Here’s yet another foam retrofit post that’s a lot like the others, but just different enough that I’m hesitating to spray without outside input (beyond that of the foam salesman).

What do you all think, should I go ahead and fill my exterior walls or let them stay “ventilated?”

Zone 4, DC metro area. This 100-yo, 2-story Craftsman has balloon-framed walls with stucco exterior on the first floor and narrow clapboard on the second floor. Nice, big roof overhangs do a lot to protect the 2nd floor clapboard, but the 1st floor stucco still gets hit with slanty rain.

There is no rain screen gap in the wall assembly, and this seems to be the critical detail that folks want to see in wood walls. Otherwise, the walls appear to be done well. From inside out it’s:
 1. painted, horse hair plaster
 2. wood lath
 3. furring strips, seemingly meant to accommodate the window frame thickness
 4. wood 2x4s (actual, not nominal)
 5. empty cavities
 6. 1×6 horizontal bracing, probably tongue & groove
 7. homemade tar paper; tar is nice and thick
 8a. wood clapboard siding (2nd level)
 8b-1. wood stucco keys (1st level)
 8b-2. two-coat, pebble-dash, lime stucco (1st level)
 9. various layers of peeling paint
 
As they say, the radiators (and air circulation) are saving all the wood in the walls from the moisture that’s being driven into it. And some will say that spray foam will stop this process and cause the walls to rot. The foam salesman, however, isn’t at all concerned. He’s just glad I don’t have OSB anywhere in the walls.

I’m hoping the tar paper is in good enough condition to keep weather-related moisture out of the walls. I’m also hoping that interior-sourced moisture will be mitigated by our typically dry winters and new HVAC systems (particularly the VAC parts) in the summer. In spring/fall, we tend to keep the windows open.

FWIW, I had a blower door test done and just got the report: 8.9 ACH in the existing condition.