Q&A - Hidden Mold in Wall Cavities, Ventless Gas Fireplaces

Hidden Mold In Wall Cavities
What evidence is there to support the advice that all mold in wall cavities must be remediated? Is there scientific data that shows how many or how few mold spores are released into the room from a wall cavity behind an otherwise normally-painted wall?
- Janet , California

Actually the evidence that has been presented at scientific meetings so far is quite mixed. In one reported case such "hidden mold" was associated with mold levels elevated above normal. In a second case it was not.
Whether "hidden mold" is a problem that requires remediation depends on a variety of factors. These include the extent of the infestation, the likelihood that it will contribute to increased airborne mold levels, the likelihood of disturbance, and in what walls it is located.
"Hidden mold" in an external wall is a problem and should be remediated. External walls are subject to air infiltration through the wall due to pressure differences caused by the wind and by indoor/outdoor temperature differences. In houses with hidden mold in exterior walls, it is not uncommon to smell mold odor on breezy-to-windy days and smell mold odor coming out of electrical outlets on such external walls. There is no doubt( based on my own testing experience) that such hidden mold increases indoor mold levels and mold exposures. As such, remediation is strongly recommended.
Hidden mold is also common in interior walls and floor cavities. These are typically associated with plumbing leaks and often involve gypsum board infestation by Stachybotrys. Is such "hidden mold" a problem? Intuitively, it is probable that in most cases it is not. Once the cavity is opened and not quickly resealed, it is more probable that an exposure risk will develop.
From a mold remediator's standpoint, it is good practice to remediate all mold, including that which is visible and that which is not. If one doesn't, and sometime in the future "hidden mold" is exposed, one will be held liable.