Slab on grade addition adjacent to full basement – insulation and vapor barrier questions

A homeowner posted a question on a building forum about a small entry addition they're building next to an existing house that has a full basement. The original plan was to build a crawl space (a shallow unheated space) under the new entry, but the builder found a large, 12-inch-thick concrete shelf built into the existing foundation wall. It looks like it once supported a stone wall that's no longer there. Because of that shelf, the plan changed to a slab on grade, meaning the new addition's floor will be a concrete slab poured directly on the ground rather than sitting over a crawl space.

The poster shared construction drawings and asked other builders and homeowners for feedback on two things: how to insulate the new slab properly, and where to place the vapor barrier, a plastic sheet used to keep ground moisture from moving up into the slab and the living space above it. These are common questions when a new slab-on-grade section connects to an older foundation with a basement, since the two structures handle moisture and heat differently.

For homeowners not involved in this specific project, the situation is a reminder that additions built against older foundations sometimes reveal unexpected structural features, like this old stone-wall support, that can change the construction approach partway through planning. Getting the insulation and moisture barrier placement right in a slab addition matters for comfort and for preventing dampness problems down the road, especially where new and old foundation types meet.