I am sure everyone has seen the windshield sun visors that are made from what looks like mylar coated bubble wrap. That product is know by the trade name Reflectix, and is commonly used as a radiant barrier in construction retrofits on single family homes. I use it in the windows of my RV when it is in storage, and it drops the inside temps by about 20 degrees F. ...
This product also works well as a field expedient sleep system when combined with duct tape to make a "sleeping bag".

All the usual disclaimers, but: I have used this product as it was designed, and in my own designs... Hasn't failed to satisfy me yet.