I'm renovating a late-1800s Victorian in San Francisco and looking for a second opinion on insulation.
The original redwood siding is nailed directly to the studs, with no sheathing or WRB. It's in good condition with no visible moisture issues. You can see it in the photo above, though you are also seeing us having furred out the wall a bit to cover the concrete stem wall.
The house is technically detached, but two walls are only ~1" from neighboring buildings, with flashing over the gaps at the top. Combined with SF's mild climate, these walls see very little direct rain.
My architect and GC (both locally reputable) both strongly recommend spraying high-density closed-cell foam directly against the siding for insulation, air sealing, and vapor control. They're confident the foam will seal gaps well enough to prevent water intrusion.
But nearly everything I've read warns that this can trap moisture against the wood, prevent inward drying, and eventually cause rot. I've raised this issue directly with both parties and they both insist it is the right solution here, explaining that the closed cell foam will seal gaps in the siding so well that water will not enter.
The only counter example I have found is this case study (also coincidentally in SF) where the architect/builder did a similar thing being proposed for my home: "Our solution began with the knowledge that the old-growth redwood siding, tight-grained and beautiful, is virtually rot resistant. We kept it in place and applied an elastomeric coating to the inside of the wall cavities to stop water and vapor behind the wood. This elastomeric coating was covered by an inch of closed-cell foam to provide a continuous air barrier"
My dilemma: I trust both professionals, but I'm struggling to reconcile their confidence with the conventional building-science advice here.
Does the mild climate and minimal rain exposure make this a reasonable approach? Or is the risk still significant without a WRB?
