Each to his own, and clothing + gear appropriate for the season. With the possible exception of the warmest summer days I never hike anywhere without my "technical" wind+waterproof jacket (similar to goretex mountaineer-type jackets, except the menbrane is not from goretex). Technical hood (i.e. one that works in foul weather), complete water proof, somewhat breathable and lots of ventilation zippers.


I also bring my bivy bag, which weighs 600 grams. Now 600 grams is a lot for the ultralight experts... but unlike "space blankets" my bivy bag is sturdy enough to be used on a regular basis. Most of the time, I unzip and unfold it into a rectangular sheet that I use as a wind break, rain tarp or heat reflector, using the pre-made rope rivets and para chord. Making lunch breaks comfortable is one of the true joys of hiking in a harsh climate... If it's very cold I unzip a ventilation opening and enjoy lunch inside my own micro climate. The bivy bag is even spacious enough that I can add my own heat source, such as a candle or a stove.


While space blankets is better than nothing, using it effectively requires a lot of practice - to the point that tossing a space blanket into your pack will be useless if you haven't practiced with it. You need to destroy a few to experience what works and what doesn't work. Also realize that wrapping a blanket is seeking a goal which is perfected in the shape of - A BAG! Why people don't use "space bags" instead of "space blankets" is certainly beyond me. A bag can be converted into a blanket with a knife in 15 seconds. Wrapping a blanket into a bag is next to impossible....Which is exactly why many survival manuals advise to bring some big plastic bags to make emergency ponchos / bivy bags.

Now uninsulated bivy bags are cold - but they are much warmer than nothing because you're now in a water proof and wind proof container. But even wind+waterproof, you still need to add insulation, both to the ground and between you and that cold bag material. Could anyone please invent some inflatable bubble wrap material that weighs and compress into next to nothing?

Another problem with non-breathing bags is that you will get condensation on the inside. Most likely, your clothes will get wet, or at least damp. Yet another reason to wear wool...