Q: We are in contract on a 70-year-old lath plaster wood frame house in the San Francisco Bay area (east bay) that had significant dry rot on one side of the house-the sellers are doing the $20,000+ remediation to remove the rot before closing. There had been recurring leaking over the past several years from a faulty flat roof which was replaced with a new bitumen built up roof this past December.

During a mold inspection this week, it was determined by air sampling by a licensed person that the aspergillus and stachybotrys levels were very high in the downstairs bedroom and adjacent laundry room (both on ground level- not subterraneous) -moisture contents were also high along some walls/window sills in those two rooms along the same wall that had the significant dry rot. The upstairs with living room, dining room, 3 bedrooms baths kitchen etc had acceptable levels of aspergillus relative to outdoor sample taken and no stachybotrys. During the tests, it was observed that the person removing the rotted wood outside came into the laundry area for a short period leading to some speculation that he brought in at least some spores-this did not explain however that the bedroom had significantly higher levels of spores than the laundry.

While there is a lot to discuss here, we were wondering what should be done to ensure, if we go forward with the home that it will be safe for our family including me (with severe allergies to mold and possible asthma) my wife and 9 month old son who also appear to have allergies. We were told that the rooms in question could be contained and air scrubbed and tested again to see if mold is penetrating through the wall as opposed to being brought in by a workman. If so, we were told that sealing off the wall after removal of the rot would prevent further mold growth. I also know that the remaining mold even if sealed off will remain dormant so if further leaks occur and the seal is broken, mold growth in the wall cavity could return. The licensed mold testing person said she was concerned (i) that even if readings were negative from the air scrub test, that we would need to know why the interior moisture readings were so high and (ii) the existence of aspergillus/stachybotrys on the dry rot wood made her very concerned. Given the history of the house with leaks etc, the sealing option does not appear acceptable to me given our health issues. Would you recommend a complete eradication of mold-ie opening up the entire exterior wall while they do dry rot removal to inspect the lath/plaster for moisture penetration and then opening up the interior wall if mold needs to be removed from the wall cavity. Will this be the only way to ensure complete remediation?

A:The airborne mold results don’t inform you as to where the mold originated or why it is present. Mold colonies do not always shed spores and one type of mold may shed spores under one condition, while another type will shed under different conditions. You can trust positive results but negative results of airborne mold testing are not convincing. Actual testing or visual inspection of the suspected source of the mold is the only absolute method.

I agree with your local investigator that the remediation isn’t complete until the building is dry and the cause of the water intrusion is repaired. If the roof was repaired 4 months ago and the heat has been operating over the winter, the walls should have dried out. The interior moisture readings may indicate flashing or caulking failure on the outside of the building. The presence of Stachybotrys is also somewhat puzzling in a plaster/lathe building finished unless there is vinyl wall covering with cellulose-based wheat paste that has gotten wet. Stachybotrys tends to grow on cellulose and is not often seen unless paper or gypsum board has been exposed to chronic wet conditions.

There has been some debate as whether sealing extensive mold growth into wall cavities is acceptable. I don’t recommend it. Pathways from the wall cavity into the occupied areas will always be present. Buildings are not and should not be air tight. Atmospheric air pressure variations from changes in the weather will cause buildings to change the direction of air movement in wall cavities. The only way it might be acceptable would be to pressurize the building so that air leaks out all the time as is done in hospital immune suppressed treatment areas. This is rarely done in residential buildings.

Response provided by: Mark E. Goldman, CMC, Senior Air Quality Scientist EFI Global, Inc.

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