Peter Yost is a building scientist and regularly blogs at Green Building Advisor. A recent post of his chronicled his adventures dealing with some sort of bio-organic growth on the recently added siding at his home — but just at the south side, not the north.

Mr. Yost’s wife first noticed the issue and began the conversation as follows:

“OK, Mr. Building Scientist, you supposedly worked your moisture magic when you re-sided the house with clapboards[…] But I am looking at little black dots all over the siding. It looks a lot like mold to me.”

You’ll want to read the full post for details, but the Reader’s Digest version is that some species of mold or mildew was relying upon the oil in the oil-based paint as a food source, rather than his wood siding. Here are some of his lessons learned:

  • Keep your eye out for oil-based primer on exterior wood trim and cladding. It’s great to order materials that have been factory-primed, but what they use can make a difference.
  • Beware of potential problems when you install a latex topcoat over an oil-based primer. If you are in any sort of “wet” climate (generally more than 20 inches of precipitation annually), these are probably not a good mix.