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The rule of threes says that a human can expect to survive for only about three hours exposed to the elements, about three days without water, and about three weeks without food. Given how space blankets can significantly help with both the exposure and the water problems, and maybe even help you catch some fish, there’s little reason not to pack one or two of these tiny guys in your emergency kit.
Read on to learn how to use yours properly.
Space blankets are thin sheets of plastic (polyethylene terephthalate) with shiny metalized surfaces. Originally invented by NASA for use in protecting space equipment, space blankets are now widely used for many purposes. Many of which support outdoor survival and first aid. Space blankets are also known as mylar blankets, emergency blankets, first aid blankets, safety blankets, thermal blankets, weather blankets, heat sheets, or shock blankets.
I chose to write this article because I’ve seen way too many websites, pictures, trainings, or advertisements that teach completely incorrect uses for mylar space blankets and I want to set the record straight on how they work and how to use them correctly to get the maximum benefit from them.
How do mylar space blankets work?
I have no idea why, but a common myth about space blankets is that they can help you generate heat. This is false. The thin sheet of metalized plastic cannot generate any heat on its own and doesn’t do anything to help you generate more heat from your own body.
So how do space blankets work?
If you remember your lessons in heat transfer from physics class, you’ll recall that heat is transferred by four main mechanisms:
- Convection – The transfer of energy between an object and its environment due to fluid motion.
- Conduction – The transfer of energy between objects that are in physical contact.
- Radiation – The transfer of energy by the emission of electromagnetic radiation.
- Evaporation – The transfer of energy during phase changes in material (like water).
So with these mechanisms in mind, let’s think through how a shiny plastic heat sheet could help us stay warm.
Read more about how to stay warm in cold weather here.
First, let’s consider convection. A ceiling fan works by convection. It doesn’t actually cool the air, but it moves the air. The fluid movement of the air pulls heat off your body and into the passing air. Under normal conditions, we typically lose about 5-10% of our heat through convection. This rate of heat loss goes up as wind speed increases or air temperature decreases. So on a windy, cold day, preventing heat loss by keeping your exposed skin out of the wind can help keep you warmer.
Since our emergency mylar blankets are windproof and can reduce wind flow across our body surface, using a space blanket as a wind barrier will reduce heat loss due to convection.
Second, we’ll examine how conduction may be impacted by emergency blankets. The bottom line here is, they don’t do anything. If you’re laying on your blanket on a pile of snow, you’ll stay no warmer with the blanket than without in terms of conduction. It has no insulating properties. Since the body loses about 2-20% of our heat through conduction in cold weather depending on how well insulating our clothing is and what surfaces we’re touching, this is a minor concern for staying warm. Especially since we’re pretty good at feeling heat loss by conduction and moving to avoid it.
Third, we’ll look at evaporation. Technically, this is mass transfer, and not heat transfer, but the human body loses heat through evaporation so we’ll consider it. The amount of heat we lose through evaporation varies greatly depending on the surrounding temperature, our rate of perspiration, respiration, and many other factors. Under normal conditions, we tend to lose about 22% of our body heat to evaporation.
Whenever you sweat, you lose heat as the water evaporates off your skin. While a mylar blanket doesn’t cause you to sweat or prevent you from sweating, it will certainly trap any moisture you give off through sweating or respiration.
This water you give off will be trapped by the blanket if it’s wrapped around you and cause the humidity in the microclimate to increase. Initially, this will reduce your perspiration rate and lessen your heat loss through evaporation, but after some time, the added humidity will lessen the effectiveness of any insulating clothing you have on by getting it damp.
That’s why rule number one for staying warm in winter is staying dry.
What this means is that evaporation can lead to decreased effectiveness of space blankets and we need to be careful to not completely seal up whatever shelter we’re building with our emergency blankets. Not only could a tight seal lead to asphyxiation, but all that humid air will condense and drip back on us and we’ll become soggy, cold, and miserable in a hurry.
This is also why any mylar emergency sleeping bag or bivvy won’t actually keep you warm for any longer than it takes to fill the bag with humid air.
If you plan to use a bivy, be sure to allow for proper ventilation to keep the humidity from condensing on the inside. This may mean cutting it open if it’s not made of breathable material or if you sweat a lot like me.
Think twice before buying that cool looking emergency bivy unless it’s made of breathable fabric like this one:
So the final heat transfer mechanism – radiation – is the big one when it comes to space blankets.
