I suspect your mechanical engineer with the dew point issue is the most credible and the likely cause based on the information I read. An 80 year old concrete, stone or even wood foundation can, and will, wick or draw moisture up into the foundation or masonry from the soil especially with damp or wet soil. I suspect the moisture is condensing into water form when the dew point conditions are right, especially as described on the north side.

However I suspect your solution will be more involved than just some silicone on the foundation and under the siding. Please understand that vinyl siding is not a water tight surface. We at Mosby discover water behind the siding-cladding materials and have found many “leaks” at the gutter line, soffit area, and at the top of windows. Here’s an example of water leak detective work.

Also, we do not recommend installing new siding over old siding. There may have already been some developing water issues that were exacerbated by the latest installation.

I suggest you discuss with the mechanical engineer a scenario where you heat (raise the temperature) and dehumidify (lower the relative humidity) on the north side basement or crawl space to affect the dew point. I believe you will not be successful “drying” or preventing wetting of the foundation which is in contact with the soil. I believe you can be more successful at changing the dew point issue from the inside and therefore reducing the number of times the joists get wet from the condensing moisture.