To a first approximation, greenhouses simply cut down on air movement, so warmed air doesn't escape. This is why covering plants at night to prevent frost works. Ditto for parkas, which keep warm air from escaping, and you can control the air exchange by controlling how loose/tight your collar is. There's lots of 'greenhouses' here in Santa Barbara County (home of flower growers, among other plants); almost all of them are covered in various kinds of plastic -- not because plastic is more (or less) effective, but because it is considerably lighter & cheaper. The major difference between plastic & glass isn't in the IR bands, but in the UV bands. Plastic is pretty good at filtering and/or downconverting UV, whereas glass tends to pass it through. Perhaps some of this downconverted UV light is converted into IR with plastic. The little bit of plastic in your car windshield still does a pretty good job of filtering out the UV. Back in the old days, only car windshields had to have the plastic, whereas the side windows didn't have it. You could tell how much UV came through each by how much bleaching action happened on the top of the dashboard v. the seat covers. At 12:00 PM 1/1/2014, Bill Gosper wrote:
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!