my thermal layer outside it wouldn't have been as much an issue in warmer weather. But unlike a Heatsheet or a mylar sheet, I didn't have the draft, so in the wind, I wouldn't have been losing my warm air.

Good information all around... I've been carrying the Heatsheet Blanket for some time now.

I did another test for comparison purposes and put the results in the original post at the end.

Thanks.

In addition to conduction, I have found I cool down a lot after half an hour or so as my metabolism drops. Setting up and getting into the bivy warms me up a bit internally, and once I've stopped moving I cool down.

We also did some testing with the bivy bags. I used the 2.0 and my wife the smaller version. It was at Hawk Mountain, PA last September. The temperature was around the freezing mark and we had no sleeping pad just a woolrich jacket beneath us. We both slept pretty warm, quite uncomfortable without a pad but we kept warm. My wife slept in her clothes and I put on a light pair of fleece pants and shirt. I did get a little condensation the first night but the second night I open the bottom up a little and had no problem.
My opinion is that these make a great emergency bag and we will always carry them. They will not replace a good sleeping bag but they could very easily save your life.
Marty Simon