I am working on my first Insulated and heated slab for a garage addition. We are focused on getting the details right. To start we are in Northern NJ (Climate zone 5/4 Marine) We installed 2″ XPS down the inside of the stem walls to the footer. Under the slab we have 2″ XPS sitting under the Vapor Barrier and wire mesh and above the crushed stone capillary break. We are running into an quandary with the edge details:

The mason is insisting that we need to

1. Stop the under-slab and edge insulation at the block at the front and back entrances so there can be a solid connection at this key stress point.
2. On the edge of the 5″ slab he is insisting the foam be only 3″ high and the final 2″ be a black expansion joint material.

I see these as 2 KEY spots for heat loss that could cause long term issues with the system. My concern is that we are, in effect, connecting the heated slab to the unheated stem walls (and outdoors) at the openings which will be a major cause of conductive heat loss. I am looking for a solution to maintain a thermal break for energy efficiency but also maintain durability.

I think it would be better to run insulation the full 5″ and simple bevel the top and eliminate the expansion joint completely. I also think we should do the same across the entrance on the inside of the block essentially creating a thermal break and separation between the outside 8″ that sits on the stem wall and the inside slab.

Please share your thoughts and advice because I need to make decisions ASAP.