I guess everyones minimum is going to differ. Me for instance when I hunt at low elevation, here on the east coast, I carry an AMK Heat sheet and some Bank line cordage. Thats it. I figure make an A-Frame, build a fire and hunker down.
Now at elevation of 10,000 add in some snow/rain and wind, that set up just would not seem adequate in my mind. It might work, but there are better options. I myself carry a bivy and a sil-tarp shelter combo pictured here. (about 3 pounds all together)

My bivy is probably a bit on the heavy side (650 grams) - but it is heavy duty. I also complement or alternate with a sturdy trash bag, a disposable rain poncho and a AMK bivy bag.

I've read this thread with great interest and I have very little to add to a subject that I find very important.

Just one note of caution: Don't have too high expectations of breathable materials:

- Breathable materials breathe at their best when you have a temperature gradient, such as a tight fitting goretex jacket.

- Even the best breathable materials are what I call "fatigued breathers". I.e. even the best materials don't really breathe nearly ENOUGH. Particular in a bivy bag setting (where the temperature gradient is much less than in a tight fitting jacket).

My personal opinion is that condensation will form inside ANY waterproof bag that you wrap around you and sleep outside in chilly weather - no matter how breathable the material claims to be. Your body is at 100F and lets out a certain amount of water vapor all the time. The inside of the bivy bag is pretty close to the outside temperature. Condensation is inevitable.

Personally I wouldn't bother with breathable bivy bags until they have a well proven track record for keeping you dry for prolonged periods that otherwise cause massive condensation.

As always, I'm more than happy to be proven wrong

Along these lines, I've started carrying an All Weather Emergency Blanket that measures 5 x 7 ft. It is a reasonable weight (285g), is durable and has grommets in the corners so it can be used easily as a tarp. At $15 its an affordable way to cover this need.

One of the other things I like about this blanket is that it is truely reusable so it can be a regular part of my camping / backpacking gear too.

having spent a cold afternoon wrapped up like a tightly rolled taco in an "All Weather Emergency Blanket", with both ends exposed a bit, i grew tired of the wind whistling thought from my feet to head. no matter how i tried to tuck it in around my legs, the problem persisted.

that was when i decided a bivi with only a single opening made a better survival shelter for me.

but i still keep a few of those grommeted mesh-reinforced laminated mylar sheets around for make-shift tarps, or as a ground cloth, or as a loaner, or in the vehicle for back-up, or tucked in a small saddle bag, or to use a red one as a signal flag/indicator.

So true about trying to stuff them back in once they have been used. The AMK Heatsheets Emergency Bivy, small and compact, weighing about 4 oz, can indeed be repacked, although it does take care and a certain amount of time to carefully fold it while compressing all the air out. It is part of my standard kit, along with something for a tarp and some light twine.

Another consideration when doing an emergency bivy is picking your spot with care. Moving just a few feet can make an enormous difference in exposure to wind, cooler temperatures, etc.

Unless you are a Cheetoh or a Potato Chip,I doubt very much you will get any sleep if you have to deploy an emergency mylar bag/blanket! I say keep what you have,& add a Frogg Toggs rainsuit,they are light,fairly durable at least for emergency purposes,you can see your feet while stepping around obstacles,& they are quiet,& inexpensive,If you should develop a hole/rip,the little roll of duct tape in your Doug Ritter PSK should take care of that,Lickety-split!The Best advice has been said,& I will reitterate too,Test what works for you in a docile environment,your backyard.Remember-Potato Chips love to be eaten fresh,cause' they can't Stand that noisy a$$ bag,lol!

That explains the snoring I heard in the snack section at the market!

Have you tested the 'space' blankets by Adventure Medical Kits? They are made of polyethylene, not mylar. They are significantly quieter and more durable.

If it is an emergency, I would have to vote for the Blizzard Bag. I carry the hunter orange and a plb. I hate the idea of a search prior to the rescue. In a non-emergency, I would leave it in the pack and use the Goretex Set and the military poncho. I pack as heavy as I can stand. You won't stay in shape unless it hurts.

someone needs to come up with a version of the military sleeping bag that was vacuum packed.nothing for super sub-zero,just normal hiking weather where you be out without a full pack of overnight camping gear.something like the foil sacks with enough insulation to keep you warm.and a real vacuum pack and not something done with a carpet cleaner.use one and toss.if you could get a bag like that into a Spam,or smaller,size can i would think it would be a life saver.what i really look forward too is a cell phone size PLB under $50