I use wood to heat my 14x48 hoophouse. With a double layer of inflated poly, the best I can do is about a 15-degree gain over the outside temp. With a single layer, it's about half that. And that is burning a large amount of firewood. My forced-air furnace is in an external building.

Anything clear that makes a good greenhouse covering makes crappy insulation, period. Double poly is something like r-1.6. Even very expensive glass only gets to about r-2.

I think the Chinese have the best design. They roll an insulated curtain down over the roof, use wood heat at night (or coal or biomass), and then roll the curtain back up at dawn every morning. That's a lot of work, I know. An automated design that did the same thing would be neat, something like a retractable roof baseball stadium.

This post was edited by Cole_Robbie on Sat, May 4, 13 at 14:23

Hi. I am really new to greenhouse growing, and new to this site, but have been reading a lot of posts, and love all that is written.
I have a 8' x 40', all metal sided greenhouse, lighted with grow lights only. My stupid question is do I need to use my heater in the summer? Do I need to totally keep it warm on colder windy days? Help the newbe, thanks.