How to Grow Mold … And How to Prevent It!

Everyone has heard of the situation where a building surface has been wetted by water intrusion (leaky pipe, damaged roof, misguided sprinklers) and mold growth has developed. This usually leads to a mold remediation exercise where mold contaminated surface materials (carpets, hardwood floors, wallboard, ceiling tiles) are removed and replaced. It is necessary to replace the structural support for these materials at times as well.

This article is not about that type of situation. In this article we will discuss two errors, one in building construction and one in building maintenance, that can cause mold growth without water intrusion directly into the building.

Usually the first thing you do in a mold investigation (after sampling the mold) is to search for the source of the water intrusion. In some cases, there is no actual water intrusion from outside the building, nor from leaky or faulty systems within the building. With the advent of airtight and watertight surfacing materials, especially flooring materials, simply enclosing a building cavity can create a terrarium effect. This allows mold to grow inside the cavity using the water from the materials used to construct the building (non-kiln dried wood studs, sheer wall plywood, floor substrates, etc.). If these cavities happen to cover open grade (bare dirt), the moisture in the dirt can also become part of an isolated ecosystem which promotes mold growth.

The worst case that Hazard Management Services, Inc. (HMS, Inc.) has dealt with involved a three foot high stage, over open grade, with a watertight and airtight flooring surface (vinyl sheeting) and no ventilation. A two-year-old building had mold growth in the sub-stage area that destroyed the 2″ x 8″ floor joists and the plywood floor substrate to the point that the vinyl flooring was turned into a trampoline. Worse, the only route of escape for the air and moisture, pulled from the wood and dirt surfaces by the terrarium effect created by the construction of this stage, was between the sheets of sheer wall plywood. This minuscule escape route for the air and moisture allowedStachybotrys mold to grow along the nail lines of the sheer wall.

Mold growth on wood, when confined to the surface as it was here, can usually be sanded down to “clean” wood in order to remove the mold growth. However, if you sand on the nail lines of sheer wall, the structural integrity of the sheer wall is lost, and the sheer wall must be replaced. Because a sheer wall is nailed in a close-together stitch pattern, removal of sheer wall from the studs in the wall leaves behind studs that are also no longer structurally sound. As a result, every stud in the wall had to be sistered or replaced, even though no mold had grown on any of the studs.

The total cost for the repair of this 500 square foot stage area damaged by mold due to improper design and construction (lack of ventilation) in a two-year-old building was over $50,000.

The second case involved improper maintenance activities on an existing heating ventilation and air conditioning (HVAC) system. Rooftop HVAC units pulled air from return air ducts and from the building exterior. These HVAC units had rooftop access hatches which allowed the system filters to be changed out. These access hatches were sealed to the HVAC ducting with ten small screws. Maintenance personnel over the years stopped attaching the access hatches with all ten screws, attaching them with two screws at diagonal corners only.

This practice saved the maintenance crew minimal amounts of time each time the filters required changing. However, it also created a system where air moved around the filters, rather than through them, and allowed water to enter the ducting whenever it rained. Though the dust and water never made it into the building, it did settle on the interior insulation within the air ducts. The combination of dirt, water and airborne mold spores allowed mold to start growing in the linings of the ducts. Once this mold growth got extensive, health complaints within the building started to be reported.

After an investigation costing over $1,000 which included dust samples, tape lift samples, and air samples, the problem was identified as the improper sealing of the access hatches on the duct work. Cleanup of the classroom building and the associated duct work cost over $40,000 all because the maintenance crew did not take the time to properly seal the filter access hatches.

Mold growth is not always avoidable, as hidden water leaks within walls, improperly insulated building cavities (condensation), and other changes within a building can cause hidden damage that is not apparent until the mold has taken root and grown extensively. However, in the first example above, the Architect, the General Contractor and the Inspector of Record all could have prevented the problem. Constructing a ventless building cavity over open grade is not only an invitation to grow mold, especially with an airtight flooring surface, but it is also against current building code.

The second scenario above, with the HVAC system access hatches that were not sealed, is an easily avoidable situation. However, because of the propensity to improperly maintain HVAC systems, it is recommended that interior duct lining be eliminated from construction. Even with proper access hatch use and timely filter changes, interior duct lining will eventually gather dust, if it doesn’t deteriorate and become an indoor air quality problem in and of itself. If there is any water intrusion into an interior lined duct, mold growth is almost guaranteed.

You can best avoid growing mold by assuring construction is conducted to code, watching for water intrusion into the building or building systems, and properly maintaining both the building and its systems.

Written by Michael C. Sharp, C.E.O. of Hazard Management Services, Inc.

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