Having shelter ready to go is a must. By the rule of 3’s, you only have 3 hours to survive without shelter. Of course that’s all situation dependant. In the summer shelter may not be so critical to have in a 3 hour time frame. Not to say that hypothermia isn’t possible in the summer, because it is. 98.6 degrees being the accepted normal body temperature, and knowing that it only takes a 3 degree drop in core body heat to become hypothermic, it is still a real possibility. I’m not sure of anyplace on earth that maintains normal body temperature 24 hours a day. It can rain for days. Think back to childhood about how many times you may have shivered in the summer as well as the conditions. Heat stroke and sun burns also need to be considered. Weakening your body in any form puts a risk on survival.
I currently do not carry a trash bag. They are worthy of carrying for a minimum for survival. I couldn’t begin to say how many people have used them at sporting events, amusement parks and the like to stay dry to continue the fun. During my young dumb years when it was not to unusual for me and some friends to go spelunking, it was recommended as a must have for the “sport”. For those that may not be familiar with spelunking, that’s cave exploring. The reason to carry a trash bag was for its ability to trap body heat. Plus it took very little room to pack in. Belly crawling through tight spaces with a backpack isn’t possible. It’s kind of creepy if you ask me today. LOL. Oh to be young and dumb again. NOT!
I do carry a cheap 99 cent rain poncho. This can do whatever a trash bag can with the bonus of greater mobility. That happens to be its greatest downfall. A poncho has open slits on the sides held closed by snaps. Wind and rain are notorious for finding even the smallest opening. Wet means cold. Cold means death. Of course that’s the worst case scenario. That is my approach though. Plan for the worst, hope for the best. I like the idea of having my head covered with the poncho. This can be done with a trash bag as well. Arm movement and exposure are also a consideration.
With the talk of using plastic sheeting for shelter, it would be prudent at this point to consider “3 minutes without air”. What ever your plan may include, make sure it takes into consideration that, “keep breathing” . Insuring that what ever step you take does not involve accidental suffocation. Kind of like not falling asleep in the bath tub. In a survival situation, sleep will likely not come easy. I always figure my first night out being huddled under a tree wrapped in that 99 cent poncho. Having a secondary covering would be a good idea.
A cheap plastic painter’s tarp would be my secondary cover/shelter. A cheap plastic painter’s tarp is light and easy to pack. It can cover multiple people as well as one. Even being hastily thrown up, it could provide fairly adequate cover for the people in a group. I am typically alone, but you never know.
A bivy is also a good small light weight consideration. They look like a sleeping bag, but a bivy is nothing more than a shelter. They provide about 5-10 degrees of protection against heat loss. A bivy is little more than a shelter. This fact brings up a good point about what expectations that could be made of an uninsulated shelter such as a bivy. 5 to 10 degrees in the dead of winter is still freezing. Plastic shelters and mylar shelters (such as the bivy and mylar ‘blankets” are good at protecting against outside wind and moisture, but with regards to moisture, these shelters will also trap moisture.
Considerations for shelter are as follows: I like to have a solution that can be put up quickly and in the dark. Most typical emergency shelters provide 5 to 10 degrees of temperature difference. Comfort is likely to be secondary to staying alive. Practice in a controlled environment will pose the least hazard to you. Make sure you are not alone when practicing cold weather survival. That way you wake up.
With all considerations of possible dangers, I will say, practice with the means you wish to enter the wilds with. Gain knowledge for your most likely scenario, and come up with your best plan to survive. Survival is survival; go out armed to the teeth with the knowledge and equipment to survive. Keep in mind the various situations that you may encounter when you come up with your plan.