A space blanket is a really good thing to have in your survival kit. They have a bunch of different uses and don't take up a lot of space. They are not expensive. You can usually buy like 10 of them for just a few dollars. I would recommend carrying more than one. The more you have, the more you can do with them. They are so useful that it just makes sense to have a few of them in your pack.
Space blankets are waterproof and windproof. They are also super reflective and have the ability reflect heat. These properties make them very useful. They are also referred to as emergency blankets. They are commonly found in first aid kits. They are also commonly included with with camping gear and in survival kits.
Space Blanket Uses
They have many different potential uses. Given their size and weight, you should probably carry them even you have other materials that are better. Even if I were carrying a tarp, a tent, a sleeping bag, and a wool blanket, I would still bring along a few emergency blankets. They are good to have just in case.
Warmth When Injured
Emergency blankets are often included in first aid kits because they are very effective at keeping the injured person warm. When a person goes into shock it is important to keep them warm. They are great when used to preserve the body heat of shock victims.
If you find yourself in a survival situation where you are stuck out in the wilderness without proper gear to keep warm, a space blanket can be a life saver. You would generally wrap yourself up in it as tightly as possible. It will radiate your heat back towards you which will help you to stay warm. You might feel like you want to just drape it over your shoulders. This might help a little but if you really want to get the most out of it you are going to have to lay down and wrap yourself up in it.
By itself, it won't keep you warm enough in cold, below freezing conditions. It will probably be sufficient either on cooler nights or when combined with other blankets. Often times people combine a space blanket with another type of blanket ( ex: fleece ) in layers. People tend to disagree on the most optimal way to do this. Some say that you should put the emergency blanket on the inside and the other blanket on the outside. Others say that you should put the space blanket on the outside instead. Some people recommend using multiple layers of each.
These blankets may keep you warm enough to get some sleep, depending on conditions. You may not necessarily even be attempting to sleep. If you know you are only stuck for one night, your goal might be to just avoid freezing to death until morning. In this case you may stay awake but use the mylar blanket for just a bit of extra warmth.
Shelter - Emergency Blanket Shelter
Space blankets can be used to construct emergency survival shelters. In the most simple configuration, you could use a single blanket to construct a lean-to. This might keep you out of the rain a bit and also may potentially even keep you warm when combined with a fire.
If you want to build something a bit more elaborate, you could construct an a-frame tent. You would first run a line between two trees or two posts of some type. You would then drape the blanket over the line and stake it down. If you try this with a single blanket, you will quickly realize that it is a bit small for this type of shelter. It will barely give you enough space. You can improve this by using two space blankets, both draped over the same line. This will give you enough space for a passable shelter.
Keep in mind that you really want to have something to keep you off of the ground. A third blanket could be used for this. So really, you want at least three space blankets. The more the better but three is about enough to give you a passable shelter. Ideally you would want to place something else under the blanket that is on the ground ( ex: leaves, pine branches, etc. ).
Another option would be to build a shelter out of logs, branches and sticks. You could use the space blanket as an additional waterproofing and heat retention layer of this shelter. For example, you could construct a debris hut and cover it with the space blanket. This option works if you have the time to construct something more elaborate. It is also better for long term use.
Tip - be careful not to rip the corners. This can really become an issue when staking down a space blanket. If you attempt to just drive a stake right through the corner of the blanket, there is a good chance that it will rip. As an alternative, you could place rocks on the corners. If you happen to have duct tape ( or any other tape ) you could reinforce the corners before staking it down. This is highly recommended.
Pre-built Space Blanket Tent Variant
If you don't want to construct a tent from an assortment of different materials, you can actually buy pre-built tents made from the same materials. You can buy actual tents, bivy sacks, and sleeping bags made using mylar. These all range in size and quality but they can save you a ton of time. They can also help you avoid potential failures and pitfalls when constructing your own.
Heat Reflector for Fires
Another great use for space blankets that you may not have thought of is as a heat reflector. These are commonly placed behind a fire to reflect the heat back toward the person next to the fire. You also might use one a reflector attached to the inside of a lean-to that is right next to a fire. You could also do both if you have more than one blanket. This is a whole lot better than just creating a standard reflector out of logs. Adding a mylar blanket to a log reflector can seriously help to reflect more heat back.
Protect From The Heat
Another, almost counterintuitive use would be to protect yourself from the heat. In a warm environment or a desert you can shield yourself from the sun very effectively with one of these.
Collecting Rainwater or Distilling Water
Another creative use for a space blanket is for collecting rainwater. Having a clean source of water is important and being able to collect rainwater is a good way to achieve this goal. You could tie it up with the center hanging much lower than the rest of the blanket. You could also dig a hole in the ground and line the hole with the blanket to catch rainwater.
If you wanted to distill water ( ex: seawater ) you could use this as one component of a still. It would be used to catch the evaporated water after it has been boiled.
What Are They Made Of?
Space blankets are made out of metallized boPET. BoPET is short for biaxially-oriented polyethylene terephthalate which is a polyester film. This film is made by stretching polyethylene terephthalate (PET). So basically once the film is produced it is then metallized. I believe they generally used an aluminum gas which is bound to the film.
Mylar is actually a brand name for boPET which is just the film. The metallized mylar is what space blankets are made from.
The material was originally developed by NASA for use in space. I think boPET was actually developed years earlier by private companies and NASA created a metalized version of it for use in space.