I have had a Rion 8x12 on a wooden deck raised about 8' above ground for about four years. I floored it with gray colored boat carpeting, which is reasonably priced and looks like it is going to last forever. No under floor insulation. The water goes right through the boat carpet. In winter I heat it with two small electric heaters, keeping it at 55F. It did OK on a 20F night a year or two ago. Here are some pix:

Here is a link that might be useful: Greenhouse

Thanks billala I bought the Rion EcoGrow about 6x8. I think this is the way I'll go but also thinking instead of decking wood maybe exterior plywood? The only problem I see is water drainage issues. Your pics are great, many thanks.