Originally Posted By: Roarmeister
The plastic sheet allows the radient heat in and then traps it like a greenhouse.


That's the key. The plastic allows heat in then holds it around you like a bubble, instead of that warm air dissipating away. Adding the space blanket has an additional effect but in my experience it's not necessary for the shelter to work well. Of course I have only used a supershelter down to the mid 20's. If you used it in significantly colder weather I suppose the space blanket's contribution would become increasingly important.