Potential Mold without Moisture!

Team, I ran into a very puzzling inspection today where there is a “potential” mold formation inside a wooden closet. Problem is that the back wall of the closet is a normal bedroom wall with no sources of pipes ducts, or any even AC outlet !
The moisture meter shows no signs of moisture…What is that??


Your moisture meter only told you that the moisture was not elevated at the time of inspection. It did not tell you about moisture that has been present in the past. Even really high humidity combined with lack of airflow can cause mold/mildew growth.

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Thanks, Ryan. Fully agree
Now, what you mean that the mold with that intensity could have been formed just because of lack of airflow and humidity in the air, without any “past” water accumulation of some sort?
And it yes, what only in this one shelf and this one spot?

Sure, have you ever been in an unvented bathroom with a shower and seen the mold/mildew growth on the ceilings?

As far as the single location, may be just where they missed when cleaning, area could have been blocked from airflow, or may have just been the only place with suitable food for growth. Also could have been a leak from the roof that ran on the interior of the wall in that area.

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The leak is not an option as it is in 3rd floor with no bathrooms or kitchen above that wall in the typical apartments above

My guess would be that the mold grows only in cold weather at a time where the outside temp is cold enough for condensation to form on the interior surface…where that surface is then wet enough but still warm enough to support mold growth. Cold outside temp is the key to when moisture (condensation) would be present. have found this same thing usually under sinks where plumbing in walls did not allow for the wall to be insulated.

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I have been in walk-in-closets where there was no supply vent and where the homeowner had the door closed. Over time surface mold developed on the walls.

In one particular instance, the RH in the closet was close to 65%, while in the rest of the home it was in the low 50’s.

With a high RH, and no air-flow, mold can start thriving on the walls.

Another scenario: where there is an attic access in a closet, many times warm humid air comes in that way (access cover not sealed well). If the a/c supply vent is within a few feet of the access, condensation happens and then mold.
Sometimes if the supply vent itself is not sealed, attic air infiltrates that way, and condensation happens on the register.

In all of these scenarios, there may or may not be elevated moisture within the wall - only surface moisture.

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Note as well the towels and clothing; I’ve seen instances when people leave damp clothing or towels under sinks, which allowed for mold growth in the cabinet. Mold just loves particle board!

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Closets are well known as havens for mold… which is why the old school builders in my area always included ventilation – for closets.

(this often was to the attic, which stopped working once the attic was insulated).

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Tell them to clean it and not to put damp towels or clothes in there. :smile:

I often see this in closets. More common on exterior walls when that humid wet closet air condenses, but I’ve seen it on interior walls as well. In fact just today. Could even storage area for mops, etc.