Spray foam insulation inside basement on walls with efflorescence

A homeowner with a basement in a house built in the early 1950s is weighing how to insulate it. Their preference was to dig out around the foundation and add rigid foam insulation on the outside, but an engineer told them this would cost much more than spraying foam insulation on the inside of the basement walls. Two engineers also told the homeowner they don't need to worry about moisture from the foundation reaching the rim joist and sill plate (the wood framing where the house meets the foundation) and condensing there, even though spray foam would block house heat from keeping that wood warm.

As a middle path, the homeowner is considering adding R-20 rigid foam on the exterior, running from about a foot above the rim joist down to ground level or a bit below, then covering it with a faux wood finish to protect it and help keep the rim joist and sill plate warm. They admit the R-value they've picked is more of a guess than a calculation.

The bigger worry is efflorescence, a white, powdery mineral deposit that shows up on basement walls when moisture moves through masonry and evaporates, leaving salts behind. The homeowner sees it on their walls, though no water has actually leaked through, and wonders whether it's safe to cover those walls with spray foam. One engineer suggested sealing the walls first with a product called RadonSeal, applied after stripping the paint, possibly using a rented dry ice blaster to do that prep work.