Hey Isaac, I dig the idea of having a loft in a greenhouse. A rocket mass heater would also be a great backup heat source.
A couple things stand out to me concerning your windows. First, you're going to want a large amount of dark-colored mass to soak up all of that direct sunlight. If all of the direct sun is getting soaked up by plants, you might find yourself charging your house with your rocket stove more often than not. Also, without some sort of awning/overhang to block the summer sun, it's going to get awful hot in there. Assuming 50 degrees latitude, you'll want to have a projection that is roughly half as wide as your windows are high. The Passive Solar Energy Book by Edward Mazria has a bunch of great information, you should check it out!
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