Airsealing and Insulation 1st Floor and Basement in Moist Century Old House

A homeowner in Massachusetts with an 1880s Queen Anne Victorian is weighing how to fix a cold, drafty first floor and a damp basement. The basement has a thin "rat slab" (a shallow concrete floor poured directly on soil, common in old basements), a stone foundation that needs repointing, and a brick wall with an air gap, all in an area with a high water table. The first floor stays below 60 degrees on cold days even with mini-split heat pumps and baseboard hot water running, partly because the old plank subfloor has gaps you can see through into the basement below.

The homeowner is considering several fixes for the basement ceiling and foundation: excavating outside to waterproof and insulate the foundation (likely the most effective but priciest, and it wouldn't address the brick skirting); spray foam on the interior of the stone and brick walls (cheaper, but risky given the high water table and the lime-based mortar's tendency to hold moisture); pulling out old plaster and lath, then insulating between the floor joists with fiberglass and sealed plastic sheeting (cheapest, doable as a DIY project, but complicated by diagonal bracing that limits how much insulation fits); or spray foam between the joists as a contractor suggested, which would still need a fire-rated covering.

This is a homeowner's question posted to a building-advice forum, not a product launch or program announcement. No rebate, cost, or timeline details were given, so there's nothing yet to act on beyond weighing these tradeoffs for a similar old, damp basement.