Hi Thomas -- what are you planning to grow inside the greenhouse during the summer? and... why? Also, you mentioned growing veggies on the north end of the GH because they would be cooler there; do you mean on the outside of the GH? Because if so, they're going to be pretty much shaded and most veggies need full sun. Inside the GH, especially in the summer, the heat is not the only problem -- things can dry out really fast, necessitating a LOT of watering. Are you maybe thinking about an automatic watering system?
Even way north where I am, my GH -- which is 8x16 and has 6 roof vents and a 4-foot-wide door on one end -- becomes too hot for most things to grow happily during the summer; and the temperature in the winter, with everything closed up, often goes over 100F on a sunny day with an outside temp close to zero. Just to give you an idea of how much they can heat up.
