Q: While remodeling a house, the roof was partially exposed, causing a brief rainwater intrusion in some walls. A test revealed mold on the bottom 2 ft. of some exterior walls. Exterior and interior walls are plaster where mold does not grow. However, the wood structure inside the wall may be susceptible to mold, although there's no moisture present now. Is there a way to destroy the mold by means other than breaking the plaster? Will mold stop growing in the absence of moisture in the walls and eventually disintegrate, or will it destroy the wood? The molds are Aspergillus, Basidiospores, Cladosporium and Stachybotrys, all small quantities. And, what level of encapsulated mold is acceptable, if any?

A: Typically mold will not become established on wood surfaces after a brief single exposure, though it happens all the time in lumberyards. Drying the wood through ventilation and heat should minimize any fungal growth. Wood contains naturally occurring chemicals (i.e., terpenes, the basis of turpentine) that resist fungal growth internally. It generally requires long-term or chronic moisture for wood to become mold-infested.

Surface mold can become established on wood framing and plaster lathe because of condensation moisture that occurs when warm, humid air contacts cold surfaces. Depending on the climate or season, an air-conditioned interior and the warm, humid exterior or a cold exterior/heated interior can allow condensation moisture within a wall cavity, depending on the quality of vapor barriers and insulation. For example, vinyl wall paper (a vapor barrier) on a gypsum board wall in an air-conditioned house in a warm, humid environment may cause mold growth if the exterior air is drawn into the building. The moisture will pass through the exterior building envelope and may condense on the cool wallpaper adhesive. Vapor barriers should be on the warm side of a wall and should not trap moisture in the wall. Positively pressurizing the building by ventilation is an ideal solution to this type of problem so that warm, humid air can’t infiltrate the building.

If mold has become established in a wall cavity, it will not continue to grow if the cavity is dry. Mold spores are resistant to desiccation and, while it will not disappear, any mold toxins present are not alive and are quite stable. Two of the fungal types you listed, Cladosporium and Basidiospores (mushrooms), are very common and dominate the outside air mold. Generally they are not considered pathogenic. Aspergillus, while a common type of mold, can contain mold toxins in some species,and one species (Aspergillus fumigatus) can infect individuals with suppressed immune systems. The mold type Stachybotrys has had a lot of press, perhaps a little more than it deserves. It can, but doesn’t always, contain mold toxins and its spores are large and sticky. They usually do not become airborne unless disturbed. Stachybotrys is also a slow grower which generally becomes established on paper or gypsum board (not wood). Its growth usually requires long-term rather than a single-event exposure to moisture. Its presence may indicate that a second source of moisture exists in the walls or that spores were transported from another location. Often the presence of a few spores without other fungal structures doesn’t indicate fungal growth. Your question did not state what kind of mold test was performed (surface, air, bulk or dust). What kind of mold test and whether only spores were present impacts the evaluation and its usefulness.

Finally, what can be done to remediate the wall with out tearing it down? Assuming the wall is plaster on wire mesh or wood lathe and the mold is only on the surface of the wood, you could fog the walls with disinfectant and fungal encapsulant. This has been successfully done in some environments. Post-remediation testing of the wall cavity is recommended to determine the efficacy of the procedure. The occupants of the house should not be present during such a procedure, and if they are chemically sensitive, the cure might be worse than the disease. Although these products are often water-based and do not impact most people, no chemical should be considered completely free of side effects, particularly when freshly applied. Guidelines from the U.S. EPA do not recognize fungal disinfectants and encapsulants in their recommendations. They recommend removal and cleaning. Although this is an ideal approach, it is impractical in wood frame buildings where cleaning, even without interior walls being present (such as behind wood framing) is not feasible.

Thanks to Mark Goldman, who is a senior air quality scientist for Engineering and Fire Investigations (EFI), a firm that provides mold investigation and indoor air quality services nationwide. Mark can be reached at mark_goldman@efiglobal.com

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