Make a black vent...

that extends fairly high over the top of the greenhouse and you could have a solar-powered ventilation.

There was a book years ago about building greenhouses the low-tech way. John Muir Publications {the same guys with the VW repair manuals}.

It seems to me though that its better to have glass on the East wall (morning light is key), strawbale insulation on the West wall (to protect from overheating in the evening), and thermal mass insulation on the North wall (thats the cold/shady side).
This for Northern temperate, it depends on how far north on how much glazing on roof is desirable...