I am installing a solarium in my house, 10 by 24 for two purposes. Passive solar heat and use as a greenhouse. I'm not sure about the best floor. I've seen one used effectively for passive solar with a brick floor. I live in Maine, and want it to be warm, impervious to water, if the two can be achieved. Do I need to build a foundation that is insulated or put it on gravel or pour a slab? Any suggestions would be appreciated.