cheap option for thermal mass
Just started reading this forum (been on the orchard forum for a long time.)
as I read thru various posting I see the need for a thermal storage mass for cold climate greenhouses.
I own a granite fabrication shop and we throw away 100's of sq. ft. of 1-1/4" granite pieces every week. the discared pieces are too small to save.
situated in a rack that holds the slab pieces upright maybe an inch or two apart the granite peices could store a LOT of heat in a relatively small space just by blowing warm air thru them with a small fan.
Any granite fabricator has a lot of this kind of waste; ask around and you can probably get pieces for free just by asking.