New Green House question

[jonnygreenthumb]

Would a gravel floor be better in a green house that's 8'x12' or would a floor of 2x4's be better. I'm thinking the gravel for higher humidity levels. Any input or other ideas in building this would help such as ventilation, covers and heating. It will be in an area that receives full sunlight until 2-3:00, then it will have "bright" shade for lack of a better term.