Foundation Box top view

Here is the top view of the insulated "foundation box". The bottom will also be fitted with a piece of 2" insulation. 

4" perforated corrugated drain pipe will be placed at the bottom, which will receive heated air blown down from inside sun heated air at the top of greenhouse. 

2ft of1.5" rough stone will be placed on top of the perforated pipe and should soak up and store any temperature from the heated air passing through it.