0Re-design 8 years ago ReplyUpvoteAluminum foil. If you want to keep the heat out it reflects it back out very well. If you want to keep the heat in it reflects it very well. It's not much of an insulator though. But it is recyclable when you need to. Best yet is the bubble wrap that has a reflective barrier built in. It's very durable and works very well. Worked very well for me once in an old almost uninsulated house turned office.