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Florida Engineering Society - Mold Article - Understanding Mold in the Indoor Environment

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We can solve the mold problem. Understanding the basic ecology of fungi will lead us down the correct path. Yes, it will take some effort to change the way we think about building design, construction, and maintenance but we can solve the mold problem. With mold claims and litigation increasing in all states, some fundamental changes must take place or we will all be affected direct or indirectly by mold.

We already know a lot about mold. Molds are microfungi with a downy, well-marked mycelium (network of filaments called hyphae) and spore mass. Fungi in general play a crucial role in nature by decomposing plant material. Without fungi the nutrients and energy bound up in plant material would not be effectively recycled and our planet would be overrun with undecomposed plant biomass. Molds require only a few crucial elements in order to grow; these include a food source, suitable temperature, and moisture.

Our buildings are chock full of suitable food sources for molds. Not only do we build most of our structures out of wood frames but we also finish the interior with wood products. We actively chop up and press together thin wood sheets (plywood), wood bits (particle board) and fibers (paper on sheetrock) which makes it progressively easier for the fungus to digest the material. Solid wood timbers are much more difficult for molds to degrade and normally it would take the more specialized wood rot fungi to colonize them effectively. It is unlikely we are going to revolutionize the construction industry by taking away wood products and replacing them with other economical, comparable products.

Can we use temperature to control fungi?

All fungi have temperature optima under which they grow best; however most fungi have a wide range of temperatures at which they can grow. Most will have an optimal growth temperature falling between 20-25 °C. There are, of course, examples of fungi that grow at very cold temperature (<5 °C) and at very high temperatures (>50 °C) but the molds commonly found growing in indoor environments grow very well at temperatures we humans enjoy (room temperature). We are not going to be able to dramatically change indoor operating temperatures as a means of controlling mold growth. Our last choice for control is moisture.

Controlling moisture is the key. Actually, moisture is currently the limiting factor staving off mold growth in 99% of our buildings. As with other parameters, fungi have optimal growth rates on substrates having specific water activities. Water activity measures the active component of water in a substrate; in simple terms, it is the water available to fungi for growth. Water activity can be calculated by dividing the partial pressure of water vapor at the surface of the substrate by the partial pressure of pure water at the substrate temperature. This ratio ranges from 1.0 for pure water to 0.0 for completely dry. Fungi that require lots of available water are called hydrophilic and grow at water activities above 0.90. Examples of hydrophilic molds include Stachybotrys sp. and Fusarium sp. They typically colonize continuously wet materials. The largest group of fungi falls in the mesophilic range. Mesophilic fungi include common indoor contaminants such as Cladosporium sp. and Alternaria sp. These fungi typically grow on continuously damp construction materials with water activities between 0.80 and 0.90. Some species of Penicillium and Aspergillus can grow on relatively dry materials. These xerotolerant fungi are able to grow at water activities below 0.80 but grow optimally above this value. The final group of fungi, known as xerophilic, actually grow best at water activity ratios below 0.80. A common xerophilic fungus is Aspergillus restrictus.

Some of these problems occur today because we build mold right into the building. Construction materials are left open to the elements at the construction site. If it happens to rain on these materials they are typically not sufficiently dried before being incorporated into the building. Material should not be stored on concrete as this can pull water out of the ground and into the material by capillary action. Materials should be stacked on plastic or other suitable material that can act as a capillary break, as well as covered from the elements. It would also help to enclose the structure as soon as possible during construction, then begin dehumidifying and acclimating materials destined for the interior inside.

Re-examining our building designs with moisture in mind is also critical for the long-term health of our next generation of buildings. A moisture barrier on the correct side of the exterior wall assembly is crucial. For Florida, this would be on the outside as moisture is drawn from hot, humid air outside to the drier, cooler environment inside. To increase the energy efficiency of our buildings, it has been necessary to make our building tighter. Making our buildings tighter has not caused mold problems, not controlling for moisture has caused the problem. Most of our buildings, if not all residential structures, are under negative pressure. In Florida, this means that moist, hot air is being sucked through every opening into the interior of the building. It has been estimated that a crack 1/16th of an inch wide by 16 inches long will allow 5 pints of water to infiltrate into a building in one day if it is under negative pressure in a humid climate. Changing our HVAC systems to create a slight positive pressure will help. Note that over pressurizing will affect energy costs.

Sudden and accidental leaks will inevitably still occur in our buildings with the best remedy being a quick drying-out of damaged materials. It is estimated that mold growth will occur on water damaged materials after being wet for only 48 hours. Deferred maintenance is not an effective strategy when mold and moisture are involved. What is clear is that we can design our buildings in a way that reduces the likelihood of moisture accumulation and hence mold growth. I will leave the details to the experts, architects and engineers. Hopefully changes to our building codes can be made with consideration for mold and moisture.

Questions? - please contact Jason Dobranic, PhD at: 800-220-3675.

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