Troubleshooting a Mold Problem on Redwood Siding

      An unusual insulated wall assembly with redwood siding over asphalt paper and studs has a mold problem but just between the studs. April 24, 2009

The exterior is built of 2x4 studs with polyurethane sprayed in foam insulation. Then a layer of 30lb black paper (roofing tar paper) and then carmel grade red wood. Over the years the 2x4's show through, they don't discolor. What does discolor is the area between the 2x4's which turns a dark color (almost black). Over the last 25 years they have simply washed the exterior with a mild bleach or soapy water solution and then used Watco exterior oil (natural clear). They have been during this about every five years to keep the exterior walls clean and a uniform natural red wood color. Is there another product we can use to solve this problem of the studs showing through and keep the areas between the studs from turning a black color? The Watco oil seems to only keep the exterior walls a uniform color for two or three years before it starts to discolor between the studs. They do not want to use a paint or stain just a clear finish that will solve the discoloration problem and leave the natural red wood color. Any recommendations?

Forum Responses
(Architectural Woodworking Forum)
From contributor H:
You have sprayed in foam between the studs and that area turns black. The cause is obvious. The solution - none unless you stain the siding with a "redwood color" opaque stain.

From Gary Katz, forum technical advisor:
Please clue me in? I'm not familiar with sprayed foam insulation. What causes the black staining? I'm very curious.

From the original questioner:
Gary, what I think is happening here is that the urethane insulation is reflecting the outside temperature (no heat loss). When it does this it causes moisture condensation. This moisture allows mold to form and build up. I don't think that it is the black tar paper bleeding through or it would also turn black where the studs are? If it is mold then that would explain why a mild bleach or soapy water solution would remove it. I was hoping someone would have a recommendation for something that would prevent the mold from forming. This is just my best guess as to what is going on with this discoloration.

From Gary Katz, forum technical advisor:
Thank you for the explanation. The cause wasn't clear to me at all. I'm still puzzled by it, but that will pass in time. The soapy water helps me visualize the discoloration much better. Personally, I think rain screen walls solve almost every water/mold/paint longevity/stain longevity problem. Plywood behind your siding might solve the problem, but it could also create new ones. An air space that would allow moisture to diffuse through evaporation and air circulation would probably be much more effective. But that would mean removing the siding. Certainly not something you had in mind. But I don't see how putting a stain on top of the siding will resolve a problem that's caused by moisture on the back of the siding. But maybe I'm still missing the whole picture.

From the original questioner:
Gary wrote "but I don't see how putting a stain on top of the siding will resolve a problem that's caused by moisture on the back of the siding."

Gary, the moisture I'm referring to is condensating on the exterior of the siding. I don't see how it could form on the backside. Your comment about rainscreen walls is correct at least with this structure. There are some redwood walls that are under four foot overhangs and those walls do not have this problem. It is confined to the walls that are exposed to weather (rain).

From Gene Wengert, forum technical advisor:
Indeed, it is condensation and mildew forming in the wet cool areas. You can use a fungicide in a finish to help control.

From the original questioner:
That was my thinking also, that it was a mold problem. Do you have a recommendation for a specific product that could be added to the Watco oil? Or would you recommend that we overspray the wood after it has been oiled? I suppose we could put the building on an annual spraying using one of those garden weed type sprayers. Do you think that would work to keep the mold from forming if done annually or would it have to be done more often?

From Gene Wengert, forum technical advisor:
The application of fungicides is tightly controlled to prevent human and environmental damage. Contact a finishing company or contact your local country extension agent to find an approved and effective chemical.

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