There comes a point in most of my backpacking trips where I think, “Who’s stupid idea was it to going into the woods anyways?” For my friend Patricia, that point hit pretty early.
Patricia hams it up for the camera
“I’m freezing!” she proclaimed. “Our water bottles are turning to ice and I can’t feel my toes. What are we doing out here?”
I unwrapped an emergency blanket and handed it to her. “Here,” I said, “wrap this around your body and over your head.”
She did (a little begrudgingly I might add) and then said. “It’s not working. Now I feel like a giant Christmas tree and I can’t hear anything anyone is saying.”
Truth be told, she did look pretty festive covered in silver next to the frosty evergreen trees, but properly used emergency blankets really do work. I’ve had them keep me safe in dangerous situations. (To read about my horror story click here.) So it is for all the Patricias out there that I write this article: How to use an emergency blanket.
To understand how an emergency blanket works, you have to examine the ways your body loses heat. Think back to physics class in High School (and I will say that I was a bit of a nerd back in the day). Your body loses heat by convection, conduction, radiation, and evaporation.
Convection is the loss of heat by contact with a fluid or gas. Rain, snow, wind, and cold air are the most pertinent examples. Used correctly, an emergency blanket creates a waterproof, windproof tube of insulated air. Because there is a limited supply of new air, your body establishes equilibrium with the warm air preventing heat loss.
Conduction is the heat transfer when two objects come into physical contact. One example would be sleeping on the cold ground. Because an emergency blanket is thin, it is not by itself effective for significantly reducing conductive heat loss. However, putting your sleeping bag over the emergency blanket forms a more effective insulating barrier. I once had to spend a cold night inside a wet sleeping bag. Because I wrapped myself in my emergency blanket (and the sleeping bag was polyfill), I was still able to stay warm enough to make it through the night.
Radiation is the simplest heat loss method to understand. Your body naturally releases heat, and emergency blankets reflects 90% of that heat back towards you.
Evaporation- As you have noticed on a hot day, your body produces sweat and the process of this evaporation creates a cooling effect. Unfortunately, your body still perspires some even in cold weather. An emergency blanket reduces the rate of evaporation. In warm weather, that would result in a buildup of fluid (think steam room), but in cold weather your body sweats less and that sweat is less likely to turn into condensation, so a vapor barrier can actually quite comfortable.
The other heat loss method that is worth mentioning is respiration. Because a properly used emergency creates a pocket of warm air, your body does not have to warm the cold air you are breathing in.
Enough with the science, let’s get down to practicalities. The most effective way to use an emergency blanket is to to create a seal around yourself. If you need to sleep, laying down and wrap it tightly around your body and over your head, being sure to not have seams where heat can escape. If possible, use the blanket under your sleeping bag and put your feet into your backpack. Leave a small opening for you to breathe through. Breathing through a shirt or scarf will help reduce heat from respiration.
Well, it looks like the explanation of why an emergency blanket took a lot longer than the instructions for use. I must admit that I’ve become a bit of an evangelist for them. I used one when I had to help out some ill prepared teenagers on the Appalachian Trail, and it made a huge difference in keeping us safe. I would not say I was toasty warm inside my cocoon, but warm enough to make it through the night. I hope this helps, and if there are things I’m missing I would enjoy hearing about them as I find the concept of vapor barriers pretty fascinating.
For a more technical explanation check out this article on thermoregulation. Also, sectionhiker.com has a lot of helpful information on this topic.
Related posts:
{ 2 comments… read them below or add one }
Thanks for checking out my blog! I love your website, too.
The bandanna is definitely essential. I had a buff that was perfect.
Excellent tips Adelaide! An emergency blanket is something I always have with me when I’m hiking, even for a super short hike. I have also found that a bandanna is another essential – it can be fashioned into a beanie to keep your head warm, and that’s where heat escapes as you mentioned. When we had some fires here in Southern CA this summer I used my bandanna to help keep out the smoke when I was returning to the trailhead.
Great site and keep up the interesting articles