A new foundation was built on top of an old foundation for a room addition. In the picture you will notice a vapor barrier sticking out between the 2 foundations. Why would a vapor barrier be installed between the 2 foundations? This wasn’t a foundation beam (no crawl space), but a foundation slab.
Hi Jeremy, what region are you in? That seems like a good idea to me but I will have to do some research. It seems that would prevent any wicking or block moisture coming through any old cracks
Maybe they had problems with dampness before and wanted a dry floor this time around. 
They might not of had a moisture barrier under the first slab and had moisture issues with what ever they had on the slab.
It would be the same as building a new slab on dirt. A moisture barrier would be installed.
What is up with the wood sandwiched in between the new slab and the stone and backup mortar or slab.? Looks like another problem for the future.
I’m not sure what that piece of wood is. Maybe they poured the new foundation short and bolted a wooden beam onto the end to extend it? It looks to thin to be a post. Who knows? I’m out here in the great state of Texas, Houston that is. Thanks for the replies!
I know yall get an incredible amount of soil expansion and contraction out there so a cracked original slab is certainly possible. I looked a little closer at the image…is the window sill basically sitting on the new slab?
I was thinking it was a door rough-in…
Sheesh. I didn’t even recognize that threshold.
This is a great example of one of those times people may think im an idiot until I open my mouth and prove it…