Humans typically lose about 60-65% of our heat through radiation, so it’s important to consider this one carefully.
Thermal radiation can be understood by considering the sun. It’s across the solar system, but it’s radiated heat warms our whole planet. Infrared heat waves emanate from anything with any heat in it, including us.
The human body is always radiating heat to the environment around us if that environment is colder than our body. You don’t radiate heat to the environment if it’s a triple-digit day, the environment radiates heat into your body. Heat always radiates toward cold so it’s really impossible to fully prevent heat loss by radiation, but just like heat waves from the sun trapped in a closed car, our heat waves can be reflected back to us by shiny surfaces and retained.
That’s where mylar blankets come in play and have benefits over a plain sheet of plastic like a garbage bag.
The metalized surface of mylar blankets is very reflective and can reflect about 90% of your radiated body heat back toward you under ideal conditions (which rarely ever exist in the field).
This reflection only works when the blanket is not directly touching your skin. It requires an air pocket to prevent the heat loss through conduction. Think about a greenhouse. No airspace, no heat capture.
So what does all this mean?
Basically, the best way to use an emergency space blanket is to set it up as a reflective surface away from your skin with enough ventilation to prevent increased humidity.
In that regard, calling it a blanket can really confuse people since it works by totally different principles than an insulating blanket. Many people try to use mylar blankets as normal blankets and it doesn’t work very well because they’re not at all insulating.
In the next sections we’ll discuss in more depth how to effectively set up a space blanket emergency shelter to make the most of the reflective properties of it to retain – or regain – more body heat or on hot days, reflect the sun’s heat to keep you cool.
How to use space blankets for staying warm and dry in cold weather
When you’re on the move:
if you’re actively hiking or traveling and need to stay warm and dry in a pinch, the best way I’ve found is to use your emergency space blanket as a poncho over your clothing. Carefully cut a hole you can fit your head through in the center being careful to keep it round so no points exist that could easily tear.
Pull the emergency blanket over your head and secure it however makes sense to your body. This may mean tucking it into your belt or under your watch or whatever you can secure it with to keep it somewhat secured around your body.
If it’s just flapping in the wind, it will help keep you dry from rain or snow, but won’t be as effective as keeping cold wind from stealing your heat and won’t be able to reflect as much heat back at you.
Just be careful to pay attention to your perspiration rate. If you start to sweat at all with the blanket over you, the moisture will get trapped and make you cold.
When you need to stay put:
If you’re sheltering in place for the night or until SAR can find you, the best method is to set up a simple lean-to shelter with the mylar emergency blanket as the roof and reflective backing and if it’s big enough, as a ground barrier.
Check out this video to see how to do this.
A few other options would be to build a lean-to frame with timber and drape this over the top with a few branches over it to secure it in place. Just be careful not to rip the mylar or it won’t take long for wind to rip it even more and you’ll be left exposed to the rain.
With the emergency blanket as a reflective backing to your shelter, you can make a fire across the front of your shelter that will radiate heat into your shelter all night. The emergency blanket will reflect the heat back toward you and keep you warm on both sides and reduce the amount of heat escaping by radiation. Even more effective would be to stack rocks or snow on the far side of the fire to form another reflective wall to keep more of the fire’s heat headed your way. If you have a second emergency blanket you could even drape that over the pile of rocks to increase the emissivity and improve radiation.
Just be careful not to get the mylar blankets too close to the fire because they will ignite.
If you’re able to use the emergency blanket as a ground cover, you’ll be warmer if you stack some insulating material under it first such as grass, dry leaves, or pine needles. This will help prevent heat loss to the ground (conduction).
If you’re hoping to be found by SAR, it’s best to always use either plain silver or blaze orange colored emergency blankets because the silver will easily reflect light and orange is the most visible color to human eyes.
Just be sure to build your shelter in the open and not buried in the woods so SAR or an aircraft can actually spot you.
Another note on that is that if aircraft are searching for you at night, they sometimes use thermal imaging cameras. The emergency blanket effectively prevents these cameras from seeing humans.
The Taliban actually used them to stay hidden from US forces in Afghanistan.
In this case, your best option is to use the blanket as a ground cover and natural materials as an insulation and waterproofing layer if needed – or just come out of your shelter when you hear any planes or helicopters overhead.
How to use an emergency mylar blanket for staying cool in the sun.
The amazing reflective properties of these emergency blankets means they can be used to reflect the radiated heat from the sun away from you as well.
