Has anyone seen this kind of result from a rain screen wall?
http://homepage2.nifty.com/arbre_d/news/news_top/DSCN1970.jpg
This is a photo on a Japanese website (http://homepage2.nifty.com/arbre_d/news/news_top/81.html) emphasizing the importance of criss-crossed furring on a rain-screen wall, to allow air to travel both vertically and horizontally behind the cladding. The photograph is said to be of a rain-screen wall built by a “major” construction company; the vertical blue lines are said to indicate where the vertical furring strips are located; the dark smudges on the wall between the blue lines are the fungus growing on the outside of the wall due to the moist air behind the wall. Several other photos on the page show the criss-crossed webbing of furring strips that are supposed to allow air to circulate more freely behind the cladding, thus preventing such patterned growth (FWIW, this is the north side of the building, where mold/fungus often grows on houses here in Japan.) Anybody seen anything like this in your corner of the world?
PS: the page doesn’t mention what the cladding is, but it appears to be some kind of cement or tile.
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I'd like to believe that, but the guy's whole point is in commenting on the criss-cross furring strip method, so this must not be the only case like this. Maybe it's a "feature" of our monsoon climate, or something to do with the specific kind of siding.
The furring pattern affects the temperature of the surface, and hence how much dew will settle and how quickly moisture will evaporate. This in turn affects how much dust/dirt settles on the surface, and also how much surface mold might occur (if that is apt to be a problem). You see similar patterns (only in reverse) in a smoker's house, or a house with a smoky fireplace. The temperature difference between hollow wall and wall with stud behind affects where the soot settles.
Criss-cross furring would reduce heat transfer between the building and the siding, reducing the temperature "contrast". This would allow the "fungus" to form evenly over the whole surface, but likely the guy didn't cherry-pick pictures of that.
If lack of air circulation were the problem you'd expect to see the worst "fungus" near the furring, where air circulation would be most severely restricted.
It looks like mildew to me, probably exacerbated by (1) the proximity of plantings and (2) the rough cementitious surfaces.
I'd want to know all about the painting - was a high-alkali primer (like Sherwin-Williams Loxon) used due the higher-alkali cementitious surface?
I see that sort of effect on improperly-painted, non-rainscreen applications of Hardiplank all the time.
Jeff
fungus on rainscreen
this could also happen when the rainscreen is "sealed" - air is trapped in the rainscreen when the top and the bottom of the rainscreen is closed. Venting the rainscreen is the key