Factory-primed wood clapboards with mold “ghosting.” The dots of mold seem to be growing through the top coat of finish paint. The green along the left-hand side is a corner board, which did not have mold ghosting.
The south side of our home is where the mold and building science discussion with my wife started. Not pictured is my wife, rolling her eyes.
My carpenter wife working with me on cladding the north side of our house (circa 2002). At the bottom of the photo, you can just see the original split-faced architectural block (making up the structure and finish of the exterior wall). At the top, the 3+ inches of closed-cell spray foam (forming the air barrier, thermal barrier, and drainage plane) and the 2x2s furring strips (providing the fastening system for the siding and the rainscreen gap) are visible.
The north side of our home. It gets very little direct sun, between its aspect and the 80-foot sugar maple just to its north.
A close-up of the clapboards on the north side of our house: no mold “ghosting.” I put up major portions of my clapboards with stainless-steel trim screws so I can easily go back and check out the condition of the closed-cell spray foam, its adhesion to the underlying concrete block, the 2x2s, jamb extensions, etc. More than 12 years later, the spray foam looks pretty much like the day it was installed.
A pin-type moisture meter (Delmhorst) showing the moisture reading in the clapboards: north or south, mold or not, the reading is 14 - 15% moisture content by weight. That's not surprising and certainly not high enough to support mold growth (which usually requires a minimum moisture content in the range of 19%).