Is it true that concrete, like stone or rock or brick will retain heat also? If so, the main cost would be in heating it up first like someone else said?
There are no free rides The only thing a great big heat sink like a concrete slab does, is to EVEN out heat over a long period of time, it doesn't save you any money or heat.
And, if the slab conducts a lot of heat into the ground, it's a loser
The parallel is cutting one end off your blanket and sewing onto the other end to make it longer. The slab might retain heat, and help "heat" the shop after the boiler is shut down, but IT ALSO took more heat to get it up to temp in the first place
