I have been thinking that it would be very beneficial to have some apparatus by which to rig a vent or louvre to open when it gets hot, and close when it cools down. Ideally, fully open at 100 degrees F, and fully closed at 60 degrees F (ideal for me anyway).
I am thinking of something like 304 stainless cable in a sort of block and tackle pulley configuration. I have found a thermal expansion calculator online and it says that a 1000' of 304 stainless cable will expand 4.6" when the temp changes +40 degrees F. Of course, when the temp goes back down it contracts back to original size...this could be tied to a lever that would open and close a vent. The diameter of the cable does not make a difference on the expansion, so something small and cheap would work fine.
I think 4.6" is enough travel to cause the vent to be completely open @ 100 degrees F, and completely closed @ 60 degrees F. Some kind of weight or spring would have to be used to keep the cable pulled tight at all times as it wraps around the pulleys...50 total pulleys would be needed for a 20' greenhouse. What a tangled mess that would be if even one of them jumped off a pulley...nasty.