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A Little Bit of Fungal Growth

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Did a town home yesterday that was one of three in the same building in a row of three buildings with the same configuration.

The building faces a park and has a street running along one side with an alley access to the garages at the back. The unit inspected is the corner unit and there is a wraparound concrete porch with a brick parapet, columns and roof that extends halfway along the street side and along the entire front side of the building.

There're individual crawlspace beneath each unit but the only vents are along the alley side of the other end unit on the opposite end of the building.

The crawlspace is power vented with a pair of electric in-line fans and 4 inch ducts. One fan brings air into the crawlspace while the other simultaneously sucks air out. The intake and exhaust ends of the ducts are about 20 feet apart near opposite ends of the crawlspace.

There is a switch nailed to the side of a floor joist near the hatch that can be used to turn off the fans; however, both were running and have been running constantly 24/7/365 since the owner moved in 10 months ago (this is a warranty inspection).

The owner wasn't aware that he had crawlspace access until I began rooting around and discovered a hatch under the carpet in the understair closet. We had to peel back the carpeting to reveal the hatch. He had no idea that the crawlspace was power vented and that he's been paying to ventilate it since he moved in. Worse, he had no idea that the underside of his OSB flooring looked like this:

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I'm guessing, but I can't say for certain, that in this case the fans probably contributed to this by pulling in outside air that was damper than the air that one would have found in the crawlspace, which has a pretty well installed barrier, because the space was essentially kept under constant but equal pressure.

The leopard spotting is far more pronounced at the end of the crawl where the exhaust outlet duct end is located than it is around the end of the inlet duct, so I think the air movement probably causes those foundation walls to act like huge humidifiers by constantly give off moisture and I think that as one moves farther away from the incoming air duct that pressure lessens, allowing some moisture to migrate up through the insulation to the underside of the floors where its soaking into the flooring and feeding this stuff.

I suppose that in order to test my theory I'd need another house with the same configuration but with an exhaust fan that runs at a higher RPM than the intake fan, so that static pressure above the insulation in that crawlspace isn't always the same. Regardless, I don't have to diagnose it, only report it and recommend they get it looked at by an indoor air quality specialist and dealt with, but I'm wondering whether anyone else has any other theories.

I can just imagine what the center unit's crawlspace looks like.




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Hi Randy,

It's certainly possible. This stuff was slick and shiny, as if it had been scrubbed off. However, that wasn't the case everywhere under the home; other places I found similar spots that when brushed with my hand were wiped around like chalk on a blackboard. I also found some light growth on the sides of a few joists.

I considered the possibility that maybe the builder sent the junior guy down there to install the barrier, or maybe the insulation contractor, at which time the growth was discovered and then the floors were scrubbed down and the fans were installed, but I don't have any way to prove that one way or the other.

Well, perhaps I do. The fans had labels on them that made it clear that they are sold by some kind of environmental "institute." That's not the kind of sub you typically see on the list of vendors routinely used by big companies (this is a D.R. Horton) home.

The worst spotting was on the OSB that had this burnt orange coloring to it. The OSB that wasn't orange like this was where it smeared like chalk when I wiped my hand over it. I can't remember ever seeing OSB with coloring like that used for flooring but the joists nearby weren't the same color so I figured the color was integral to a finish put on the OSB and didn't have anything to do with any kind of sealant used on the underside of the floors after someone had tried to mitigate mold.



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Hi Mike. I don't have any scientific theories other than it doesn't seem like a good idea to force air into a crawl year round when, at times, that outside air might have extremely high humidity. The exhaust fan would also be sucking air in from the other vents and it seems unlikely that the air exchange would be very even. Do you know if the other two units also had fans?

I've only seen powered vents once. It was also a new home (2007, client had lived there for a month), but a single level SFR. In that case there were 3 fans located "evenly" around the otherwise normally vented crawl. All were arranged to exhaust, and all were controlled by humidistats. There was nothing else unusual about the house and I don't know why they were installed in this particular home, but there was a plate near the crawl access from a "mold" company. I vaguely remember discussing them with the client who was unaware of the fans or any special reason for them. As there were no indications of moisture problems, or any visible mold, I reported their presence but made no other recommendations.

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