Welcome to the forums!
As I write this, pinballlooking is in the process of building a greenhouse from the ground up. Currently, he is fine-tuning his automatic venting he installed. I would guide you towards reading his thread, as he has documented the whole build with lots of useful details, including product selection and trials.
http://ecorenovator.org/forum/lawn-g...use-build.html
Not quite sure when he selected the auto-vent cylinder arms, but he started installing them around Christmas, maybe page 50 of his chronicle.
As to a particular product you are considering, what brand or model of windows or doors are you looking at? You make no mention in your original post.