sub slab and basement insulation

This is a homeowner forum post, not a news story, so there's no program or policy update to report. A homeowner near the New Jersey shore who is rebuilding is asking for advice on foundation and wall insulation for a high-performance, near-passive house. The site sits between rivers and close to the ocean, with a water table at 6 feet and a planned footer depth of 4 feet.

The poster is weighing two exterior wall assemblies, both using continuous rigid mineral wool insulation (comfortboard) outside the framing, either with 2x4 or 2x6 studs, plus mineral wool batts inside the wall cavity. The goal is a wall that lets moisture pass through while a humidifier and dehumidifier manage indoor humidity, since the household keeps the house humid for a piano and instruments and to help a child's eczema. For the foundation, they're asking whether the high water table means using rigid foam board (XPS) instead of mineral wool below the slab, how deep the insulation should sit relative to the footer, and whether mineral wool against poured concrete is still discouraged.

This is a technical construction question aimed at getting engineering-level answers from other builders, not a decision an average homeowner needs to weigh in on. If you're planning a similar rebuild, the specifics here (foam type, depth, and vapor control near a high water table) are the kind of details worth discussing directly with your own architect or foundation engineer, since local soil and water conditions vary a lot by site.