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Building Science

Pete’s Puzzle: Mold on Painted Clapboards is Food for Thought

There is mold on the factory-primed, latex top-coated wood clapboards on the south but not the north side of our house

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.
Image Credit: All photos: Peter Yost
View Gallery 6 images

Whenever my wife starts a conversation with, “OK, Mr. Building Scientist,” I know I am in some kind of trouble. That proved to be the case one day when we were out hanging laundry on the south side of our house.

“OK, Mr. Building Scientist, you supposedly worked your moisture magic when you re-sided the house with clapboards.” (See Images #2 and #3, below, and this GBA Green Home case study). “But I am looking at little black dots all over the siding. It looks a lot like mold to me.” (See Image #1 at right.)

Sure enough: mold seemed to be growing through the paint rather than simply growing on the surface; very strange. And stranger still: no mold of this sort (or any sort) growing on the north side of the house, nearly always shaded by our large sugar maple (see Images #4 and #5).

How else are the two sides of our house different?

The deep-energy retrofit of our house — including the recladding of our exterior walls — has been a 12-year project, mostly “weekend warrior” work of mine. That is quite a bit of time for things to change from one side of the house to the next.

And change they did: the north gable of our house I did two years earlier than the south, with what turns out to be a completely different source of pre-primed, finger-jointed wood clapboards. My local building materials supplier — without any notification — switched sources, from local cedar pre-primed, finger-jointed clapboards to Paulownia clapboards from the Philippines (which I purchased because I was told that the Paulownia is equal or better quality, a “greener” product, and more cost-competitive). See .

Ah, OK. The mold culprit has to be this new wood, I thought. Sure, the Paulownia seemed as robust as the cedar when it first went up, but look at what happened over time…

The problem is not the wood…

As I talked around about this problem, a guy smarter than me said, “How could the wood be the problem? The mold is not growing in the wood.” I felt like he had just said, “OK, Mr. Building Scientist…” (See Image #6.)

I have to admit, I was stumped (pardon the word choice). So, I did what I always do when I have a question about wood building materials: I called my good friend at the Forest Products Laboratory (FPL), Bob Falk.

Bob was nearly as puzzled as I was, but he knew exactly who to ask at the FPL: Research Chemist Chris Hunt:

“Growth of mildew is determined by three factors: food, water, and mildewcide. Availability of spores or colonization is not a problem: the entire outdoor world is covered in spores, and everything gets wet once in a while allowing them to germinate. What makes for visible growth is good conditions on a regular basis.

Enough water for mildew to grow occurs with RH is in the 85%+ range. Food might come from the wood but more likely comes from oil-based primer. Oil is wonderful food. Oil-based paint/primer is also. Modern oil-based paints are not as tasty as the older formulations but still have some food value.

Mildewcide/biocide is commonly put in oil-based paint but not so much in latex because bugs have a hard time digesting latex paint. Latex paint is very porous, however. Mold will burrow down through many coats of latex to get at alkyd layer or other food source underneath.”

Chris also suggested this resource: .

NOTE: If you go back and carefully read the EPrime brochure I linked to, you’ll find that, sure enough, the factory-applied primer is oil-based.

So, the solution?

Chris Hunt from the FPL continues:

“My recommendation is clean thoroughly and then topcoat with another latex. However, this time buy mildewcide and add it to the paint before application. Another approach that worked in the past is top-coating the existing paint job with a solvent-borne water repellent preservative to give about 5 years of protection… Beware of a glossy appearance, however.”

My 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.
  • Don’t ever let anyone cut the FPL budget; these folks know more about wood than all of us builders and remodelers and architects put together, and we need their thorough and practical knowledge.

In addition to acting as GBA’s technical director, Peter Yost is the Vice President for Technical Services at in Brattleboro, Vermont. He has been building, researching, teaching, writing, and consulting on high-performance homes for more than twenty years. An experienced trainer and consultant, he’s been recognized as NAHB Educator of the Year. Do you have a building science puzzle? . You can also to get a free report on avoiding toxic insulation, as well as regular posts from Peter.


  1. John Clark | | #1

    ROFL !!
    First two paragraphs had me rolling. haha

  2. Doug McEvers | | #2

    Is it the oil primer?
    In Minnesota I have been insisting raw wood siding be primed on all sides (ends also) with oil primer. In this climate at least, oil priming has worked very well. Two top coats of latex have lasted as long as 18 years on one project. Walls are house wrapped and well air sealed so moisture moving through the walls is minimized. I also prefer a rabbeted bevel cedar siding over plain lap as no cupping occurs over time.

  3. Brendan Meyer | | #3

    Cheaper for a reason
    Stories like this remind me why pre-primed finger jointed stock is cheaper than unprimed solid wood stock. It's cheaper for a reason and it's not because it's the better product.

  4. Malcolm Taylor | | #4

    Surely it's cheaper because the product is made out of unusable mill ends that would otherwise be discarded? Finger-joints may weaken the piece, but I can't see how they would have any role in mold growth.

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