By Stephen Andrews

A few months after I began inspecting buildings for mold I was asked by a real estate agent representing a potential buyer to test for mold in a four year old home. The home was bricked on all sides. On two sides of the home the finished basement was completely below grade with a third side about fifty per cent below grade. The basement on the back side of the home opened out onto a brick patio. Although the patio was about the same elevation as the concrete slab in the basement, the patio sloped away from the house at about one-quarter inch per foot. This should have been sufficient to provide adequate drainage away for the home. But because to ground was almost level with the concrete slab on the backside of the home, other problems were being created.

I visually inspected the entire home and the HVAC (heating ventilating air conditioning) system and could find no sign of mold anywhere. I took moisture readings in the hardwood on the first and second floor as well as relative humidity readings in the basement first and second floor. The basement was fully carpeted so moisture measurements were not taken in the concrete slab. Conditions to sustain mold growth were not evident. I took air samples for mold test in the basement, first and second floor and an outside sample for comparison. When the laboratory test came back they showed elevated levels of Stachybotrys (black mold) as well as Penicillium and Aspergillus. This was perplexing because Stachybotrys requires a food source that is almost saturated.

The test results dictated a second and more comprehensive mold investigation. The ground level around the concrete slab should have been at least six the eight inches below the concrete slab floor and since the ground outside was about the same elevation as the slab, I thought the source of moisture may be coming from the edges of the concrete slab. I proceeded to pull the carpet back and check the moisture levels in the concrete around the concrete edge. Sure enough, the moisture in the top one inch of the slab and within about two feet of the concrete edge measured thirty to forty per cent. The moisture five feet from the concrete edge measured only twenty two per cent. After finding the source of moisture it wasn’t hard to find the mold growth. There was an adjoining exercise work-out room with rubber mats covering the carpet on top of the concrete floor. The carpet under the rubber mats was literally black with mold and damp with moisture that migrated through the concrete slab.

The potential buyer visited the house on three different occasions and said she got a headache each time – a common mold symptom. The solution was to replace the carpet and lower the grade on the back side of the home to eight inches below the concrete slab and install a ducted dehumidifier on the basement air conditioning system to keep humidity below fifty per cent at all time. This allowed the slab moisture to continuously dry and prevent humidity levels from rising to levels that would support mold growth. Of course they were also instructed to refrain from using any type of floor covering that would trap moisture on the slab.