Here is a useful link:
http://www.builditsolar.com/Referenc...s/HeatLoss.htm
The R value for 6 mil plastic is .6 and an inflated double layer is about R1.6 I think. Using the calculator will make it sink in how difficult it is to heat a greenhouse. The best design I have seen is Chinese. It's only clear on the south side, and they roll a big insulated blanket down every night and back up every morning. For my greenhouse, I have to build a little low tunnel around the plants, drape that in plastic, and put the heater under there.