If you’re stuck in a hot, sunny location and can’t find adequate shade, you can do your best to rig the mylar sheet up with the shiny side facing directly at the sun. Keep the sides open to allow for air circulation (remember the convection discussion above?).
You may have to adjust the angle of the shelter as the sun moves across the sky, but staying under this will keep up to 90% of the sun’s energy from reaching you.
you could even tuck a corner of it under your hat and use it as a sort of reflective cape as you hike.
First aid uses for space blankets
Not only should you have a mylar emergency blanket with you for shelter to protect you from the elements, but keeping one in your first aid kit can open up all kinds of options for the creative – or the properly trained. Here’s a list of first aid uses for mylar emergency blankets.
- Use strips of an emergency blanket as a tourniquet.
- Use an emergency blanket as a sling to immobilize a broken bone.
- Tie a strip of an emergency blanket as a compression bandage around a wound
- Tie a splint in place using multiple strips cut from the emergency blanket
If you look at the weight savings you can gain by packing a simple mylar blanket instead of a tourniquet, a sling, and an ace bandage, it’s clear they’re worth more than their weight in your pack.
Other uses for space blankets
For passive signaling, while you’re resting or busy with other tasks the reflective surface of the mylar blankets is perfect for spreading out in an open field as a signal when you’re lost and search and rescue crews are looking for you. Blaze orange is also a great color if you want to get one with dual colors.
If you’re trying to actively signal someone, you can wrap the emergency blanket tightly around something like the end of a water bottle or phone and point it where you want to signal.
If you’re in need of fresh water and it’s raining, you can simply spread the space blanket out and funnel the water into a container to harvest the rainwater from it. You can collect a lot of water in a short amount of time with this much surface area.
In winter, you can spread a thin layer of snow on the blanket and allow the sun to melt it and collect the water.
Not only can you protect yourself from the elements with your space blanket, but you can wrap it around your backpack as you hike or when you get to camp to keep it dry.
The mylar material used in space blankets is heat resistant up to about 400-500°F, but if exposed to direct flames, it’s very flammable and will ignite.
Tip: when you build a shelter with a mylar blanket, don’t build your fire too close.
The flammability of mylar also means you can shred it up in a little ball and use it as tinder to catch a spark. It also burns quite slowly so it provides for a good fire starting material when needed. Just be warned that it’s plastic and will put off nasty smoke.
You can improvise cordage in a pinch by cutting the blanket into 1” strips and twisting them until a tight string forms.
The shiny plastic can be cut into small bits and attached to fish hooks to lure fish.
If you’re stranded in a cold car in winter, you can line the windows with pieces of mylar blanket to reflect the heat back into the car and prevent heat loss.
A note on the durability of mylar emergency blankets:
The ultrathin mylar emergency blankets are definitely not very durable and not meant to be used more than a few times.
As long as you’re careful with them and are prepared to replace them after each use, this typically isn’t a problem. They’re super cheap, so just buy a bunch and replace them as needed. The goal is to get back home alive and if they help with that, they’re doing the job you need them to.
Should you carry an emergency blanket?
Yes, you should pretty much always carry an emergency blanket with you into the backcountry or in your car. They’re cheap, lightweight, packable, and readily available. The cost is very low for the potential value they could provide.
Which emergency blanket should i carry?
So the next question is if you’re going to carry one, which one should it be? My current recommendation is to get an oversized silver or blaze orange one for increased visibility and a more comfortable shelter.
The regularly available sizes are below:
79” x 39” – This is the small size. These aren’t great at building shelters with, but will work for a short huddle to pass a storm or such.
82” x 52” – This is a standard size blanket but still not big enough for much.
82” x 62” -This size is considered extra-large. It is ten inches longer than the standard size and fits taller people, but still not great for shelters.
59” x 87” – These are larger than most emergency blankets on the market. If you want one for more than just first aid and such, don’t bother with this one and just go for the oversized.
71” x 142” – This is the oversized blanket that I like to carry. It’s perfect for shelters but still doesn’t take up much space in a pack. They’re harder to find, but here’s a link to buy some if you want:
I carry the oversized one in my ARK (Always Ready Kit) and I have a small one in my first aid kit. Whenever I go out in the backcountry for things like skiing, mountain biking, canyoneering, or trail running, I pack my ARK with various modifications to match the adventure, but for shelter, the oversized mylar space blanket seems to be my best lightweight option.