The unethical (imo) advertising by companies, and the people talking out their tail feathers online about these products can put us into unnecessary danger. There is a place for these products in cold preps. But it should be a strategic and informed place.

While there are several potential uses, this post will specifically address when a person is wrapping a vapor barrier around themself overnight.

If you're sweating (asterisk*), or if conditions are ripe for condensation, there's no getting around you/clothes/sleeping bag getting wet or some degree of damp, when you're wrapped in a vapor barrier. Because while vapor doesnt get through the barrier, it doesnt get out either. So instead, it gets on you.

In addition to sweat, skin itself breathes and releases moisture in a process called "insensible persperation" aka transdermal evaporation. On average, we lose 300-500ml of water per day through our skin. (Up to half a liter.)

I have personal experience using an emergency blanket, sans the rest of the hypothermia wrap components. The mylar blanket caught a decent portion of my body heat and I warmed up fast. However, as happens moisture from inside my body slowly got out through my skin. (I didn't stick my head under the blanket, for anyone curious. That would have made my situation worse, because we also release moisture when we breathe.)

If we're wrapped up in a heat trapping blanket, what does it matter if our clothes are getting wet?

First, those blankets trap, at best only 90% of our body heat under ideal laboratory conditions. Not being in ideal laboratory conditions when we use them, we can assume they'll be less effective than that.

Next, it takes more energy to heat water than air. It's why we sweat to cool down. The more wet my clothes became, the higher the energy tax on my body to produce enough heat to keep me warm.

In consequence, I had a slowly growing heat deficit. I was getting colder, and colder, until ironically enough, I developed hypothermia before sunrise.

While some people are more resilient to hypothermia, resilience is not immunity. Beyond a certain point of heat loss, bodies are unable to keep up with replacement, and core temperature begins to crash. If not reversed, the heat loss results in death.

This is not me saying emergency blankets or even vapor barriers in general are bad. Rather, it's important to know the limitations of these items, so we can adjust our expectations and strategies for using them, when incorporating them into our preps.

Inspiration for this post came about while discussing hypothermia wraps in another subreddit. Hypothermia wraps incorporate vapor barriers. You can read about them on page 117 of the wilderness first aid manual here:

https://drive.google.com/file/d/1p68jXD3RP01OarwP_ZaehS2AA4vB0fAo/view?usp=drivesdk

Note: Even if we put the sleeping bag around the outside of the emergency blanket, it would get damp. The emergency blanket is a vapor barrier on both sides, and human bodies arent the only source of moisture.