Another thought:

I noticed a lot of condensation on your greenhouse walls, which also reduces light intensity inside a GH by reflecting light away. This is often much worse in the winter due to low night temps and higher humidity. I also read that certain kinds of plastics used in GHs can make this effect much worse. Not sure what you might be able to do about it, but again, something to consider.

Found this article after a quick search:
http://www.onhort.com/onhort/main.cfm?r ... cleID=3308

Gary