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Mold on cedar siding

| Posted in General Questions on


Three years after installation we are seeing increasing black mold growth on our Cedar cladding. Climate in 4c, near Vancouver, BC. 

The cedar is treated on all sides with Osmo UV-Protection Oil 420 that is supposed to contain biocides to prevent the growth of algae, mildew and fungal attack. We have attempted to clean with a proprietary Osmo wood cleaner to no avail, the growth appears to be under the oil coating. 

This is a Western facing wall, but due to tree shading doesn’t get sun in the winter. The wall makeup is Zip, 2″ Rockwool, 1″ strapping rain screen then the siding. This elevation has a generous 6′ overhang. 

I have read on here that all oils can actually feed mold growth. So are we better off using another product? If so, any suggestions on removing this and what to reapply? We are trying to avoid the orange tint many ‘Cedar’ stains give, we were otherwise happy with the colour the Osmo provided. 

My second concern is the potential for lack of ventilation on the rain screen I built. Further reading has lead me to believe I should have vented at the top of the rain screen more, under windows the siding is tucked into the J channel and similarly at the top of the wall assembly there isn’t a specific gap, the cladding is simply finished with a J channel against the soffit. 

Is this lack of top ventilation causing excessive moisture build up perhaps? If so, any retrofit ideas for increasing the ventilation? I can only think of drilling a few 1″ holes and covering them with a mesh louvered grill such as those used on boats, but I am not sure that would look particularly great.

Thank you very much. 

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  1. quinnowen | | #1

    Attaching photo again as it doesn't appear to have attached on original post. Thnx

  2. Expert Member


    Unfortunately we live in a rain forest one of whose aims is to turn any wo0d back into soil as quickly as it can. Mold on shaded north and west faces of houses here is inevitable. Penetrating stains seem to allow the mold to get deeper into the wood that surface treatments, but are much easier to maintain. I've tried Behr, Sikkens and couple of other brands and have experienced the same mold growth under the finish. All I have found to be effective is frequent removal of the mold before it can gain too much of a foot hold - or of course using a solid colour stain - but that unfortunately sort of defeats the purpose of using cedar in the first place. All this is a long way of saying I don't have any useful advice beyond reassurance it's something everyone in the Lower Mainland faces.

    I can assure you it is nothing to do with how you constructed your rain-screen. Venting a the top of the cavity and at windows has no appreciable effect on the longevity of the siding or how much mold it takes on the PNW.

  3. ERIC WHETZEL | | #3

    I've had mixed results with this product:

    I've had better luck with this product:

    It's basically glorified chlorine bleach, so it needs to be used with caution, especially in regards to skin, eyes, clothing (anything you want to protect from overspray). I would start in a small area first, just to see how the wood and the Osmo responds. After a few minutes, I always rinse off the treated surface with water as well.

  4. joenorm | | #4

    I live in the same area and am experiencing small doses for the same issue. I do not think this is a rain screen or venting issue. Has everything to do with the shade/side of house and living in a moist place. Looks like a good excuse for a blackish stain.

  5. quinnowen | | #5

    Thank you all for the feedback. I am pleased it is not an inherent construction issue. But the cleaning and recoating sounds challenging! I think we'll sand it back, clean with a mold product as suggested by Eric and see what happens. Thanks again.

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