So what you have right now are the rafter vents (soffets). I suggest putting in a few gable vents at the roof peak, as high and close to the roofline as you can get them. The soffet vents will pull in air, and the gable vents will expel air. Relying on windows for ventilation can be problematic, especially if you have to close them due to weather.
Thanks for the input! If we added gable vents, those would sit on the East & West walls, right? I did a Google search for gable vent, and that seems how most of them are set up.