I hope it helps some of your plants, and your heating bill, Karen!
I read about that trick decades ago, but I no longer live in a really cold area. We get a few snow days, and many freezing days, but very few days much below 30 F. Our 20-year average LOWEST annual temp is 15-20F, yet I see few days below 30 F.
The authors (in Maine) claimed to be able to get another 10-15 degrees out of each hoop-film layer (two LOW tunnels under their big hoop houses). They said they were able to grow commercially salable salad greens all winter. I don't recall them supplying extra light, though.
