zero radon??
At high altitude greenhouse temperature will not be a problem
Radon from soil is dangerous, when concentrated.
In my country there are companies who provide measurement of radon level in building plot, when level is low, regular ventilation is enough. When high, special isulation of base is needed when you build a house. When you cover house with greenhouse, you will encrease radon concentration.
Greenhouse temperature can easy reach 50°C durig summer, high greenhouse can help, but water will condense on house walls and windows.