WDS>You can (and I have) built a small greenhouse yourself, mine is made out of wire, sheet plastic, and a wood frame. It works pretty well to keep the inside warm. I forget but I think it keeps inside about 20F warmer than the outside on a sunny day. The reason is the same -- plastic and glass both are pretty transparent to visible but opaque to IR. ----------- This can't be right. The outbound, reradiated IR heats the plastic? No matter how thin? Bounces off? (There are hundreds of ridiculous diagrams in Google images depicting this.) Claim: Greenhouses work just as well sheathed in material 100% transparent to IR. They work because they are made of AIR! http://www2.gi.alaska.edu/ScienceForum/ASF8/817.html --rwg